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.
This report accompanies a new map that revises the original bedrock geologic maps of the Delaware Piedmont compiled by Woodruff and Thompson and published by the Delaware Geological Survey (DGS) in 1972 and 1975. Combined detailed mapping, petrography, geochemistry, and U-Pb geochronology have allowed us to redefine two rock units and formally recognize eleven new units. A section of the Pennsylvania Piedmont is included on the new map to show the entire extent of the Mill Creek Nappe and the Arden Plutonic Supersuite.