The Geology of Delaware

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Rockford Park Gneiss

Geologic Time Period: 
Piedmont Unit

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.

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 < 10 modal percent pyroxene. Mafic layers contain subequal amounts of plagioclase, pyroxene, and hornblende. Penetrative deformation and granulite facies metamorphism have obscured igneous fabrics and contact relationships.