Geology
A tremendous variety of geology is evident along Wissahickon Creek. Three of the geologic regions that the stream passes through are the Newark Basin of Triassic sandstone and shale, the limestone and dolomite of the Chester Valley, and the Wissahickon Formation where the waters of the stream flow into the Schuylkill and eventually the Delaware Rivers.
A unique and very distinctive rock of the Wissahickon Creek valley is Wissahickon schist, the predominant bedrock underlying the Philadelphia region, found over a broad swath of southeastern Pennsylvania from Trenton into Delaware and Maryland. This Precambrian to Cambrian stone, first studied in the Wissahickon gorge, has flecks of glittery mica, small garnets, and many-toned shadings of gray, brown, tan, and blue, and is attractive enough to have become a common building material in the 19th and early 20th centuries.
In addition to Wissahickon schist, there are layers of quartzite in the valley. Both schist and quartzite are metamorphic rocks formed from sedimentary deposits of mud and sand that one time were washed from ancient continents into a shallow sea. These sedimentary deposits were over time compressed into shale and sandstone. During long periods of mountain building, the shale and sandstone were slowly transformed into the schist and quartzite found today. In some places, the compression and heat were extreme enough to fuse the schist with emerging igneous rocks into hard-banded gneiss.
Other rocks in the valley are layers of igneous pegmatite and remains of granite plutons, embedded crystals within the schist. A few locations close to Devil’s Pool and along Bell’s Mill Road have a talc schist which contains the mineral talc, so soft it can be scratched with a fingernail.
A virtual geologic tour of Wissahickon Creek is available at this site.
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