Piedmont GeologyPiedmont Geology johncallahan Thu, 06/24/2010 - 22:49
Overview of the PiedmontOverview of the Piedmont johncallahan Sat, 06/27/2009 - 00:08
The Appalachian Piedmont and Atlantic Coastal Plain are physiographic provinces that are separated by the fall zone. The fall zone (also called the Fall Line) is the contact where the hard crystalline rocks of the Piedmont dip under and disappear beneath the sediments of the Coastal Plain. The landscape and rock types shown in northern Delaware are classical examples of the larger geologic features that dominate the geology of eastern North America.
There are reasons why the major cities line up parallel to the coast and in accordance with the trend of the mountains. The fall zone was the limit of navigation to the European explorers. This and the availability of fresh water suggested the location of the initial settlements. Earth materials in the regionâstone, sand and gravel, brick and china clay, mica, and feldsparâsustained economic development for several hundred years. Waterpower provided energy essential to the Industrial Revolution. Commerce and communication by ship benefited by access to tidewater. Northeast-southwest land travel and communication benefited from the relatively flat topography of the inner Coastal Plain. No wonder that our initial population concentrated along the fall zone. Delaware's Piedmont provides outstanding illustrations of the influence of geology on our history and society. Our forebears lived close to the land and understood its basic dictates. Pressures of land use and environmental protection now prompt us to rediscover the essentials of geology and apply modern science to advance applications appropriate to todayâs needs.
Image of the Fall Line from the USGS National Atlas: http://nationalatlas.gov/articles/geology/features/fallline.html
Geologic History of the Delaware PiedmontGeologic History of the Delaware Piedmont johncallahan Mon, 09/21/2009 - 14:44
The Delaware Piedmont is but a small part of the Appalachian Mountain system that extends from Georgia to Newfoundland. This mountain system is the result of tectonic activity that took place during the Paleozoic era, between 543 and 245 million years ago. Since that time, the mountains have been continuously eroding, and their deep roots slowly rising in compensation as the overlying rocks are removed. It is surprising to find that although the Delaware Piedmont has passed through the whole series of tectonic events that formed the Appalachians, the mineralogy and structures preserved in Delaware were formed by the early event that occurred between 470 and 440 million years ago, called the Taconic orogeny. This event was triggered by the formation of a subduction zone off the coast of the ancient North American continent that slid oceanic crust on the ancient North American plate beneath oceanic crust on the overriding plate, produced magma, and fueled an arc-shaped chain of volcanoes. This volcanic arc existed in the late Precambrian-early Paleozoic along most of the eastern margin of the ancient North American continent (Fig A). In Delaware, there is some evidence in the Wilmington Complex to suggest that the overriding oceanic plate included a small island cored by continental crust.
As convergence continued, most of the sediments deposited on the subducting plate were scraped off to form a thick pile of deformed and metamorphosed rocks. In Delaware this accreted pile of sediments became the Wissahickon Formation. The many amphibolite layers in the Wissahickon suggest that these sediments may have been mixed with ash falls, basalt flows from the volcanoes, or slivers of underlying oceanic crust that were broken off during scraping (Fig B). Eventually, continued convergence dragged the ancient North American continent into the subduction zone where it collided with the volcanic arc and pushed up a gigantic mountain range (Fig C). The creation of this range signified the end of the Taconic orogeny along the Appalachians. Today the once lofty mountains have eroded away leaving their roots exposed in the rolling hills of Delawareâ€™s Piedmont. The intense metamorphism that occurred when the root zone was deeply buried in the base of the mountain range has obscured most of the rocksâ€™ original features; however, careful study has recognized a series of rock units that represents the ancient continental margin.
The amphibolites and "blue rocks" of the Wilmington Complex were formerly a volcanic island that existed seaward of the ancient North American continent about 500 million years ago. The gneisses of the Wissahickon Formation represent sediments deposited in a deep ocean basin between the volcanic island and the continental shelf. The pure white crystalline marble of the Cockeysville Marble is the metamorphosed equivalent of a carbonate bank or reef that formed just off the ancient shoreline. The impure quartzites of the Setters Formation were certainly dirty beach sands, and the Baltimore Gneiss that forms the basement under the Setters and Cockeysville formations is billion-year-old rock, assumed to be a remnant of the ancient North American continent (Figs A, B, and C).
The rocks in the Delaware hills are still eroding (Fig D); their surfaces are fractured, broken, and covered with moss and lichen. As the rocks disintegrate, small pieces wash into the creeks and rivers to begin a journey that may take them to the Atlantic Ocean where they will be buried on the continental margin. Millions of years from now subduction may begin again off Delawareâ€™s shore, and these sediments will be caught in another cycle of mountain building and erosion.
Common Rocks and Minerals of the Delaware PiedmontCommon Rocks and Minerals of the Delaware Piedmont johncallahan Thu, 06/24/2010 - 23:27
The Red Clay Creek has flowed through the rolling hills of northern Delaware for many thousands of years, cutting a deep valley into the old deformed rocks of the Appalachian Piedmont. The Red Clay valley contains many of the common rocks found throughout the Delaware Piedmont.
Igneous rocks are those that form by the crystallization of a hot molten liquid called magma or lava. We can see igneous rocks form today where lava erupts from volcanoes and cools to form solid rock. If it was not for volcanoes, it might be difficult to convince anyone that rocks can form from molten lava. Igneous rocks that form on the Earthâs surface are called volcanic rocks or extrusive igneous rocks.
Not all molten rock rises from deep within the Earth to erupt in a volcano. Sometimes the molten rock, or magma, does not reach the surface, but is held in big underground chambers where it slowly solidifies to form intrusive igneous rocks. We can see this type of igneous rock only where erosion has removed the overlying rocks.
Extrusive and intrusive igneous rocks can be distinguished by the size of their mineral grains. If the individual crystals are too small to be seen without magnification, the rock is fine-grained and probably extrusive. If you can easily differentiate the grains, it is considered coarse-grained and intrusive. Extrusive rocks are fine-grained because lava cools quickly and large grains do not have time to form. Intrusive rocks cool slowly deep inside the Earth and have time to grow large mineral grains.
The igneous rocks exposed in the Red Clay Valley are mostly coarse-grained, intrusive rocks that are named granites, granitic pegmatites, diorites, and gabbros. These rocks form in large masses usually without the layering that is characteristic of sedimentary and metamorphic rocks.
|Basalt||A fine-grained, dark-colored, extrusive igneous rock that forms
by the crystallization of lava flows. Most basalt flows in the Red Clay Valley have been metamorphosed to amphibolites and are now composed of plagioclase, pyroxene, and amphibole.
|Granite||A coarse-grained, light-colored rock composed of quartz and two feldspars (plagioclase and orthoclase), with lesser amounts of mica or amphibole.|
|Gabbro||A coarse-grained rock composed of greenish-white feldspar (mostly plagioclase) and pyroxene. Gabbro is usually very dark in color. It is the intrusive equivalent of basalt.|
|Pegmatite||An igneous rock with very large (usually > one inch), well-formed crystals. A granitic pegmatite has the mineralogy of a granite and abnormally large grains, whereas a gabbroic pegmatite has the mineralogy of a gabbro and very large grains.|
|Diorite||A coarse, uniformly grained rock composed of a feldspar and less than 50% amphibole or pyroxene. A quartz diorite has the composition of a diorite plus quartz and biotite, whereas a granodiorite has the composition of a diorite plus quartz and two feldspars.|
Metamorphic rocks are sedimentary or igneous rocks that have been changed. These changes usually occur deep within the Earth, by processes we cannot observe; however, we do know that under the lithosphere the mantle is a slowly churning reservoir of fiery hot rock. Thus, when rocks are deeply buried, they are heated from the reservoir below and squeezed from above by the overlying rocks. At these high temperatures and pressures, some minerals will become unstable and change into new minerals. For example, clay will change into mica, mica plus quartz will change into sillimanite, and chlorite will change into garnet. The mineral changes that occur in solid rocks as they are heated and deeply buried are known as metamorphism.
Common metamorphic rocks are slate, schist, gneiss, quartzite, marble, and amphibolite. The dominant rocks in the Delaware Piedmont are gneisses and amphibolites, rocks that were highly metamorphosed by heating deep within a subduction zone.
|Gneiss||A course-grained rock commonly having imperfect, but prominent light-dark layering. In the Delaware Piedmont the light layers are composed of feldspars and quartz and the dark layers of mica, garnet, sillimanite, amphiboles, and pyroxenes.Gneisses are formed by the high-grade metamorphism of either igneous or sedimentary rocks.|
|Schist||A sharply layered, commonly crinkle-folded rock, that can easily split into flakes or slabs due to a well developed parallelism of platy minerals such as micas or amphiboles. Schists commonly form by the medium-grade metamorphism of igneous and sedimentary rocks.|
|Amphibolite||A rock composed primarily of amphibole and feldspar. The amphibole grains are commonly elongated with long axes parallel. In the Delaware Piedmont most amphibolites are formed by the metamorphism of igneous rocks.|
|Serpentinite||A greenish-yellow, greasy soft rock composed essentially of the mineral serpentine. It may be soft enough to carve with a pocketknife. Serpentenites are formed by the metamorphism of ultramafic (iron-magnesium rich) rocks. Ultramafics originate deep in oceanic crust and occur on land only as slivers of rock that have been thrust faulted onto the continental margin.|
|Quartzite||A massive rock composed essentially of interlocking quartz grains. Quartzites are formed by metamorphism of sand or sandstone.|
|Vein Quartz||A rock composed of sutured quartz crystals that formed by precipitation from a solution or melt. In the Piedmont vein quartz commonly fills ancient fractures.|
|Marble||A massive, coarse-grained sparkling blue-white rock composed mostly of calcite and/or dolomite. Marble forms by the metamorphism of limestone.|
Sedimentary rocks are made up of the debris from weathering and erosion of rocks, from chemical precipitates, or from the remains of living things. Most sedimentary rocks are formed from particles of older rocks that are carried by rivers and streams to lakes or oceans where they are deposited, deeply buried, and then consolidated into solid rock. They cover most of the ocean floor and three-quarters of the land. The most common solid sedimentary rocks are shale, sandstone, conglomerate, and limestone.
The only sedimentary rocks in Delawareâs Piedmont are modern sediments (sand, silt, and clay) that are being eroded, transported, and deposited in the local streams as the rocks within the watersheds weather and erode. The Piedmont has been a source of sediment that is deposited elsewhere, and has been for a long part of geologic time.
At some time almost everyone has picked up and examined a rock. It may have been round and smooth and you liked the way it felt; it may have been just the right size to skip across a pond; or it may have been beautiful or unusual. Whatever your reason for picking up a rock, we hope you observed that it was made up of many small individual grains. These small grains are minerals. Most common everyday rocks, such as granite, slate, or gneiss, are made up of several different minerals, but it is possible for a rock, such as quartzite, to be composed of only one mineral. The dictionary broadly defines a mineral as a naturally occurring solid with a definite chemical composition and an ordered (crystalline) atomic arrangement.
Minerals can form in many ways, such as crystallization from a lava or magma, by recrystallization when a rock is heated or compressed, or by precipitation from water. Usually new minerals crystallize in a medium where they are competing for space with other minerals that are forming at the same time, and they end up as a maze of interlocking grains. However, if the minerals are allowed to crystallize without competition, such as in water or molten magma, the minerals will crystallize into geometric shapes that are strikingly beautiful and often valued by collectors. There are thousands of different minerals that form in the Earth, but only a few are found in the Red Clay Valley.
|Quartz||A glassy, transparent to translucent mineral that breaks and fractures like glass. Its color is usually white to gray. Quartz is present in almost all Piedmont rocks.|
|Feldspar||In weathered rocks or granitic pegmatites, feldspars occur as milky white or pink porcelain-like minerals that often break into rectangular shapes with shiny flat surfaces. In fresh, unweathered amphilbolites or gneisses, the feldspars are glassy and transparent. In the 18th and 19th centuries, feldspar was quarried in the Red Clay Valley for use in porcelain, china, and glazes. Orthoclase and plagioclase are two types of feldspar found in the Delaware Piedmont.|
|Mica||A mineral with perfect basal cleavage that easily separates into sheets. The varieties are black biotite, white muscovite, bronze phlogopite, and green chlorite. Micas are common in all Piedmont rocks except the high-grade gneisses of the Wilmington Complex.|
|Garnet||Most Piedmont garnets are a dark-red, iron-rich variety called almandine. They usually occur as 12-sided crystals that vary in size from crystals so small they can be seen only under a microscope to crystals of an inch or more across. Garnets are considered semi-precious stones, but in the highly deformed rocks of the Piedmont they are usually fractured and not suitable for jewelery. Garnet is also used as an abrasive.|
|Sillimanite||Sillimanite, or fibrolite as it is commonly called, occurs as aggregates of thin fibers, nodules, or veins. Its color is either gray blue or dull white. It is a high-grade metamorphic mineral that occurs in the gneisses and granitic pegmatites. Sillimanite is the Delaware State Mineral.|
|Calcite and Dolomite||The major minerals in marble. In the Delaware Piedmont they occur in the Cockeysville Marble as blue-white, coarsely crystalline interlocking grains. Years ago the marble was quarried, converted into quick lime, and used as a soil conditioner.|
|Serpentine||A secondary mineral that forms by the alteration of magnesium-rich minerals. Serpentines are always shades of green, they are soft, and have a slightly soapy or greasy feel|
|Amphibole||A large family of minerals. In the Delaware Piedmont, they are usually black or dark green. Amphiboles usually have one good cleavage that will sparkle on fresh surfaces. Arock containing around 50% or more amphibole is called an amphibolite.|
|Pryoxene||A group of dark minerals that are common in the Piedmont rocks. They usually occur as interlocking grains in the highest-grade gneisses, amphibolites, and gabbros.|
Deformation in the PiedmontDeformation in the Piedmont rockman Tue, 06/30/2009 - 15:38
All of the rock units in Delawareâs Piedmont are highly deformed. Deformational features, such as folds, faults, and/or joints, are present in almost every outcrop.
The folds are a remarkable assortment of sharp folds, angular crinkle folds, and round gentle folds that may be upright, inclined, or turned upside down. The variety can be attributed to several distinct episodes of folding, and to the different mechanical properties of the rocks. For example, the soft mica-rich gneisses of the Wissahickon were crinkle-folded, whereas during the same deformation the more rigid amphibolites were bent into rounded folds. Overall, the folds in the Piedmont suggest a long compressional event in soft rocks that were hot and deeply buried.
Although folding styles in the rocks vary dramatically, the trend of the folds is remarkably consistent across the Piedmont, and parallels the trend of the Appalachians as a whole, which is northeast-southwest. Folds permit determination of tectonic trends and are convenient indicators of crustal movements. Thus the folds in the Piedmont suggest a geologic setting at colliding plate boundaries, and the orientation of the folds indicates convergence from the southeast.
Today in the Delaware Piedmont there are no large active faults. Delaware is positioned on the trailing edge of the North American plate in a moderately active tectonic area. Several hundred million years ago, when Delaware was caught between two colliding plates, deep earthquakes were frequent and probably violent as regional faults stacked the various units in the Piedmont into a high mountain range. These ancient faults are difficult to identify, having been largely obscured by metamorphism and deformation.
Faults in cool brittle rocks may offset folds or layering across the fault surface, form a smooth slick surface called a slickenside, or grind up the rocks to produce fault gouge. These indicators of brittle faulting are present in the Piedmont rocks, but they are less common than folds and joints, and are younger features.
Almost all the rocks exposed in Delawareâs Piedmont are broken and fractured to form joints. If joints form in deeply buried rocks, they are normally healed with vein material, such as quartz or mixtures of quartz and feldspar. Because the Piedmont rocks were once deeply buried, healed veins are a prominent feature of these rocks. Most of these veins were healed before the major deformational events, and are now folded, stretched into thin layers, or pulled apart into segments.
Surface exposures of Wissahickon and Baltimore Gneiss rocks are riddled with very young horizontal and vertical joints, most likely the result of unloading and expansion as the overlying material is removed by erosion. Wilmington Complex rocks will joint and weather by peeling off a curved shell leaving round rocks. This jointing and weathering style is typical of massive, unlayered rocks. Thus, to a first approximation, it is possible to distinguish the rocks of the Wilmington Complex from those of the Wissahickon by the shape of the rocks at the surface. The Wissahickon rocks are angular and sharp whereas the Wilmington Complex rocks are round. The brittle fractures in the Piedmont rocks are important because they provide storage reservoirs for ground water. To produce water, the wells in northern Delaware must tap a fracture zone.
Piedmont Rock UnitsPiedmont Rock Units johncallahan Tue, 06/30/2009 - 16:34
The Piedmont occurs in the hilly northernmost part of the state and is composed of crystalline metamorphic and igneous rocks. These include a variety of rock types that were formed deep in the earth by metamorphic processes, mostly in the early part of the Paleozoic Era (app. 400-500 million years ago), and later uplifted. The rock units of the Wilmington Complex in the Piedmont are subdivided into geologic units called lithodemic units. These bodies of rock are identified by distinctive geological characteristics and are sufficiently thick and areally extensive to be mapped at the earth's surface and/or in the subsurface. Other rock units are mapped as formations. The age of the geologic units that are recognized in the Delaware Piedmont by the Delaware Geological Survey are summarized in the chart below.
Abbreviations are those used on Delaware Geological Survey maps and cross sections. Geologic time scale not to scale.
For more details (breakdowns) of geologic time, please refer to:
Piedmont Rock Unit DescriptionsPiedmont Rock Unit Descriptions johncallahan Thu, 06/24/2010 - 22:56
Ardentown Granitic SuiteArdentown Granitic Suite johncallahan Mon, 07/27/2009 - 14:47
The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.
Medium- to coarse-grained granitic rocks containing primary orthopyroxene and clinopyroxene; includes quartz norites, quartz monzonorites, opdalites, and charnockites. Feldspar phenocrysts common. Mafic enclaves locally abundant in proximity to gabbronorites.
Baltimore GneissBaltimore Gneiss johncallahan Tue, 07/28/2009 - 11:34
The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.
Granitic gneiss with swirling leucosomes and irregular biotite-rich restite layers is the dominant lithology and constitutes approximately 75 to 80 percent of the exposed rocks. The remaining 20 to 25 percent comprises hornblende-biotite gneiss, amphibolite with or without pyroxene, and pegmatite. Granitic gneiss is composed of quartz, plagioclase, biotite, and microcline. Minor and accessory minerals are garnet, muscovite, magnetite, ilmenite, sphene, apatite, and zircon. The hornblende gneiss contains plagioclase, quartz, hornblende, and biotite with/without orthopyroxene. Accessory minerals are garnet, muscovite, clinozoisite, perthitic orthoclase, iron-titanium oxides, sphene, and apatite. Amphibolites are composed of subequal amounts of hornblende and plagioclase with minor quartz, biotite, clinopyroxene, and orthopyroxene.
Barley Mill GneissBarley Mill Gneiss johncallahan Tue, 07/28/2009 - 10:01
The following description is published in GM10 Bedrock Geologic Map of The Piedmont of Delaware and the Adjacent Pennsylvania, Schenck, W.S., Plank, M.O., and Srogi, L., 2000.
Coarse-grained, foliated tonalite gneiss. Major minerals are biotite, hornblende, plagioclase, and quartz. Includes mafic enclaves or layers composed of subequal amounts of hornblende and plagioclase. Also includes a coarse-grained granitic lithology composed of biotite, microcline, plagioclase, and quartz.
Biotite TonaliteBiotite Tonalite rockman Wed, 06/02/2010 - 13:05
Fine- to medium-grained, equigranular biotite tonalite usually occurring as rounded boulders. Tonalites are leucocratic (15 to 25% modal mafic minerals), light gray to buff on fresh surfaces, and locally contain mafic enclaves with reddish rims, the result of iron hydroxide staining. Possibly intrusive into the Perkins Run Gabbronorite Suite.
Brandywine Blue GneissBrandywine Blue Gneiss johncallahan Mon, 07/27/2009 - 16:37
Medium to coarse grained granulites and gneisses composed of plagioclase, quartz, orthopyroxene, clinopyroxene, brown-green hornblende, magnetite, and ilmenite. Mafic minerals vary from
Bringhurst GabbroBringhurst Gabbro johncallahan Mon, 07/27/2009 - 14:44
Coarse- to very coarse-grained gabbronoite with subophitic textures. Primary minerals are plagioclase, olivine, clinopyroxene and orthopyroxene. Olivine, where present, is surrounded by an inner corona of orthopyroxene and an outer corona of pargasitic hornblende, both with spinel symplectites. The gabbronorites locally contain abundant xenoliths of mafic Brandywine Blue Gneiss. (GM13 Geologic Map of New Castle County, Delaware, Ramsey, K.W., 2005)
Christianstead GneissChristianstead Gneiss johncallahan Tue, 07/28/2009 - 10:08
Coarse-grained, foliated granodioritic gneiss. Major minerals are biotite, microcline, plagioclase, and quartz. Includes thin layers of fine-grained foliated amphibolite plus large pegmatites.
Cockeysville MarbleCockeysville Marble johncallahan Tue, 07/28/2009 - 11:18
In Delaware, predominantly a pure, coarsely crystalline, blue-white dolomite marble interlayered with calc-schist. Major minerals in the marble include calcite and dolomite with phlogopite, diopside, olivine, and graphite. Major minerals in the calc-schist are calcite with phlogopite, microcline, diopside, tremolite, quartz, plagioclase, scapolite, and clinozoisite. Pegmatites and pure kaolin deposits and quartz occur locally.
Faulkland GneissFaulkland Gneiss johncallahan Tue, 07/28/2009 - 10:10
Predominantly fine- to coarse-grained amphibolites and quartz amphibolites with minor felsic rocks, probably metavolcanic. Major minerals are amphibole and plagioclase with or without pyroxene and/or quartz. Amphibole may be hornblende, cummingtonite, gedrite, and/or anthophyllite. Halos of plagioclase and quartz around porphyroblasts of magnetite, orthopyroxene, and garnet are common features.
Iron Hill GabbroIron Hill Gabbro johncallahan Mon, 07/27/2009 - 14:41
Black to very dark green, coarse- to very coarse-grained, uralitized olivine-hypersthene gabbronorite and pyroxenite with subophitic textures. Primary minerals are calcic plagioclase, orthopyroxene, clinopyroxene, and olivine. Amphibole is secondary, a pale blue-green actinolite. Olivine, when present, is surrounded by coronas similar to those in the Bringhurst Gabbro. The gabbronorite is deeply weathered leaving a layer of iron oxides, limonite, goethite, and hematite, mixed with ferruginous jasper. The jasper contains thin seams lined with drusy quartz. Contacts with the Christianstead Gneiss are covered with sediments of the Coastal Plain.
Metapyroxenite and metagabbro (undifferentiated)Metapyroxenite and metagabbro (undifferentiated) johncallahan Tue, 07/28/2009 - 10:57
Light-colored coarse-grained rocks composed of interlocking grains of light colored, fibrous amphiboles, most likely magnesium-rich cummingtonite and/or anthophyllite with possible clinochlor. These rocks become finer grained and darker as hornblende replaces some of the Mg-rich amphiboles. Associated with the metapyroxenites are coarse-grained metamorphosed gabbros composed of hornblende and plagioclase. The metapyroxenites and metagabbros are probably cumulates.
Mill Creek MetagabbroMill Creek Metagabbro johncallahan Tue, 07/28/2009 - 09:56
Coarse-grained gabbroic and metagabbroic rocks, variably metamorphosed and deformed. Primary minerals are hornblende and plagioclase.
Montchanin MetagabbroMontchanin Metagabbro johncallahan Tue, 07/28/2009 - 09:58
Coarse-grained gabbroic and metagabbroic rocks, variably metamorphosed and deformed. Primary igneous minerals include olivine, clinopyroxene, orthopyroxene, and plagioclase.
PegmatitePegmatite johncallahan Tue, 07/28/2009 - 10:29
Coarse- to very coarse-grained granitic pegmatite with tourmaline crystals locally. Where outcrop is present, pegmatite is tabular and concordant with the regional trend of the underlying Wissahickon Formation. Lenticular xenoliths of Wissahickon gneisses occur locally in the pegmatite.
Perkins Run Gabbronorite SuitePerkins Run Gabbronorite Suite johncallahan Mon, 07/27/2009 - 16:34
Fine- to coarse-grained gabbronorite and minor diorite with subophitic to ophitic textures, variably foliated or lineated. Plagioclase, orthopyroxene, clinopyroxene, and hornblende are major minerals; biotite and olivine locally present. Olivine typically surrounded by corona structures as described for the Bringhurst Gabbro. Contemporaneous with the Ardentown Granitic Suite.
Rockford Park GneissRockford Park Gneiss johncallahan Tue, 07/28/2009 - 09:53
Fine-grained mafic and fine- to medium-grained felsic gneisses interlayered on the decimeter scale. Layers are laterally continuous, but mafic layers commonly show boudinage. Felsic layers are composed of quartz and plagioclase with
SerpentiniteSerpentinite johncallahan Tue, 07/28/2009 - 11:03
Massive fine-grained dark to light yellow-green serpentinite. Contacts with the Wissahickon Formation are not exposed.
Setters FormationSetters Formation johncallahan Tue, 07/28/2009 - 11:28
In Delaware, predominantly an impure quartzite and garnet-sillimanite-biotite-microcline schist. Major minerals include microcline, quartz, and biotite with minor plagioclase, and garnet. Muscovite and sillimanite vary with metamorphic grade. Accessory minerals are iron-titanium oxides, zircon, sphene, and apatite. Microcline is an essential constituent of the quartzites and schists and serves to distinguish the Setters rocks from the plagioclase-rich schists and gneisses of the Wissahickon Formation.
Windy Hills GneissWindy Hills Gneiss johncallahan Tue, 07/28/2009 - 10:12
Thinly interlayered, fine- to medium-grained hornblende-plagioclase amphibolite, biotite gneiss, and felsic gneiss, possibly metavolcanic. Felsic gneisses contain quartz and plagioclase with or without microcline with minor pyroxene and/or hornblende and/or biotite. Metamorphic grade in this unit decreases from granulite facies in the northeast to amphibolite facies toward the southwest. Correlated with the Big Elk Member of the James Run Formation in Cecil County, Maryland.
Wissahickon FormationWissahickon Formation johncallahan Tue, 07/28/2009 - 10:36
Interlayered psammitic and pelitic gneiss with amphibolite. Psammitic gneiss is a medium- to fine-grained biotite-plagioclase-quartz gneiss with or without small garnets. Contacts with pelitic gneiss are gradational. Pelitic gneiss is medium- to coarse-grained garnet-sillimanite-biotite-plagioclase-quartz gneiss. Unit has a streaked or flasered appearance owing to the segregation of garnet-sillimanite-biotite stringers that surround lenses of quartz and feldspar. Throughout, layers of fine to medium-grained amphibolite composed of plagioclase and hornblende, several inches to
Selected Outcrops of the Delaware PiedmontSelected Outcrops of the Delaware Piedmont johncallahan Thu, 06/24/2010 - 23:06
Map of Selected Piedmont OutcropsMap of Selected Piedmont Outcrops johncallahan Wed, 09/09/2009 - 17:22
The Delaware Geological Survey maintains detailed records on outcrops located throughout the state. These records are completed by geologists when visiting the outcrop locations during field work. Many outcrops are on private property; however, we have selected several in the Piedmont area that are easily accessible and allow you to see some of the fascinating geology underlying northern Delaware. Each of the outcrops shown on this map has a small write up describing the mineralogical and structural details that can been seen in the rock outcrop. Please make sure to be respectful of property while observing these geologic treasures.
Outcrop Ba14-a: The Setters Formation at Avondale QuarryOutcrop Ba14-a: The Setters Formation at Avondale Quarry rockman Tue, 09/15/2009 - 13:52
The Setters Formation is located in southeast Avondale, PA. Huge slabs of rock have been exposed by a gravel company that has been removing the hillside: quarrying for quartzite to sell as building stone and grinding pelitic rock into gravel and stone. These slabs have a foliation with a strike of 45 degrees East of North and a southeastern dip off of the Avondale Anticline. They also display quartzite, schist, and pods of pegmatite, containing large garnets (1-2" in diameter) and schorl tourmaline, that appear to be â€œsweated out of schist.â€ A dramatic contrast in rich type-shelf facies reflects beach sand and bogs or inlets.
Outcrop Bb25-c: The Yorklyn Railroad CutOutcrop Bb25-c: The Yorklyn Railroad Cut johncallahan Fri, 09/11/2009 - 10:43
Wissahickon gneisses and amphibolites are exposed in the railroad cut near Yorklyn. Here the rocks are unusual because the layering is accentuated by the presence of fault gouge between the layers. Fault gouge forms as movement along a fault in hard, brittle rocks crushes and grinds the rocks into a powder. Gouge was a term used by miners because they could easily "gouge" it out of the rock. Here the gouge "weathered out" leaving deep indentations that emphasize the layering and the tilt, which is to the southeast at an angle of about 45 degrees.
Outcrop Bc32-a: The Mt. Cuba Picnic GroveOutcrop Bc32-a: The Mt. Cuba Picnic Grove johncallahan Fri, 09/11/2009 - 09:57
The Mt. Cuba Picnic Grove provides an opportunity to look at the gneisses and amphibolites of the Wissahickon Formation. The large boulders of gneiss lying beside the steps are peppered with dark-red garnets and elongated nodules of dull-white sillimanite. These sillimanite nodules (1/4" to 3/4" long) are abundant in the gneisses at Mt. Cuba and are an interesting feature of these highly metamorphosed sedimentary rocks. Alternating layers of gneisses and amphibolites crop out on the east side of the track. The gneisses show some typical upright folds and fractures. Contacts between the layers trend northeast, parallel to the regional trend of the Appalachians.
Outcrop Bc32-b: The Mt. Cuba Railroad CutOutcrop Bc32-b: The Mt. Cuba Railroad Cut johncallahan Fri, 09/11/2009 - 09:52
The Mt. Cuba railroad cut is narrow and deep, and much of the rock is covered with dirt and soot from the train. The rocks are interlayered gneisses and amphibolites, with gneisses predominating in the south end of the cut and amphibolites in the north end. Folding is well developed, but the angle of the sunlight as it shines on the walls of the cut will determine which of the folds will be the easiest to see. A 4" amphibolite layer outlines the fold in this part of the outcrop. Wonderful examples of the effects of rock type on folding styles can be seen in the cut and in many of the rocks piled north of the cut.
Outcrop Bc44-f: The Tatnall Preschool GroundsOutcrop Bc44-f: The Tatnall Preschool Grounds rockman Thu, 12/10/2009 - 12:11
The Tatnall Preschool Grounds contain many light-colored, coarse-grained, igneous-looking rocks (Barley Mill Gneiss) with mafic enclosures. These mafic enclosures make up only a small part of the rock. They may either be random in slope or they are elongated. When the Upper School and Preschool were built in the 1970s and 1980s, a lot of rock was removed from the foundations. The rock is either scattered around as large boulders in the landscaping, or in the back of the athletic fields in a large dump. The rocks in the dump show examples of mafic rock (greenish in color), part mafic and part pegmatite, and granitic rock.
Outcrop Bd21-a: Boulder Field at Brandywine Creek State ParkOutcrop Bd21-a: Boulder Field at Brandywine Creek State Park rockman Thu, 10/01/2009 - 12:18
In the patch of woods north of the upper parking lot in Brandywine Creek State Park are large outcrops of amphibolite. The outcrops are rounded from exfoliation, and are black with few structural features. The mafic hornblende grains are elongated parallel to a few thin felsic bands. This lineation strikes east-west and dips to the north. These boulders are located on the northwest facing slope of the valley and are probably a paraglacial feature left over from a colder period in Delaware's geologic past.
Outcrop Bd41-b: Rockford Park Gneiss Boulders at Rockford ParkOutcrop Bd41-b: Rockford Park Gneiss Boulders at Rockford Park rockman Tue, 09/22/2009 - 15:20
The Rockford Park boulders can be found just beyond the Rockford Tower on the slope facing the Brandywine Creek. Some areas of the Rockford Park Gneiss actually display some banding of felsic gneiss and mafic gneiss which are interlayered on a scale of 4" to 2'. This banding strikes 30 degrees east of north and dips 60 degrees to the northwest. The mafic layers are boudinaged and broken, some of which are weathered away into a prominent relief. Between some layers, the rock is intruded by a coarse-grained and apparently undeformed gabbro.
Outcrop Bd42-e: The Cliffs of Alapocas WoodsOutcrop Bd42-e: The Cliffs of Alapocas Woods rockman Tue, 09/22/2009 - 13:32
Located in Wilmington, DE, the Cliffs of Alapocas Woods are opposite the old Bancroft Mills across the Brandywine Creek. Along the creek you will find large exposures of Brandywine Blue Gneiss. Compared to other outcrops in the Piedmont of Delaware, the rock examples here are massive. When observed closely, the felsic gneiss displays a medium grain size. Most of early Wilmington was built from the stone from these quarries. These impressive rock features are enjoyed by local rock climbers as well as many who use the Northern Delaware Greenway.
Outcrop Bd44-b: Bringhurst Gabbro boulders in Shellpot CreekOutcrop Bd44-b: Bringhurst Gabbro boulders in Shellpot Creek rockman Tue, 09/22/2009 - 15:55
Found in the creek bed and flood plain, the large boulders in Shellpot Creek are excellent examples of Bringhurst Gabbro. The gabbro is very coarse-grained with crystals up to 2" long; however, variations in the grain size exist over a scale of a few inches. While observing this rock closely, one can occasionally find grains of orthopyroxene (possibly bronzite) up to 4" long. Some of the boulders have grains of olivine surrounded by double coronas of orthopyroxene, spinel, and hornblende.
Outcrop Be22-e: Ardentown Railside BouldersOutcrop Be22-e: Ardentown Railside Boulders rockman Wed, 10/07/2009 - 14:48
Located in Ardentown are a few silicic boulders just on the northwestern side of the railroad bridge that crosses the South Branch of Naaman Creek. These boulders are part of the Ardentown Granitic Suite. Some have very large (several cm) feldspar phenocrysts. Some display contacts between granitic rock and quartz-rich rock, which is probably metasedimentary rock due to the granular nature of quartz.
Outcrop Be21-e: Hanby Park QuarryOutcrop Be21-e: Hanby Park Quarry rockman Thu, 10/01/2009 - 15:09
On the south side of Chestnut Hill is an outcrop of very large boulders in the woods of Hanby Park near Arden, DE. This area of the park seems to be the site of an old quarry. The rocks here are very similar to the rocks found down the South Branch of Naaman Creek (Ardentown and Ardencroft) as they both share the same fine-grained, mafic properties with traces of coarse-grained charnockite.
Outcrop Be22-k: Charnockite Boulders at ArdentownOutcrop Be22-k: Charnockite Boulders at Ardentown rockman Thu, 10/01/2009 - 11:59
In the valley of the South Branch of Naaman Creek, through Ardentown, is a group of charnockite boulders and fine-grained mafic rock (probably amphibolitized gabbroid). The mafic rock is mostly non-megacrystic with some coarse-grained and equigranular charnockite. On the ground around the boulders are small pieces that contain a clear example of a contact between coarse-grained and fine-grained rock types.
Outcrop Be23-g: Charnockite Boulders in the South Branch of Naaman CreekOutcrop Be23-g: Charnockite Boulders in the South Branch of Naaman Creek rockman Thu, 10/01/2009 - 13:16
Running through Knollwood Park in Claymont, DE is the South Branch of Naaman Creek. This stream is laden with fairly mafic, medium to coarse-grained charnockite. Some of the charnockite samples here may be mylonitic. A few boulders contain xenoliths as well. Other gabbro boulders display charnockite veins in a gradational zone over about 1-2 meters.
Outcrop Be32-g: Lesher Park StreambedOutcrop Be32-g: Lesher Park Streambed rockman Wed, 10/07/2009 - 15:08
In Claymont, DE, the intersection of Marvel Avenue and Parkside Boulevard occurs at Lesher Park, which contains Perkins Run, a creek West of Harvey Road. In the streambed of this creek is an outcrop of Perkins Run Gabbro, which is part of the Arden Plutonic Supersuite. The gabbro displays joints that are oriented 10 degrees west of north. Along these joints, veins of charnockite (orthopyroxene-bearing granite of the Ardentown Granitic Suite) can be found.
Outcrop Ca44-d2: The Christianstead SubdivisionOutcrop Ca44-d2: The Christianstead Subdivision rockman Thu, 10/15/2009 - 14:33
Outcrops between Hidden Valley Drive and Farmhouse Road. The Christianstead subdivision is underlain by interlayered mafic and felsic gneiss with large pegmatites. The felsic gneiss, in the northwestern half of this subdivision, is deformed granodiorite, seen as massive igneous layers with only rare crinkle folding. There are a few bright eyes textures on the west end of this subdivision, which is all underlain by granodiorite.
Outcrop Cb15-c: The Confluence Quarry at North PointeOutcrop Cb15-c: The Confluence Quarry at North Pointe rockman Thu, 10/22/2009 - 13:54
Just northwest of the confluence of Mill Creek and an unnamed tributary is an abandoned quarry. This quarry sits off the greenway trail, across from a ruined foundation wall, and displays samples of black, coarse-grained, foliated amphibolite. The unnamed tributary and Mill Creek are choked with amphibolite rocks and boulders. The amphibolite here weathers with a rusty rind and has a foliation strike of 10 degrees east of north and an obviously steep to vertical dip.
Outcrop Cb42-c: Windy Hills Bridge OutcropOutcrop Cb42-c: Windy Hills Bridge Outcrop rockman Thu, 10/22/2009 - 14:16
Considered one of Delaware's most famous Piedmont outcrops, the Windy Hills Bridge outcrop is composed of mafic and felsic gneiss of the Windy Hills Gneiss. Much of the layering in the outcrop is regular and is 8 to 10" thick. At the contact between these layers there is evidence of partial melting. In terms of mineralogy, this rock contains mainly hornblende, plagioclase, quartz, biotite and epidote. This outcrop shows tight folds that plunge steeply 70-90 degrees to the northeast and southwest. The gneiss is cut by a long lens of pegmatite, which intruded after the folding and metamorphosing that yielded the gneiss. There is also an interesting layer of cobble just above the bedrock in this area presumed to be the contact with the Coastal Plain sediments. These newer outcrops to the southwest display a 4-10" pelitic layer which becomes more extremely magmatic, with 1" leucosomes and Â½" mafic selvages. Overall, strikes of foliations of the mafic and felsic layering in these outcrops are 70-75 degrees east of north and the dips are a steep 80-85 degrees to the southeast, or almost vertical.
Outcrop Cc12-a: The Cave at Brandywine SpringsOutcrop Cc12-a: The Cave at Brandywine Springs johncallahan Wed, 09/09/2009 - 16:17
Approximately 100 yards east of the tracks is one of the largest outcrops in the park. Here along the hillside, a thick layer of crinkle-folded, yellow-weathering gneiss overlies a layer of garnet-bearing quartzite and amphibolite. At the contact between the quartzite and the schist, a large piece of the quartzite has fallen out creating a small cave. Maybe Indians used this cave, but it is not very inviting. If you hit the black rocks with a hammer they will ring. Look for the tiny lavender garnets in the quartzite.
Outcrop Cc12-c: The Red Clay Creek EdgeOutcrop Cc12-c: The Red Clay Creek Edge rockman Tue, 11/17/2009 - 15:16
Along the edge of the Red Clay Creek exists a large outcrop that extends out into the stream. This rock is part of the Wissahickon Formation, with pelitic facies, ½" elongated sillimanite nodules, and disharmonic folds. The compositional layering of this rock is 1/8“ ½" of biotite rich layers alternated with fine-grained psammitic layers (not quartz-feldspar layers). Some of these layers are sheared (shear zones). The sillimanite nodules, pegmatite pods, and shear zones in this rock are all parallel to fold axes. The axial plane of these folds is 20 degrees east of north, plunges 42 degrees northeast, and dips 90 degrees. Within this large outcrop are several 2-3' layers of â€œrock that ringsâ€ (when hit) and are folded with petitic gneiss. This pelitic gneiss shows more intense folding while the rest of the rock is gently folded. The â€œrock that ringsâ€ is also peppered with small lavender garnets.
Outcrop Da15-h: The Paraglacial Boulder Feature of Chestnut HillOutcrop Da15-h: The Paraglacial Boulder Feature of Chestnut Hill rockman Tue, 09/22/2009 - 11:55
Prime examples of Iron Hill Gabbro can be found in the area surrounding Chestnut Hill at Rittenhouse Park. The gabbro here is considered coarse to very coarse grained. Boulders of Iron Hill Gabbro are located on the northeast facing slope southwest of the Christina Creek. This gabbro boulder field is probably a paraglacial feature left over from ice age times deep in Delaware's geologic past.
Piedmont Field Trips - GeoAdventuresPiedmont Field Trips - GeoAdventures johncallahan Thu, 06/24/2010 - 23:11
What are GeoAdventures?What are GeoAdventures? johncallahan Sat, 06/27/2009 - 00:15
GeoAdventures are designed to allow the reader to learn about a particular geologic point of interest in Delawareâs Piedmont province and then take a short field trip to that area. Want to know more about the Wilmington blue rock or Brandywine blue granite? Take the Wilmington Blue Rock GeoAdventure and go see just what the blue rock looks like.
GeoAdventures are great for a family education outing, Boy or Girl Scout training, mineral or rock-collecting club, or Earth science school trips. See the whole Piedmont by reading Special Publication 20 and riding the Wilmington and Western Railroad steam train all along the Red Clay valley following the field trip guide in the back of the book.
Check these pages as new GeoAdventures will be continually added.
Exploring the Wilmington Blue Rocks: A GeoAdventure in the Delaware PiedmontExploring the Wilmington Blue Rocks: A GeoAdventure in the Delaware Piedmont johncallahan Tue, 07/14/2009 - 12:03
The Wilmington blue rock, Delaware's most famous rock, underlies both the city of Wilmington and the rolling upland north and east of the city. It is best exposed along the banks of the Brandywine Creek from south of Rockland to the Market Street Bridge. Along this section the Brandywine has carved a deep gorge in the blue rock. The water fall along this four mile gorge is approximately 120', and in the 17th and 18th centuries provided water power for one of the greatest industrial developments in the American colonies. The field trip stops described below are chosen as good examples of blue rock along the Brandywine Creek, and to illustrate how the geology has influenced the development of this area. It is not necessary to visit every stop to become familiar with the blue rocks, you may choose to visit only a few.
The specific objectives of this adventure are to:
- Examine the igneous and metamorphic rocks of the Delaware Piedmont that have been called the Wilmington blue rock by quarrymen and the Brandywine Blue Gneiss of the Wilmington Complex by geologists.
- Investigate the role played by the blue rocks in the ancient geologic history of northern Delaware which involves subduction of tectonic plates, formation of volcanoes, and the progressive collision of the North American, European, and African plates to form a huge mountain range. All of this tectonic activity occurred sometime between 570,000,000 and 250,000,000 years ago. Since that time northern Delaware has remained tectonically quiet as the mountains have slowly eroded their debris of clay, sand, and gravel, onto the continental shelf of the Atlantic Ocean.
- Recognize how the bedrock and accompanying land forms have influenced land use and industrial development. At its peak in the 18th century, the Brandywine Creek flowing across the blue rocks provided energy for some 130 flour mills, paper mills, and textile mills. Later in the 19th century, in the gorge above Wilmington, the duPonts began the manufacture of gun powder, and from their beginning along the Brandywine, they have grown to be one of the giants of American industry.
The rocks you will see on this trip are locally called the Wilmington blue rocks or Brandywine Blue Granite. When found in stream beds, yards, or old quarries, the rocks are black or dark gray, however when freshly broken during quarrying the rocks are a bright royal blue. Although weathering changes the color, construction workers have always called this rock the "blue rock". Recognizing the importance of these rocks to the city, Wilmington's original baseball team called themselves the blue rocks, a name that has since been adopted by the city's new baseball team.
Geologists map the blue rock by its geologic name "the Brandywine Blue Gneiss" and assign the rocks to a geologic unit called the Wilmington Complex. The Wilmington Complex forms the bedrock under the much of the city of Wilmington and Brandywine Hundred (Figure 1). The rocks are mostly a mixture of metamorphic gneisses and plutonic igneous rocks. The gneisses, which are the most abundant rock type, are the true "blue rocks". However when you see them today along the Brandywine, they are massive, solid, blue-gray rocks with few visible features to indicate their long history. Since their formation approximately 570,000,000 years ago, these rocks have experienced a long history of burial, high-grade metamorphism, deformation, uplift, and erosion. The metamorphism has totally recrystallized the rock to produce a monotonous body of rock that is wonderfully suited for building houses and fences. It is useless as road ballast as it breaks rock crushers so today the large boulders dug up during construction are usually buried off site.
The mineralogy of the blue rocks is simple, the rocks usually contain only four minerals; quartz, feldspar, pyroxene, and magnetite. Geologists have described this rock as a banded gneiss, even though the light-dark banding is weak and not always present. There are large areas that consist of only light gneiss or dark gneiss. The gneisses weather to form a white rind. It is only then that streaks of minerals up to one inch long can be seen on the white weathered surface. The dark streaks are usually pyroxene or magnetite and the lighter streaks are quartz and feldspar. The banding and the mineral streaks are the only features that are commonly seen in the blue rocks.
The tectonic setting proposed for the origin of the Wilmington Complex is thought to be the deep part of a volcano that developed over an east dipping subduction zone. The subduction and volcanism were early in a series of tectonic events that produced the Appalachian Mountain System. Later, probably between 480,000,000 and 440,000,000 million years ago, the volcanoes collided with the ancient North American continent. Because of this collision, the rocks of the ancient continent, the rocks in the volcanic range, and the rocks lying in the ocean between the continent and the range, were all folded, sheared and buried to depths of 10 to 12 miles where they were metamorphosed by extreme heat and pressure. For many years these buried rocks remained at very high temperatures, somewhere between the temperatures required for high-grade metamorphism and melting (around 1,300Â°F). Today, after uplift and erosion, the highly metamorphosed rocks are exposed in Delaware in what is recognized by geologists as the metamorphic core of the Appalachian Mountain System.
Coarse-grained igneous rocks are exposed in Bringhurst Woods Park and in the communities of Arden and the Timbers. These rocks probably intruded into the blue rocks and may be younger. They are undeformed and only slightly metamorphosed, thus it is good site to study intrusive igneous rocks (Bringhurst Gabbro GeoAdventure).
Use Figure 1 as a guide to where the 5 stops on this adventure are located.
Stop 1. Brandywine Creek State Park
Park in the lot on the east side of the Brandywine Creek just south of Thompson's Bridge Road. At this stop we will see the contact between the blue rocks of the Wilmington Complex and the metamorphic sedimentary rocks of the Wissahickon Formation. The contact runs northeast at 45 degrees parallel to the regional trend of the Appalachian Mountains, and is exposed along Rocky Run.
There are two options for this stop. Walk (1) follows the southeast side of Rocky Run and will take approximately one and one half hours. Some of the walk includes bushwacking off existing trails so this trip is not suitable for young children. The exposures on Walk (1) are abundant and are good examples of both the Wissahickon and Wilmington Complex rocks. Walk (2) follows the dirt road from the parking lot to the south and will take about one half hour. This is an easy walk and you will be able to see both the metasediments of the Wissahickon and the black boulders of the Wilmington Complex.
- Walk south along the Brandywine creek. The hillsides on the east of both the parking lot and the road expose large outcrops of the metamorphosed sediments of the Wissahickon Formation. Many of the outcrops are covered with fungus, making it necessary to look carefully to see the features of these rocks (Area marked A in Figure 2). Cross the bridge over Rocky Run. Take one of the paths that lead northeast parallel to Rocky Run (Figure 2B). A few Wilmington Complex boulders are strewn along the hillside, however approximately one quarter of a mile to the northeast you will encounter a large swale that is literally choked with hundreds of rounded boulders of Wilmington Complex blue rocks (Figure 2C). The boulders are dark, rounded, and show light-dark layering. If you look carefully you may see a few "bright eyes". The bright eyes are grains of black magnetite surrounded by white grains of feldspar and quartz. If you use your imagination, you can see the rocks are looking at you!. Geologists believe this field of boulders is to be a paraglacial feature, formed by freeze and thaw action. The boulders slowly worked their way downslope during the last glacial period, about 10-40 thousand years ago.
- Cross the boulder field, turn left, and walk toward Rocky Run. Look for a wall of rock bordering the northwest side of Rocky Run (Figure 2D). Wissahickon rocks form the wall and the streambed while the rounded boulders of Wilmington Complex gneisses clog the stream, litter the southeast banks and lie scattered in the flood plain. The layering in the Wissahickon wall rock is irregular and defined by stringers of garnet, biotite and sillimanite in a mass of quartz and feldspar. The garnets are dark red, either oval or round, and may be as large as three quarters of an inch in diameter. The stringers, and any folds that are present, are best seen by standing in the stream and looking upstream. The contact between the Wissahickon and Wilmington rocks is hidden beneath the flood plain.
- To see the contact, you need to follow the stream to the confluence of Hurricane Run and Rocky Run and stay on the northeast side of Rocky Run. (Figure 2E). The exposed contact is difficult to recognize and probably interesting only to geology students at the high school or college level. It is exposed in a ten foot area along the northeast side of Rocky Run where dark, fine grained Wilmington Complex gneisses are interlayered with light colored Wissahickon gneisses. The Wissahickon rocks appear to have been melted and recrystallized to form granites with thin layers of garnets. The biotite and sillimanite that occur in the Wissahickon gneisses are replaced by tiny garnets. This reaction in which garnet replaces biotite and sillimanite occurs only at very high temperatures. The Wilmington Complex layers vary in thickness between 3 inches and 2 feet, and are dark solid, massive rocks.
- The nature of this contact is controversial. Geologists are unable to find any substantial evidence in the rocks that will allow them to determine how these two units were placed next to one another. The possibilities are: (1) the original volcanic pile that became the Wilmington Complex rocks was thrust up and over the Wissahickon sediments during subduction of the tectonic plates, (2) the Wilmington Complex slid down from the northeast, maybe from as far northeast as New York City, on a large regional strike slip fault such as the San Andres in California, or (3) that the contact is intrusive and the Wilmington Complex igneous rocks intruded the Wissahickon sediments before the metamorphism.
- Return to the parking lot.
- Walk south along the Brandywine creek. The hillsides on the east of both the parking lot and the road expose large outcrops of Wissahickon rocks. Many of the outcrops are covered with fungus, making it necessary to look carefully to see the individual minerals and the layering (Area marked A in Figure 3). Look for large garnets and curving stringers of biotite and sillimanite.
- Walk down the road and cross the bridge over Rocky Run. The contact between the Wissahickon and the Wilmington Complex occurs approximately 450 feet south of the bridge. At the contact the rocks in the roadbed change from the light colored, mica-rich rocks of the Wissahickon to dark, rounded boulders of the Wilmington Complex. These Wilmington Complex boulders dot the hillside east of the road. Most boulders are banded and some will contain "bright eyes" The "bright eyes" are grains of magnetite surrounded by light colored quartz and feldspar. If you use your imagination, you will see the rocks winking at you!
- Return to parking lot.
Stop 2. Rockford Park
This is the most easily accessible stop and will take between fifteen minutes and a half an hour to observe the blue rocks at this location. Follow the main road in Rockford Park to the parking lot at the tower. Park and walk toward the Brandywine Creek. Along the ridge are large outcrops of sharply banded Wilmington Complex gneisses (location of "star" in Figure 4). The banding runs 40 degrees east of north, parallel to the regional strike of the Appalachian Mountain System. The layers are vertical, orientated perpendicular to the land surface.
The bands are 9 to 12 inches thick. During intense metamorphism, around 440,000,000 years ago, these rocks were totally recrystallized and stretched. During stretching, the dark bands were more rigid than the light bands and separated. The light bands were plastic and flowed between the separations. French geologists named this texture boudinage. It is caused by intense squeezing or stretching of the rock while it is warm and plastic.
The light bands are composed of quartz and plagioclase feldspar, with minor amounts (Stop 3. Quarries on Brandywine Creek, Alapocas
This quarry has recently been given to the county as part of its park system and can be accessed on the Delaware Greenway (location of "star" in Figure 5). Good exposures of Wilmington Complex gneiss or blue rock are found on the exposed back wall of the quarry. The rock is a monotonous, light-colored gneiss with a few thin dark bands. The dark bands appear to have been deformed by stretching or pulling apart and often occur as pieces about a foot long . Thicker dark bands may persist for the extent of the exposure. The dark bands probably represent original lava flows. This rock looks as if it has been squeezed and stretched. The stretching occurred many years ago when the rocks were hot and plastic. Today these rocks in the quarry are hard and brittle. They will no longer bend or fold, but they will fracture and break during earth movements such as earthquakes or erosional unloading.
Stop 4. Brandywine Park
Large boulders line the banks of the Brandywine as it flows through Brandywine Park. The boulders along the creek are blue rocks, but the banding is replaced by irregular layering and, in some rocks, the mafic bands are replaced by clots or pods of mafic rock (location A, B, C in Figure 6).
This stretch of the Brandywine was the location of many of the mills, thus the bedrock is much disturbed. A large mill race still exists on the southwest side of the creek, however in the 18th century mill races bordered both sides of the stream. The races carried water to turn water wheels and provide energy for the many mills built below the great falls near the Market Street Bridge. Below the Market Street Bridge the Brandywine is navigable, allowing ships to sail up the Christina and lower Brandywine to pick up the flour, cotton, and snuff from the mills that lined the stream.
The rock removed from the mill races was used to build homes for the mill owners and workers. Many of the houses and churches in Brandywine Village that have been built from blue rocks are now beautifully restored.
Stop 5. Swedes Landing
This stop will take about one half an hour and is an easy interesting walk through the park at Old Swedes Landing to "The Rocks" in the Christina River (Figure 7).
In 1638 the Kalmar Nyckel and the Fogel Grip sailed up the Christina River past the entrance to the Brandywine to "The Rocks" where a large flat slab of blue rock protrudes into the main channel of the river. This rock slab was a convenient place to unload the weary passengers that were aboard the ships. The passengers, mostly Swedes and Finns, stayed and settled on the Christina near this site.
The large flat slab of rock on which the early settlers landed, although reduced to make room for river travel on the Christina, is still a present in Swedes Landing Park. "The Rock" is a slab of Wilmington Complex gneiss or blue rock, and marks the eastern edge of exposure of the Appalachian mountain system where the hard rocks of the Piedmont Province plunge beneath the soft sediments of the Coastal Plain. The boundary between the Piedmont and the Coastal Plain is defined in most places by a well-marked change in topography, usually an abrupt transition from rolling hills to a flat smooth lowland. Geologically it defines the transition from the hard crystalline rocks of the Piedmont to the gently dipping beds of younger clays, sands, and gravels of the Coastal Plain. This boundary is called the Fall Line, and extends along I-95 from Newark, through south Wilmington, toward the Delaware River. It is but a portion of the line or zone that extends unbroken from New York to Georgia. Many of the great cities of the east such as new York, Trenton, Philadelphia, Wilmington, Baltimore, Washington, Richmond Raleigh, and Macon are built on the Fall Line.
Woodlawn Quarry: A GeoAdventure in the Delaware PiedmontWoodlawn Quarry: A GeoAdventure in the Delaware Piedmont johncallahan Tue, 07/14/2009 - 11:04
A visit to Woodlawn Quarry is suitable for ages 10 to adults and provides an interesting opportunity to observe common mineral specimens, identify the quarry as an early mining site, appreciate the physical work necessary to quarry rock with hand tools, and discuss the economic importance of the minerals found in the quarry. The minerals that can be readily found and identified in the quarry are feldspar, quartz and mica.
This area was bought in 1910 by William Bancroft as a wild flower preserve. It is now part of the First State National Monument, a Federal National Monument within the National Parks System.
Feldspar was actively quarried at this site from 1850 to 1910. There were many feldspar quarries or spar pits as they were commonly called scattered throughout the Delaware Piedmont in the early eighteen hundreds. The feldspar recovered from this spar pit was transported by horse and wagon to a factory in Philadelphia where it was used for making porcelain products such as dishes, figurines, false teeth, or sinks. The quarry eventually closed because machinery made other sites more accessible.
The rock quarried is an intrusive igneous rock called a granite. Intrusive rocks do not flow or explode from a volcano onto the earth's surface, but solidify deep within the earth. Molten rock called magma flows slowly through cracks or other zones of weakness in the local rock and cooled slowly to solidify into a rock made up of large mineral grains. The intrusive rock quarried here at Woodlawn names a graphic granite because the feldspar grains contain inclusions of quartz in geometric shapes that look like the cuneiform writing of the ancient Arabs. The graphic granite also contains white mica (Muscovite) and the accessory minerals garnet and beryl.
The graphic granite cooled and crystallized slowly within preexisting rock, called the country rock. The so-called country rock surrounding the graphic granite is part of the Wissahickon Formation, a formation made up of highly metamorphosed and intensely deformed rocks that formed in the core of the ancient Appalachian Mountains. The magma from which the granite crystallized probably formed during the metamorphism. This is a common occurrence in metamorphic terrains where the coarse grained granites are called pegmatites.
The minerals found in this quarry can be distinguished by their physical properties, color, cleavage or fracture, and luster. Cleavage is the tendency of some minerals to break along definite surfaces that are parallel to possible crystal faces, and provides a means of identifying these minerals. Minerals without cleavage will break by fracturing or breaking in all directions. Not all minerals show good cleavage, most show fracture.
FELDSPAR occurs as two varieties, one is pink and one is white. All the feldspar grains a re opaque, that is light does not shine through the mineral. The feldspars break with good cleavage in two directions. The pink feldspar has better cleavage than the white and often breaks into small perfect rhombohedrons. The fresh cleavage surfaces have a pearly luster. The pink feldspar is a variety called microcline, and the white feldspar is plagioclase. Both feldspars form similar crystals, but have different elements in their crystal lattices. Plagioclase grains display surface striations due to exsolution during cooling.
QUARTZ grains are transparent to translucent, that means that light will pass through the grains. They occur here as crystalline masses that fracture like glass. The masses show a transition from clear white quartz to smoky quartz.
Quartz is the most common mineral in surface rocks. It is the principal constituent in many igneous sedimentary and metamorphic rocks and forms the sand on most of our beaches. It has many uses such as a gemstone, as an electronic component, as the principal component of glass.
MICA is easily recognized because it has perfect one directional cleavage and separates into thin elastic sheets. A cluster of sheets if referred to as a book and appears block and opaque. The sheets are clean and transparent, but may contain hexagonal-shaped inclusions (reticulated inclusions) of a black iron mineral. Separating the books into thin sheets illustrated the prominent basal cleavage. This colorless variety of mica is called Muscovite.
The sheets obtained from large books were use to make heat proof windows for old stoves and ranges. Because of their electrical resistance, the iron-free micas are widely used in many kinds of electrical equipment. The isinglass, popular years ago as shatterproof windows in automobiles was made using a sheet of mica and clear glass.
GARNET occurs here as tiny dark red crystals with 12 sides, called a dodecahedron. The crystals are rare and small and it is necessary to look carefully to find crystals. The garnets are hard, have a glassy luster and no distinct cleavage. When broken they look like dark red glass.
BERYL or aquamarine as it is commonly called, is pale blue-green. It has no cleavage and occurs here as irregular masses in the graphic granite. Beryllium is a rare element, and most granitic pegmatites do not contain beryl, however this occurrence is part of a group of beryl-bearing granitic rocks that have been identified in southern Chester and Delaware counties in Pennsylvania and northern New Castle county in Delaware. Both garnet and aquamarine are semiprecious stones.
This map shows the location of Woodlawn Quarry. As previously stated, it is now part of the First State National Monument, a Federal National Monument within the National Parks System and no mineral collecting is allowed.
The Bringhurst Gabbro: A GeoAdventure in the Delaware PiedmontThe Bringhurst Gabbro: A GeoAdventure in the Delaware Piedmont johncallahan Tue, 07/14/2009 - 11:51
A field trip to Bringhurst Woods Park is appropriate for students in grades 5 and up (10 years and older), and provides an opportunity to observe intrusive plutonic igneous rocks that have intruded into country rock, which in this case is the blue rock or what geologists call the Brandywine Blue Gneiss. In addition, the minerals in the pluton are large, easily identified, and interesting. Mineral collecting is not allowed within the park, however permission may be obtained to collect along Shellpot Creek southeast of the park. Please do not use rock hammers on the rocks in the park.
The specific objectives of this adventure are:
- To observe an intrusive igneous rock and the country rock (Wilmington blue rock) it has intruded
- To identify the individual minerals in an igneous rock
The rocks along Shellpot Creek in Bringhurst Woods Park are intrusive igneous or plutonic rocks. Because of the good exposure in this park, geologists have named these rocks the Bringhurst Gabbro and mapped the pluton as a geologic unit within the Wilmington Complex (Figure 1). The Bringhurst Gabbro represents a magma flow that flowed into the Wilmington Complex and cooled deep underground. The rocks of the Wilmington Complex underlie the most City of Wilmington and Brandywine Hundred. During the 18th and 19th centuries all rock units within the Wilmington Complex were extensively quarried for building houses, fences, retaining walls, schools, churches, and factories. They were used wherever a building material was needed. The most common rock unit in the Wilmington Complex is a high-grade metamorphic rock called the Brandywine Blue Gneiss (commonly called the Wilmington blue rock). This "blue rock" was named for the bright blue color of the rock when it is freshly exposed. It is the Wilmington blue rock that the Bringhurst Gabbro intruded.
The Bringhurst Gabbro exposed along Shellpot Creek has not been deformed or recrystallized by metamorphism, thus the rocks of the Bringhurst pluton lack the layering found in most of the other metamorphic rocks of the Delaware Piedmont. Because there are no fine-grained "chilled margins" at the contact between the pluton and the Wilmington blue rock, the pluton probably intruded the gneisses while they were still hot, sometime in the early Paleozoic between 500,000,000 and 400,000,000 million years ago.
Shellpot Creek in Bringhurst Woods Park is choked with large rounded boulders of Bringhurst Gabbro that have eroded out of the surrounding hills. A close look shows the minerals in the gabbro are between 1/4 and 2 inches in length and 1/4 to 1 inch in diameter (Figure 2). Blobs of fine-grained dark rock are common in the Bringhurst pluton. These dark blobs are chunks of Wilmington blue rock that were picked up and incorporated into the magma as it intruded into the gneiss. These inclusions are called xenoliths, a word derived from the root xeno- meaning foreign and lithos- meaning rock. Thus, a xenolith is a foreign rock enclosed within another rock. In this case the xenoliths are derived from the country rock, the Wilmington blue rock. Although the Wilmington blue rock is composed of both dark layers and light layers, all the xenoliths are derived from the dark layers. This is possibly because the light-colored inclusions melted at a lower temperature than the dark inclusions, and the light inclusions melted in the hot gabbroic magma of the pluton becoming commingled and no-longer recognizable.
A contact between the coarse grained rocks of the pluton and the Brandywine Gneiss occurs approximately 700 to 800 ft east of the park entrance (Figure 1). The gneiss at the contact is contorted and contains clots of quartz.
Before a field trip to Bringhurst Wood Park, it is recommended the group visit one of the Wilmington Complex stops described in the Wilmington Blue Rocks Geologic Adventure, so the participants can recognize the Wilmington blue rock.
Minerals of the Bringhurst pluton (Figure 2) are plagioclase feldspar, pyroxene and olivine. The plagioclase is dark gray and glassy. Feldspar has two distinct cleavages, thus when a feldspar crystal is broken along a cleavage plane it will present a smooth shiny surface. The pyroxene crystals are elongated, black or bronze colored, and may have a distinctive schiller or iridescent luster on a fresh surface. The olivine grains are less common than the pyroxene, and in this pluton, the olivine grains are usually rusty and have a black rim. Individual minerals in the rims cannot be recognized in hand specimens, but microscopic study has identified an inner rim that is an intergrowth of orthopyroxene with spinel and an outer rim that is an intergrowth of hornblende with spinel. The olivine-bearing rocks are more abundant southwest of the park entrance.