Rugget outcrops of Wissahickon Formation, a regional rock formation consisting of schists, genisses, metaconglomerate, and metagraywackes of Late Precambrian age (about 750 million, according to the Maryland Geological Survey).
The URL is: https://gotbooks.miracosta.edu/gonp/grfa/html2/grfa046.htm
Last modified: 3/11/2019
