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USGS Geology in the Parks

Geologic Provinces of the United States: Appalachian Highlands

Delaware River

Ordovician and Silurian rocks

Delaware Water Gap National Recreation Area

Photo by Jack B Epstein, USGS

Exposure of Ordovician (Martinsburg) and Silurian (Shawangunk) rocks in Delaware Water Gap, New Jersey side. The slates of the are Martinsburg are separated by an angular uinconformity of about 10 degrees from the resistant quartzites of the Shawangunk, representing the Taconic orogeny. A heavy apron of talus are derived from the Shawangunk cliffs. On the opposite (Pennsylvania) side of the gap the Shawangunk does not dip so uniformly and is arched (shown by the projected dashed line), allowing for the interpretation of a flexure at the gap site. This has suggested structural control for the location of the gap, a fact contradictory to the hypothesis of regional superposition. Mt. Tammany is named after a Lenni-Lenape Indian Chief who had a secret society named after him which, through time, became the infamous Tammany Hall.

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