The Delaware Water Gap: A Window Into Earth’s Early Oceans

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Article by Ariana Miranda—GeoCorps Paleontology Intern, Delaware Water Gap National Recreational Area

Mount Minsi
Figure 1. Mount Minsi as seen from the parking lot on the New Jersey side of the Delaware Water Gap. It is primarily composed of Shawangunk Formation.

NPS photo by Ariana Miranda.

When most people think about fossils or paleontology, they may recall the famous dinosaurs of the Late Jurassic found in the rocks at Dinosaur National Monument. The towering, long necked dinosaurs Apatosaurus and Camarsaurus, and nearby, the nimble Allosaurus fragilis awaiting its prey. Or, they may conjure scenes of impressive herds of woolly mammoths (Mammuthus primigenius) traversing the vast snow-covered plains of North America. Ancient herds like these are preserved at Waco Mammoth National Monument in eastern Texas. Unbeknownst to most people, the northeastern United States holds an immense wealth of marine fossils.

One such park, the Delaware Water Gap National Recreation Area (DEWA), straddling the lower Delaware River along northwestern New Jersey and eastern Pennsylvania, boasts extensive ancient reefs. These reefs showcase prehistoric marine life that predates the dinosaurs of the Mesozoic Era by 200 million years. The rocks in DEWA range from the Late Ordovician to the Late Devonian, providing a relatively continuous record of 360 million years on the east coast and a vast repertoire of the evolutionary history of early marine life (Epstein 2001). While Delaware Water Gap National Recreation Area has provided picturesque scenes and refuge for endangered species in the modern day, its best kept secret is its paleontology and geology (Figure 1).

A cross section of a preserved reef
Figure 2. A cross section of a preserved reef from the Coeymans Formation with rugose corals and stromatoporoid sponges. Pencil is about 7 inches.

NPS photo by Ariana Miranda.

One of the most famous reefs in the park can be found in the Coeymans Formation. This limestone rock is dated to around 300 million years ago (Early Devonian) (Epstein 2001). In one certain area of Coeymans Formation, a preserved reef of rugose corals can be seen. Rugose corals were the main architects of Silurian and Devonian reefs, reaching sizes that are comparable to today’s reefs (Figure 2). They can easily be identified by their distinctive horn shaped body. Another major reef builder that can be seen in these reefs are stromatoporoid sponges. They can be distinguished by their layered growth bands when viewed in cross section. Various sections of this rock unit are entirely made of these corals and sponges.

While DEWA is chock full of invertebrate fossils (animals without a spinal cord), there are some areas where the remains of a few early fish have been found. These early fish would have looked alien compared to modern day fish. These ancient fish, collectively named heterostracans, were tadpole shaped with a covering of thick bony plates (Figure 3) and lacked jaws. Inevitably, these early fish went extinct by the end of the Devonian. Curiously, these fossils have only been found in the Bloomsburg Formation, possibly because the environment favored the preservation of their bodies after death.

the bony plates of a fish
Figure 3. The bony plates of Vernonapsis sp. (NJSM 11931), a heterostracan fish. It is currently reposited at the New Jersey State Museum.

NPS photo by Ariana Miranda.

Perhaps the most famous rock layer is the Mahantango Formation because of its diverse preservation of marine life that existed during the Late Devonian. The most recognizable fossils are found in these rocks, including trilobites, brachiopods, bivalves, and crinoids. Occasionally, the cigar shaped shells of prehistoric cephalopods can be found. Just outside the park, several reefs propagated by rugose and tabulate corals, which lived in these shallow sea environments, can be observed (Epstein 2001).

Within DEWA there is a dedicated fossil trail maintained by the Pocono Environmental Education Center (PEEC). The short trail offers a summary of the fossils found along the trail with a collection of fossils on display (Figure 4). There is another display with diverse assemblage of fossils of the sea creatures that lived during the Middle Devonian in the PEEC visitor center.

The Fossil Trail maintained by the Pocono Environmental Education Center
Figure 4. The Fossil Trail maintained by the Pocono Environmental Education Center, crossing through the Mahantango Formation. The end of the trail is marked by a small exhibit showing fossils found nearby

NPS photo by Ariana Miranda.

Delaware Water Gap National Recreation Area provides visitors and scientists alike the opportunity to learn about the early and diverse marine life of Earth’s oceans. Ultimately, our understanding of life in this crucial period, allows us to understand our place in Earth’s history as humans.

Suggested Readings
Epstein, J. B. 2001. Stratigraphy in the region of Delaware Water Gap National Recreation Area. Pages 1–13 in J. D. Inners and G. M. Fleeger, editors. 2001: a Delaware River odyssey. Field Conference of Pennsylvania Geologists, Harrisburg, Pennsylvania. Guidebook for the Annual Field Conference of Pennsylvania Geologists 66.

Miranda, A., J. S. Tweet, V. L. Santucci, and K. E. Deutsch. 2018. Delaware Water Gap National Recreation Area: Paleontological resources inventory. Natural Resource Report NPS/DEWA/NRR—2018/1721. National Park Service, Fort Collins, Colorado.

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Last updated: October 4, 2019