Geologic mapping was conducted at 1:12,000 with a 1-ft contour basemap. In some instances, stratigraphic boundaries drawn at topographic breaks reflect detailed mapping using LiDAR data. Elevations of stratigraphic contacts along stream valleys are projected from subsurface data. Except for a few erosional bluffs, these contacts are covered by colluvium. This map supersedes this portion of Geology of the Chesapeake and Delaware Canal Area, Delaware: Delaware Geological Survey Geologic Map Series No.

The complex geologic history of the surficial units of the Harbeson Quadrangle is one of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology is further complicated by periglacial activity that produced dune deposits and Carolina Bays scattered throughout the map area.

The geologic history of the surficial units of the Fairmount and Rehoboth Beach quadrangles is that of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology reflects this complex history both onshore, in Rehoboth Bay, and offshore. Erosion during the late Pleistocene sea-level low stand and ongoing deposition offshore and in Rehoboth Bay during the Holocene rise in sea level represent the last of several cycles of erosion and deposition.

Rising and highstands of sea level during the middle to late Pleistocene deposited swamp to nearshore sediments along the margins of an ancestral Delaware Bay, Atlantic coastline, and tributaries to an ancestral Chesapeake Bay. These deposits are divided into three lithostratigraphic groups: the Delaware Bay Group, the Assawoman Bay Group (named herein), and the Nanticoke River Group (named herein). The Delaware Bay Group, mapped along the margins of Delaware Bay, is subdivided into the Lynch Heights Formation and the Scotts Corners Formation.

The surficial Pliocene and Quaternary sedimentary deposits of the Atlantic Coastal Plain of Delaware comprise several formal and informal stratigraphic units. Their ages and the paleoenvironments they represent are interpreted on the basis of palynological and lithologic data and, to a lesser degree, on geomorphology.

The purpose of this report is to provide information on the mining industry of Delaware as an essential component of a growing economy. The industry, particularly in sand and gravel mining, must deal with uneven regulation, land use competition, and environmental pressures. It is hoped that the information gathered here will assist planning and regulatory agencies as well as an interested general public in evaluating the role of the extractive mineral industry.