Geology of the New York City Region

32. Delaware Water Gap National Recreation Area

Several miles west of Exit 12 on west-bound Interstate 80 is a "scenic overlook" that provides a westward view across a half dozen miles of the Great Valley to the ridge line of Kittatinny Mountain. One's gaze is drawn to a great notch in the high ridge where the Delaware River has carved a great gorge through the mountain. The drive westward is filled with anticipation as one approaches the high cliffy mountainsides that rise above the Delaware River. The fast ride through the Delaware Water Gap on Interstate 80 is most impressive. However, For most people this is the only portion of the Delaware Water Gap National Recreation Area that most visitors see.

Kittatinny Mountain exists because its core consists of the highly erosion resistant Silurian Shawangunk Formation (Figures 73 and 74). The top of this long ridge maintains a nearly even elevation of between 1,400 to 1,600 feet for much of its extent from southern Pennsylvania northward to the New Jersey/New York border. This mountain top surface corresponds with the Schooley Peneplain, the remnants of an ancient erosional surface that can be traced throughout the Appalachian region. The Schooley Peneplain corresponds to all the mountain top levels of the Highlands region to the east, as well as the upland regions of the Pocono Plateau. As the land has steadily risen through Late Tertiary time, stream erosion has kept pace with the uplift, with regional streams probably maintaining base levels close to their current gradients. The nearly pure quartz content of the Shawangunk Formation (including both its aggregate grains and its cement) ensures that the rock is extremely resistant to both mechanical and chemical weathering and erosion. In contrast, the carbonate rocks and shales of Cambrian and Ordovician age of the Great Valley region of New Jersey to the east of the ridge are more easily eroded. The same is true of the Early and Middle Devonian rocks of the Port Jervis Trough to the west. As a result of these weathering and erosion characteristics, Kittatinny Ridge endures relatively unchanged as the surrounding landscape continues to wear away.

Geologic cross section of Kittatiny  Mountain
Figure 73. Generalized cross-section of Kittatinny Mountain the Delaware Water Gap area (after Lyttle & Epstein, 1987).

Delaware Water Gap as seen from 24,000 feet
Figure 74. A west-facing jetliner view of the Delaware Water Gap.

The origin of the Delaware Water Gap has been a topic of debate for well over a century. The chasm of the Delaware River through the high ridge of Kittatinny Mountain is one of many "water gaps" and "wind gaps" through high ridges in the Appalachian Mountains. Water gaps are where rivers currently cut through ridges. Wind gaps are where a valley has been carved through a ridge, but no stream currently flow through. There are several hypotheses regarding the age of the gaps. Early researchers suggested the gaps date to a time during the Late Cretaceous when the landscape in the region was greatly subdued and the coastal plain extended far inland of its present extent. Gradual uplift during the Tertiary resulted in the subsequent down-carving of wandering streams on this ancient surface (the Schooley Peneplane discussed above). Later researchers noted that the slope of stream drainages on the Atlantic side of the Appalachian Mountains is much steeper that the gradient on the midcontinent side, and that the gaps are perhaps the result of stream capture processes as the steeper, higher energy streams carve back into their headlands. Other evidences suggests that the gaps occur in locations where a higher degree of fractures, jointing, and faulting have occurred. For example, the Shawangunk Formation is dipping more steeply on the north side of the Delaware Water Gap (Mt. Tammany at around 50 ) than on the south side (Mt. Minsi at around 25 ). The difference in dip angles no doubt would suggest that the brittle Shawangunk Conglomerate was highly fractured in the vicinity of the gap, and therefore more easily eroded. Perhaps an unresolved question is the role that continental glacial played in the development of the gaps in the region. There are almost a dozen gaps in the Stroudsburg, Pennsylvanian region between the Delaware Water Gap and Wind Gap, located about ten miles to the south. Glacial till deposits of the most recent Pleistocene glacier extend about 10 miles south of the Delaware Water Gap. Other glaciers in the Early Pleistocene also advanced into this region. The blockage and ponding of northward flowing streams, and the subsequent breakout of streams across new divides probably played a roll in the development of the landscape with its numerous wind and water gaps in the Stroudsburg area.

Delaware Water Gap National Recreation Area encompasses 70,000 acres of land in both the New Jersey and Pennsylvanian sides of the Delaware River extending for 35 miles upstream from the water gap. The park was established in 1965 by President Johnson. This followed a period of intense public outcry after Congress in 1962 which authorized the Tocks Island Dam project under the Flood Control Act. The dam would have turned the park area into a reservoir. It is now managed by the National Park Service.

There are two visitor centers: the Kittatinny Mountain Visitors Center is located just off I-80 in the Delaware Water Gap just east of the Delaware River Bridge. The other is located at Dingman Falls near Dingmans Ferry on the Pennsylvania side, just east of Route 209. The park's central attraction is the river which is one of the cleanest rivers for it size in the United States. It is a popular attraction for canoeing and fishing enthusiasts. The park has over 200 miles of scenic roads, and an equally extensive trail system that includes 25 miles of the Appalachian Trail along the crest of Kittatinny Mountain. Be aware that while hiking, camping, or driving through the park it is possible to encounter wildlife. The black bear population in the park area numbers in the thousands, and they are not an uncommon sight.

There are many scenic destinations to enjoy in the park that are of geologic interest. Perhaps the most popular hike in the park is the moderately strenuous hike to the top of Mount Tammany along the Appalachian Trail. The trailhead is located in the 1-80 rest area (west-bound side), but can be easily reached from a road connecting to the visitor center located on the east-bound side. The trail climbs 1,200 feet over a distance of 2 miles, and leads to overlooks on the high cliffs overlooking the river. The steeply dipping ridges of rock along the eastern slope consist of tightly cemented sandstone and conglomerate of the Silurian Shawangunk Formation (Figure 75). Beneath the cliff the hillsides are covered with large blocks of the rock plucked from steep mountainside over the millennia by freeze-thaw cycles of weathering. The rocks accumulates as great talus slopes beneath the cliff.

Mt. Tammany (Kittatiny Mountain), Delaware Water Gap Nation Recreation Area
Figure 75. The Delaware Water Gap from riverside. Cliffs of Silurian Shawangunk Formation crop out above I-80 on Mount Tammany (part of Kittatinny Mountain).

Return to Valley and Ridge Province Main Page.
Continue to the next page. Return to Main Page
The URL is:
Last modified: 3/11/2019