1: DELAWARE WATER GAP, A GEOLOGY CLASSROOM
Jack B. Epstein
U.S. Geological Survey
Introduction
The Delaware Water Gap National Recreation Area (DEWA) contains a rich geologic and cultural history within its 68,714 acre boundary. Following the border between New Jersey and Pennsylvania, the Delaware River has cut a magnificent gorge through Kittatinny Mountain, the Delaware Water Gap, to which all other gaps in the Appalachian Mountains have been compared. Proximity to many institutions of learning in this densely populated area of the northeastern United States (Fig. 1) makes DEWA an ideal locality to study the geology of this part of the Appalachian Mountains. This one-day field trip comprises an overview discussion of structure, stratigraphy, geomorphology, and glacial geology within the gap. It will be highlighted by hiking a choice of several trails with geologic guides, ranging from gentle to difficult. It is hoped that the "professional" discussions at the stops, loaded with typical geologic jargon, can be translated into simple language that can be understood and assimilated by earth science students along the trails. This trip is mainly targeted for earth science educators and for Pennsylvania geologists needing to meet state-mandated education requirements for licensing professional geologists. The National Park Service, the U.S. Geological Survey, the New Jersey Geological Survey, and local schoolteachers had prepared "The Many Faces of Delaware Water Gap: A Curriculum Guide for Grades 3–6" (Ferrence et al., 2003). Portions of this guide, "The Many faces of Delaware Water Gap" appear as two appendices in this field guide and is also available by contacting the Park The trip will also be useful for instruction at the graduate level. Much of the information presented in this guidebook is modified from Epstein (2006, 2010).
Please note: It is illegal to collect rocks, plants, or parts of animal life in National Park lands.

1. Delaware Water Gap, A Geology Classroom

View of Delaware Water Gap on right.
Crossing into Pennsylvania.

Exposure of fluvio-glacial outwash in road cut on left.

Arrow Island Overlook on right.
Arrow Island Trail (Stop 4 on left).

Martinsburg outcrop on left, note low dip of slaty cleavage.

Cold Air Cave on left (Stop 3).

STOP 1. Point of Gap Overlook
Geologic Synopsis Structure, Stratigraphy, Geomorphology, Glacial Geology
The following questions are typically asked—or at least thought about—by laymen visitors to Delaware Water Gap:
What is a water gap?
What is the layering in the rocks?
What are the rocks made of?
Why are they of different colors?
Why are they tilted and curve in different directions?
Why is the cliff irregular and not smooth?
Why does the mountain top look flat from a distance?
Why did the Delaware River cut through the mountain here?
Did it cut through anywhere else?
Possible answers to these questions will be discussed with trip participants. But first, let us go through the jargon! The NPS has prepared a curriculum guide for school grades 3– 6, "The Many Faces of Delaware Water Gap" (Ferrence et al., 2003). In that guide, the Point of Gap Overlook is discussed (see Appendix 1), and the guide includes exercises in how the rocks formed, how the mountains were built, and glaciation.
Delaware Water Gap is often cited as the classic water gap in the Appalachian Mountains. Figure 6 portrays its geology. Anyone who studies the area is compelled to contemplate the history of the gap and why it is where it is. Is the structure seen in the gap (Fig. 2) as simple as it looks? What story do the satellitic folds in the Shawangunk tell? Why does the cleavage dip to the northwest within several hundred feet of the contact with the Shawangunk? These issues were summarized in the last GSA trip to this area four years ago (Epstein, 2006) and may be discussed at the stop.
The spectacular Delaware Water Gap has inspired people for generations, and created wonder on how this magnificent chasm through Kittatinny Mountain could have been cut by the Delaware River. Its story goes back in geologic time many hundreds of millions of years, although the actual cutting took place within the last few million years.
Our planet has had a dynamic history for much of its existence. Mountains have risen and fallen as continents have shifted and collided with each other during the earth’s more than 4.5 billion year history in a process called plate tectonics. Kittatinny Mountain is part of the Appalachian Mountain chain that extends for more than 2000 miles from Maine to Georgia. The birth of the Appalachians dates back several hundred million years. About 550 million years ago during the Cambrian period, a carbonate bank (A in top of Fig. 7) lying on the shelf of the old North American continental crust (B) faced eastward toward an ancient ocean, Iapetus (C). Africa lay across the Iapetus Ocean far to the east. A volcanic island (D) formed in the middle of the ocean as the African continent began to drift westward, beginning the closure of Iapetus. Thick sediments were deposited in the basin ahead of the volcanic island arc that were later consolidated into the Martinsburg Formation. As the ocean basin continued to close during continued plate convergence, sediments were deformed (middle figure) and later, not shown in Figure 7, uplifted ∼450 million years ago during the Ordovician period of geologic time, resulting in the buckling of the earth’s crust and uplifting of the ancient Appalachian Mountains. This period of mountain building is called the Taconic Orogeny. Sands and pebbles from the rising mountains were shed to the northwest during Silurian time, deposited across the warps and folds in the underlying Martinsburg. These clastic rocks were to become the Shawangunk Formation that holds up the mountain at the gap. Other orogenies variously affected the rocks in the Delaware Water Gap area. During the Devonian period, the Acadian orogeny uplifted mountains to the southeast which resulted in deposition of thick sediments seen in the Pocono Mountains today, but it had little or no structural effects on the rocks here. The Alleghenian orogeny at the end of the Paleozoic era, probably beginning in the Pennsylvanian period, folded and faulted all the rocks in eastern Pennsylvania, and was responsible for the folds seen in the Gap. It was at this time that Africa finally collided with North America.
For many millions of years erosion has worn down these rocks–the harder rocks stand as mountains while the softer rocks were eroded to form valleys. The sandstones and conglomerates of the Shawangunk Formation (shown by the stippled pattern in Figure 7) are among the hardest rocks in the entire Appalachian Mountains and form some of the highest ridges, extending all the way south to Alabama.
How the Gap was Formed
Some millions of years ago, the headwaters of a southeastward flowing river that was still south and east of here (perhaps near present-day Trenton, New Jersey) eroded backward toward the Kittatinny Ridge (A in lower Fig. 7). The hard rocks of Kittatinny Mountain presented an obstacle to erosion, and the Delaware found a place in the ridge where the rocks are more highly fractured and less resistant to the erosive power of the river. Thus, the river eventually worked its way headward through the mountain (A), ―capturing‖ waters on the other side (B) and establishing the present course of the Delaware through Kittatinny Mountain (C). It carved it way through the rocks that make up the Pocono Mountains to the north. Carrying sand and pebbles, which eventually wind up in the Atlantic Ocean, the Delaware River continues to downcut and widen what was originally a small cleft in the rock. That cleft, over time, has been enlarged into today’s mile-wide chasm: The Delaware Water Gap.
At this stop we may discuss the several intricacies of structural geology in this area, such as origin of slaty cleavage, pressure-shadow mechanism affecting cleavage, and ages of deformation. Maybe not!

Contact between the Shawangunk and Bloomsburg
Contact between the Shawangunk and Bloomsburg dipping 35°NW.

Southeast-dipping rocks in the Bloomsburg Red Beds.
US 611 traverses the Bloomsburg for the next 0.8 miles in a series of small undulating, low-amplitude folds. Note the southeast-dipping cleavage.

Crest of the Cherry Valley anticline
Crest of the Cherry Valley anticline in the Shawangunk Formation at top of road. The contact between the Shawangunk Formation and Bloomsburg Red Beds is conventionally placed at the base of the lowest red bed. However, at this locality this color change migrates up and down section by as much as 700 feet, making for a peculiar map pattern (Epstein, 1973). Enter upstream side of Delaware Water Gap.

Northwest-dipping rocks of the Shawangunk Formation on left.

STOP 2. Lake Lenape
We will follow the Appalachian Trail along an abandoned blacktop road that once led to the top of Mount Minsi to a fire tower. At the north edge of the pond is a memorial to Sean Dolan, A Delaware Water Gap fire fighter who died in a nearby car accident in 2004.
Lake Lenape is near the trough of one of many small folds in Delaware Water Gap (Fig. 8). It is in an area where the contact between the Shawangunk Formation and Bloomsburg Red Beds is based on change from gray in the Shawangunk to reddish in the Bloomsburg. In this immediate area, this color change variably cuts across ∼700 feet of stratigraphic thickness, making for the complex boundary seen on the map.
Lake Lenape is a marvelous area to give students an opportunity to examine several rock types, including sandstones, siltstones, and shales, to determine differences between bedding and rock cleavage, and to make a crude clinometer to prepare a map of the local gentle syncline.
The curriculum guide handout (Appendix 2) will provide instructions on how to use a compass, prepare grain size charts in order to determine sedimentary rock type, and encourage students to examine the local biota. Be wary of poison ivy, which covers the southernmost outcrop that will be measured. Here, have the students eyeball the dip with the clinometer rather than placing it on the rock. There is also an exercise that asks the geologic question, "How Do Lakes Form?"
"The Many Faces of Delaware Water Gap" is a 125-page geology curriculum (grades 3– 6) available from the Delaware Water Gap National Recreation Area. It includes field trips for five locations in the park in Pennsylvania and New Jersey, as well as a 20-page introduction to general geologic topics for the park. Teachers are invited to contact the park for a copy
An interesting exercise would be to have the students walk from the lowest point along the road for a little more than 100 feet up the trail to the south where the outlet is located, ∼4 feet higher in altitude. Then ask them these questions: Why is the outlet not at the lowest point along the road? Did the lake formerly drain at the lowest point? (Have them look to the east of the road to determine where the most gully erosion occurred.)
Point out the location of rhododendrons–on the north slopes in the shade and on acid, organic-rich soil.
There is a splendid overlook and view of the Delaware Water Gap by following the Appalachian trail for a short distance where the trail leaves the road ∼600 feet south of the pond (see Fig. 8). From here you can discuss some of the folds in the Bloomsburg Red Beds across the river.

STOP 3. Cold Air Cave
This talus cave (Fig. 9A) was formed by juxtaposed alignment of large talus boulders derived from conglomerate and sandstone blocks of the Shawangunk Formation above. Some of these blocks are nearly 30 feet long. A large talus floe overlies the cave (Fig. 9B) and a large cleft in the cliff above (Fig. 9C) confirms the potential for generating these blocks during periods of freeze and thaw. The cave may be as long as 70 feet, although only ∼30 feet of the cave is currently accessible to normal-sized individuals. According to Snyder (1989), the cave was discovered ca. 1870 when very cold temperatures, approaching 30 °F, were reported coming from the opening. The cave became an attraction sometime thereafter with a building erected at the entrance (Fig. 9D); tours given. Beginning in September 2000, a cooperative effort to understand the phenomenon of Cold Air Cave was undertaken by the U.S. Geological Survey and National Park Service. Readings for temperature, wind speed, wind direction, and weather conditions were recorded. The data indicates significant fluctuations in cave temperature; they are warmer in summer and cooler in winter. There are significant differences in temperature depending on whether the air is blowing into or out of the cave. Air blowing out is cooler than when it is blowing in the reverse direction, indicating that there is storage of cold air in the scree system and possibly the bedrock above. The average temperature in the parking lot is only slightly cooler than underground temperatures found at this latitude—normal is 54–56 °F. The average temperature in the cave is surprising, however, and was unexpected. In conclusion, Cold Air Cave continues to attract and mystify people with its local folklore, blowing cold air, and mysterious scientific explanation.
NOTE: Since the time of the preparation for this stop, this cave and others throughout DEWA, have been closed to the public because of a virulent fungus, "white nose syndrome", that threatens the local bat population (http://www.nps.gov/dewa/parknews/upload/newsBATCAVES.pdf).

Small outcrop in the Martinsburg Formation on right
Small outcrop in the Martinsburg Formation on right (Figure 10). Note the gently northwest-dipping (5°) cleavage due to fanning in a pressure shadow of the syncline defined by the Shawangunk Formation that will be discussed at Stop 1. Bedding here dips 25° NW.

STOP 4. Arrow Island Overlook and Trail.
Arrow Island, a streamlined bar, is presently being modified by the Delaware River. This occurs due to a decrease in the velocity of the river as it emerges from the narrow confines of the gap to the north. Such a feature has been termed an expansion bar.
The trail is about one mile long (Figs 11 and 12), passing over glacial deposits and outcrops of slate and greywacke in the Martinsburg Formation, ending at a kettle hole pond near National Park Drive. There are several abandoned slate quarries and prospects along the trail, one of which we hope to visit.

Very coarse gravel in late Wisconsinan outwash terrace
Very coarse gravel in late Wisconsinan outwash terrace to right exceeds 80 feet in thickness. The gravel was either laid down between the valley's wall and stagnant ice forming a kame terrace, or it is the eroded remnant of a valley train that formerly filled the Delaware Valley.

Rock fence on right
Rock fence on right is composed of blocks of the Allentown Dolomite with abundant fine sedimentary structures.

Cross Slateford Creek.
In 1805, the Pennsylvania Slate Company developed a slate quarry south of the water gap near Slateford Creek. It is abandoned.

National Park Drive on right leads to Slateford Farm
An example of a National Historic Site maintained by the National Park Service.

Faulted and overturned beds in the Bushkill Member
Faulted and overturned beds in the Bushkill Member of the Martinsburg Formation in ravine to right.

Flat-lying slate in the Bushkill Member
Flat-lying slate in the Bushkill Member in a 100-foot-long abandoned quarry overlain by 10 feet of glacial drift in ravine to right. A dolomite concretion, characteristic of basal beds of the Martinsburg elsewhere, lies in the bottom of the quarry.

Crossing the concealed Portland fault
Crossing the concealed Portland fault, which juxtaposes Martinsburg against the Jacksonburg Limestone and rocks of the Beekmantown Group where it is exposed.

Enter Portland, Pennsylvania.

Cross Jacoby Creek
Deglaciation of the Jacoby Creek valley resulted in the formation of several proglacial lakes that became progressively lower as the ice retreated from the northeast-draining valley and lower lake outlets were uncovered (Ridge, 1983).

Allentown Dolomite crops out on right.

Cross the axis of the Ackerman anticline
Cross the axis of the Ackerman anticline (Drake and others, 1969) in the Allentown Dolomite approximately at point where ramp bears off to 1-80 West.

Coarse kame-terrace (outwash) deposits on right.

Unconformable contact - Martinsburg and Shawangunk
Unconformable contact (<5°, Beerbower, 1956) between the Martinsburg Formation (Ordovician) and Shawangunk Formation (Silurian) covered by talus on right. If you are quick, lucky, and can avoid a car crash, look up about 50 feet and see mudcracks in an overhanging shale in the Shawangunk. (Fig. 13).

STOP 5. Geologic Features Along the Red Dot-Blue Blaze Dunnfield Creek Trails
The Red Dot–Blue Blaze–Dunnfield Creek Trail circuit takes the hiker to the top of Mount Tammany in Kittatinny Mountain, New Jersey (Fig. 14), and in less than four miles, passes through many geologic phenomena, including a variety of rock types, landforms and glacial and structural features. The points of interest along the way (Fig. 15; Table 2), provide insight into these natural elements that influenced the formation, history and composition of Delaware Water Gap. Note that the sedimentary layers that make up the cliffs in the main part of the gap dip to the left, but in Dunnfield Creek valley to the left the beds are horizontal. Trip time is ∼4 hours.
Table 2. Localities of geologic interest along the Red Dot-Blue Dot- Dunnfield Creek trails at Stop 5.
1: Near contact between the Shawangunk Formation and Bloomsburg Red Beds.
2: Eight-foot-long boulder with slickensides.
3: Glacial kame terrace on silt, sand and gravel.
4: Glacial striae.
5: Rotted limestone glacial erratic.
6: Rib of Bloomsburg bedrock.
7: Series of greenish-gray and red siltstone, sandstone and shale of the Bloomsburg.
8: Large erratic, Schoharie Formation.
9: Overlook of the Delaware Water Gap.
10: Red sandstone and siltstone of the Bloomsburg have been polished by the last glacier (20,000 years ago), producing glacial striae.
11: Springs.
12: Beginning of Shawangunk Formation on steep slope.
13: Talus.
14: Rib of quartzite with joints.
15: Glacial cobbles and glacial striae on Shawangunk.
16: Gentle slope underlain by some shale.
17: Forest fire.
18: Overlook, many sedimentary structures in the Shawangunk.
19: Blue Blaze Trail descends slope through laurel and blueberries.
20: Exposure of Bloomsburg bedrock with glacial striae.
21: Soil erosion by boots and rain exposing Bloomsburg bedrock with glacial striae.
22: Erosion has removed about three feet of glacial till.
23: Dunnfield Creek falls over flat beds of the Bloomsburg in bottom of syncline.
24: Three large erratic boulders of slightly cherty limestone.
25: Intersection with Appalachian trail. Several more boulders in creek.
26: Plunge pool formed where the creek drops over hard sandstone and gouges out a rounded pool in softer shale below.
27: The creek erodes along a joint surface here forming a 30-foot sluiceway.
28: Large boulders fallen from adjacent Bloomsburg outcrop have sharp edges compared to the rounded and eroded edges of erratics.
29: Cleavage present in horizontal shale layers but not in sandstone. Erratic in creek.
30: Beginning of the terrace deposit that was first seen at locality 3.
31: 25-foot-long limestone erratic limestone.
32: Bridge. Flat alluvial fan towards parking lot made up of rounded cobbles.
33: Parking lot. Note 6-foot boulder 40 feet above the creek in terrace to right.
GLOSSARY
Alluvial fan: Gently sloping mass of sediment fanning out from a river mouth.
Cleavage: Closely spaced fractures along which a rock may split.
Erratic: A rock that was carried some distance by a glacier from its place of origin.
Kame terrace: Flat-topped hill formed from sediment that was deposited along a valley wall by streams that flowed from an adjacent melting glacier.
Slickensides: Polished and striated rock surface caused by one rock mass sliding past another.
Striae: Narrow parallel scratches cut into a rock surface by rock debris embedded in the bottom of a moving glacier.
Syncline: U-shaped downfold of rock layers.
Talus: An apron of irregular rock fragments derived from and lying at the base of a cliff.
Till: Unsorted mixture of clay, sand, and boulders deposited beneath a glacier.

Splendid folds in the Bloomsburg Redbeds
At the ramp turnoff from I-80 to Old Mine Road there are splendid folds in the Bloomsburg Redbeds showing the relations of cleavage and bedding (fig. 16). The southeast dip of cleavage in all limbs of the folds indicate that it was rotated by bedding slip after they formed.

STOP 6. Guide to the Geology of the Karamac Trail
The trail begins in the parking area at the traffic light and barrier along the Old Mine Road 400 feet north of the I-80 overpass (Fig. 17). This a level trail, nearly one mile long, along an old railroad grade, and along which a few different types of rocks (sandstone, shale, siltstone and conglomerate), two different geologic formations of Silurian age (Bloomsburg Red Beds and Shawangunk Formation), a landslide retained by a concrete structure (gabion), a few folds, joints, rock cleavage, and glacial boulders (erratics) can be seen. Terms in italics should be fully explained to students. The distance along the trail north from the parking area is shown in feet.
LOCALITY DESCRIPTION
-350 On the east side of the road is an excellent example of a small thrust fault and ramp in the Bloomsburg Red Beds (fig. 18). The floor of the thrust is at the base of a thick sandstone bed lying on top of shale and siltstone that are less resistant to erosion.
0 Parking area. Looking to the right up the steep slope are joints, the result of the tectonic stress that folded the rocks in the Delaware Water Gap area. Similar vertical fractures can be seen along the trail for the next 400 feet.
400 The smooth face in a 6-foot high bed of sandstone ahead is an excellent example of a joint (fig. 19). Note the cleavage in the silty beds at arrow.
500 The closely spaced fractures in some of the greenish and reddish beds are tilted into the hill in a much gentler angle than are the joints (fig. 20). This is cleavage which forms in shale and siltstone by reorientation of the mineral grains in the rock.
600 The interlaced segments of concrete is a 100-foot long gabion (fig. 21) which was constructed at this site to hold back a landslide that removed part of the Old Mine Road above the trail. Because of the slide the road is now a single lane with a three-minute traffic light to control traffic along this stretch, and it is unwise to walk along it. The concrete blocks are filled with cobbles to stabilize the road. Immediately above this site along the Old Mine Road there is the potential for another rockfall because blocks of rock are being separated along irregular joints (fig. 22).
1000 The islands in the middle of the Delaware River and terraces along its banks are made of sand that was deposited when the river flowed at a slightly higher level after retreat of the Wisconsinan glacier which left this area ∼20,000 years ago. The sand islands are mute testimony to a former higher level of the river.
1075 Rivulet tumbling down over dark red rocks. This is the Bloomsburg Red Beds. Note that the rock layers are nearly flat at this spot. These rocks were deposited by rivers flowing to the sea. The soft sediment, composed of mud and sand, was hardened or lithified into rock and the layers are termed bedding. The process of lithification results from a variety of causes, including cementation (the rock is cemented by minerals that precipitated out from ground water) and compaction (due to the weight of thousands of feet of overlying sediments that were deposited on top).
1300 Water emitting from a drain pipe. Just beyond a rivulet the bedding is tilted much more steeply to the south than previously (fig. 23), showing that rocks have been folded. The rocks here are no longer red, but shades of gray. Thus, we have left the Bloomsburg Red Beds and are into another rock unit, the Shawangunk Formation. It would be worthwhile to discuss the fact that the Bloomsburg overlies and is younger than the Shawangunk based on the dip of the beds here (law of superposition).
1800 This small anticline (fig. 24) interrupts the general south dipping layers along this stretch of the trail.
2000 Note that the bedding layers are tilted back towards the south, indicating that there is a syncline between here and the previously seen northward dipping beds. This is the site of a spring, where water, flowing through cracks in the underground rock (termed ground water), appears at the surface. During the summer when rainfall is less plentiful, the spring may dry up.
2400 Bedding in the Shawangunk Format ion dips (is tilted) gently to the south (fig. 25). Farther north along the trail and to the end of the trail it will be dipping northward, a structural configuration which is termed an upfold or anticline, named the Cherry Valley anticline because it is a major anticline that extends for several miles to the northeast and southwest. This outcrop is also a good example of crossbedding, which is defined by layers of rock that dip more steeply than the beds above and below and is caused by large ripples that formed in a rapidly moving stream during the Silurian Period.
3400 100-foot long outcrop through the Shawangunk Formation (fig. 26). A close look at the different beds show that some have grains the size of sand (sandstone), in others the grains are very fine (shale and siltstone), and some of the beds contain pebbles, perhaps 0.5 inch long, in a rock termed conglomerate. The beds here are tilted ∼20° toward the north.
4200 The gray rocks of the Shawangunk Formation here are more steeply dipping (33°) than to the south.
4500 So far all the loose boulders we have seen are quite angular in shape, having been derived by breaking off from the adjacent bedrock. Notice the large rounded boulders here. That shape implies that they have been water worn. But water needs to be very rapidly flowing to move boulders of that size. Because these rounded boulders are made up of a variety of rock types, some of which are foreign to the immediate neighborhood, it implies that they have been carried here by glaciers which invaded this area ∼20,000 years ago. Now, fortunately, the glaciers are gone. Could they return some time in our geologic future? This would be a good place to discuss climate change, both natural and anthropogenic. These boulders, because they have come from afar, perhaps several miles away, are termed erratics. In the ground underneath they are mixed with clay, a geologic deposit called till. Till forms much of the surface further northward along the trail.
4850 The rocks of the Shawangunk Formation are cut through here along this old railroad grade (fig.27). The beds are tilted more steeply than before; 62o at this spot. This is a good outcrop to show the different lithologies in the Shawangunk Formation and the cleavage in the shales that dip 85o northwest. It is also a good outcrop to illustrate the various orientations of joints. It would also be an excellent locality to demonstrate the use of a Brunton compass to acquire structural orientations and also to demonstrate the orientation of cleavage in folds (fig. 28) and also to discuss the relationship of cleavage to overturned beds in a fold.
4950 Trail ends at bridge abutment, part of the Lackawanna Railroad bridge that was removed during the flood of 1955 (Fig. 29). Excellent blocks of conglomerate at the abutment.