In the patch of woods north of the upper parking lot in Brandywine Creek State Park, there 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.
Coarse-grained gabbroic and metagabbroic rocks, variably metamorphosed and deformed. Primary igneous minerals include olivine, clinopyroxene, orthopyroxene, and plagioclase.
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.
All the rock units in Delaware’s Piedmont are highly deformed. Deformational features, such as folds, faults, and/or joints, are present in almost every outcrop.