Upper Delaware River Geology: Devonian Rock, Glacial Landforms and Early Land Plants

Upper Delaware Scenic and Recreational River protects a 118 km (73.4 mi) stretch of the Delaware River on the Pennsylvania–New York border, in the Appalachian Plateau. The National Park Service's Geodiversity Atlas — part of its Geologic Resources Inventory — collects what is known about the park's rocks, minerals, sediments, fossils, landforms, soils and the processes behind them, for teaching, geoconservation and managing living and non-living resources together.

Image from the National Park Service's Geodiversity Atlas page for the Upper Delaware

The park

Rolling hills and riverfront villages line what is one of the finest fishing rivers in the northeastern United States.

The rocks

Age. Mostly Paleozoic rock from the Devonian Period, roughly 420–360 million years ago.

A puzzle. The Upper Devonian rocks of the upper Delaware valley are a baffling mix of clastic strata, for two reasons: the layers themselves are confusing across the region, and geologists have defined, named and ordered them in different, even conflicting, ways. From oldest to youngest, the park's units are the Mahantango, Trimmers Rock and Catskill formations.

Landforms. The river corridor holds a striking variety:

• glacial deposits and glacial outwash terraces;
• bedrock knobs;
• cut-off incised meanders;
• island complexes;
• narrow, deep gorges.

Sandstone cliffs, well seen at the Hawks Nest, are old river-channel deposits. They show many bedding features: crossbedding, ripple marks, current lineation, tool marks, mud cracks and conglomerates.

Fossils. Most fossils found in the park are Devonian plants — some of the earliest land plants.

Roebling's Delaware Aqueduct. Image from the National Park Service's Geodiversity Atlas page for the Upper Delaware

Maps and inventories

The Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks, from a scoping summary of the mapping plan through GIS map data, a report on the park's geology and management issues, and a poster. A Soil Resources Inventory for the Upper Delaware is also complete. Both are on the NPS DataStore.

Sources

Based on "NPS Geodiversity Atlas—Upper Delaware National Scenic and Recreation River, New York and Pennsylvania," National Park Service; a work of the United States government in the public domain. The source's account of the confusing strata draws on a personal communication from Frank Fletcher (2021).