The Geology of Delaware Water Gap National Recreation Area

Delaware Water Gap National Recreation Area takes in the gap and the valley upstream of it on the Delaware River, in northeastern Pennsylvania and northwestern New Jersey. For centuries the gap has channeled people through the otherwise forbidding Kittatinny Mountain — American Indians, the resort crowds of the late 1800s, and today's highway and rail traffic. About 5 million people visit each year, many from New York City and Philadelphia.

The park's 27,009 ha (66,740 ac) of rolling hills, ridges and valleys run along 64 km (40 mi) of the Middle Delaware National Scenic and Recreational River, the longest free-flowing river in the eastern United States. The valley sits between Kittatinny Mountain on the east and the Pocono (Allegheny) Plateau on the west.

The National Park Service's Geodiversity Atlas summarizes the park's geology, drawing on the Geologic Resources Inventory.

The gap

Near the park's southern boundary the river passes through the gap itself: a V-shaped notch about 1.6 km (1 mi) wide and 370 m (1,200 ft) deep in the narrow ridge of Kittatinny Mountain, part of the Valley and Ridge province of the Appalachians. Mount Minsi in Pennsylvania and Mount Tammany in New Jersey stand on either side, and the gap has been called the most attractive in the country.

It formed through headward erosion and stream capture. Folding and faulting during the Appalachian mountain-building episodes, hundreds of millions of years ago, left a zone of weakness in the rock; erosion concentrated there and opened the gap. Other gaps in the region formed the same way but are nowhere near as large.

Rocks and structure

The bedrock was laid down in the Ordovician, Silurian and Devonian periods as lime, mud, sand and gravel in a basin west of highlands raised along the eastern edge of North America during Appalachian mountain building. Much later, over the past 2 million years, ice-age glaciers planed off the highlands, carved troughs and dropped outwash, till and moraines. The youngest deposits are stream alluvium, colluvium gathered at the foot of slopes, and swamp and marsh deposits.

Mountain building bent the rocks into folds — both anticlines ("A"-shaped) and synclines ("U"-shaped) — and broke them along normal, reverse and thrust faults. The limbs of major folds hold up Kittatinny Mountain's high ridges and show in the walls of the gap, and later folds and faults are stamped over earlier ones.

The sedimentary rocks keep signs of where they formed: turbidites from underwater landslides, mudcracks, soft-sediment deformation and concretions. The Taconic Unconformity, a regionally important gap in the rock record, separates Ordovician from Silurian rocks and marks the uplift and erosion of an ancient mountain range.

Waterfalls

The park's steep relief makes many waterfalls. Raymondskill and Dingmans falls are the two tallest in Pennsylvania, and Buttermilk Falls is among the tallest in New Jersey. Bushkill Falls, the "Niagara of Pennsylvania," is just outside the park. Waterfalls mark places where resistant rock gives way to weaker rock, or where a river has adjusted to a change in base level or a big increase in flow. Over time they retreat upstream, cutting a gorge into the resistant ridge.

The upper part of Dingmans Falls, flowing over Devonian rocks of the Mahantango Formation. National Park Service photo by Ron Karpilo, Colorado State University.

Along Dingmans Creek, west of Dingmans Ferry, Pennsylvania, Factory, Fulmer, Deer Leap and Dingmans falls are successive knickpoints, sharp breaks in the stream's slope. They were cut not by glacial ice but by meltwater as the glaciers melted back. The faces of the falls are knickpoints from before the ice ages; the series of notches in their lips, cut by quarrying and plucking, formed during high meltwater flows after three or four Pleistocene deglaciations. Pinchot, Raymondskill and Bushkill falls formed the same way, while Silverthread Falls, also near Dingmans Creek, follows a narrow joint in the rock.

The ice ages

Glaciers advanced and retreated over the region again and again during the Pleistocene Epoch, from 2 million to about 20,000 years ago. In the last, the Wisconsinan glaciation, the Laurentide Ice Sheet covered northern North America, and its Kittatinny and Minisink Valley lobes stopped just 4 km (2 mi) south of the park's southern boundary. The park's glacial features are well exposed and well studied: those deposited or carved by moving ice, those laid down by meltwater rivers (glaciofluvial) and those laid down in lakes near the ice (glaciolacustrine). The landscape changed considerably after the ice last withdrew.

Fossils

The park's Ordovician, Silurian and Devonian rocks hold a rich fossil record, mostly marine invertebrates in its claystone, siltstone, sandstone, conglomerate, limestone and dolomite. Fossils have been studied here for more than a century. In 1903 Stuart B. Weller of the New Jersey Geological Survey collected and described fossils from many sites in the area; his specimens are among more than 1,300 from park collections at the New Jersey State Museum in Trenton. All fossils in national parks are protected by the Paleontological Resources Preservation Act of 2009.

Caves

Karst is limited to the park's limestone-bearing Silurian and Ordovician rocks and other units with carbonate minerals. On the New Jersey side, dozens of small sinkholes, sinking streams, springs and small caves line Wallpack Ridge, mostly in the Devonian Onondaga Limestone; Dingmans Ferry Spring and Brau Kettle are typical karst springs. All caves and mines in the park have long been closed to the public to slow the spread of white-nose syndrome in bats.

Cold Air Cave is not a solution cave but a talus cave: a space among huge, jumbled sandstone boulders of the Shawangunk Formation that fell from the slopes above. Blocks nearly 9 m (30 ft) long roof it and look deceptively stable; the talus around it shows that freeze–thaw can still bring rocks down. It was found and named around 1870, when very cold air, near –1°C (30°F), was reported coming from rocks along what is now State Highway 611, and it drew tourists in the late 19th and early 20th centuries. It may run up to 21 m (70 ft), though only about 9 m (30 ft) is passable for adults. Air blowing out is colder than air blowing in, suggesting a store of cold air in the spaces of the scree and perhaps the bedrock above.

Mines

Mining and quarrying here go back at least to the 1750s. The Pahaquarry copper mine may have been the first copper mine in the United States. Its origins are wrapped in lore: one story credits a Dutch operation of the 1600s with building the 167-km (104-mi) Old Mine Road. The mine worked the thinly scattered copper of chalcocite beds in the Bloomsburg Red Beds, on and off across three centuries, but never paid. Several quarries and sand and gravel pits also lie in the park. Abandoned mines can be valuable cultural sites and habitat but can endanger people and wildlife: stay out and stay alive.

Maps and reports

The park's digital geologic map. Image from the National Park Service's page.

The Geologic Resources Inventory's scoping summary, digital geologic maps, report and poster for the park are on the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Delaware Water Gap National Recreation Area, New Jersey and Pennsylvania," citing Sevon and Inners (2001), Witte and Monteverde (2006), Epstein (2001, 2010), Snyder (1989), Parris and others (1990) and Kopczynski (2004).
• The source dates the bedrock to 450–359 million years ago but its fossils to more than 480 million to about 360 million years old, which cannot both be right; neither range is given here. It spells the Onondaga Limestone "Onandaga," corrected here. Its image of the inventory report's cover is not reproduced.