Without question the deepest, longest, and most magnificent Wissahickon Creek water gap is the seven-mile long Wiisahickon Gorge eroded across a 420-foot high ridge of tightly folded metamorphic and igneous rocks and which is located entirely within the Philadelphia city limits. The erosion resistant ridge extends in an east-northeast direction across southeast Pennsylvania and is also crossed by other drainage routes including the Schuylkill River , which has eroded a narrow valley or gorge across this ridge extending from Conshohocken to near the Philadelphia city center (downstream from where Wissahickon Creek joins the Schuylkill River).
Figure 1: The Wissahickon Gorge extends in a south-southeast direction across map center and turns in a southwest direction to join the southeast oriented Schuylkill River Gorge. Note southwest and northeast orientation of Wissahickon and Schuylkill Gorge tributaries. The Andorra Wind Gap is at location 1 and has an elevation of 266 feet. The Marble Hall Through Valley is at location 2 and has an elevation of less than 220 feet. Elevations at locations 3 and 4 on either side of the Wissahickon Gorge north entrance exceed 420 feet. United States Geological Survey map digitally presented using National Geographic TOPO software.
The Whitemarsh Valley is located upstream or north on the south oriented Wissahickon Creek route from the metamorphic and igneous rock ridge and is underlain by much less erosion resistant carbonate rocks and provides through valleys linking the Wissahickon Creek valley with Spring Mill Creek, which drains to Schuylkill River at Spring Mill, and Plymouth Creek, which drains to the Schuylkill River near Conshohocken. Floors of these through valleys are at least 200 feet lower than metamorphic and igneous rock ridge crest elevation. Further upstream in the Wissahickon Creek drainage basin there are additional through valleys with floor elevations lower than the metamorphic and igneous rock ridge crest elevation and those other through valleys also link the Wissahickon Creek valley with other Schuylkill River tributaries. East of the Wissahickon Creek valley, and also upstream from the gorge, are through valleys with floor elevations of approximately 350 feet linking the Wissahickon Creek valley with the south oriented Tookany-Tacony-Frankford Creek valley, which drains to the Delaware River, and also through valleys with elevations only slightly higher than 300 feet linking the Wissahickon Creek valley with south oriented Pennypack Creek, which also flows to the Delaware River.
Wissahickon Creek flows at an elevation of about 120 feet where it enters the Wissahickon Gorge near Chestnut Hill College and flows in a south-southeast direction through the gorge’s northern six miles and then turns to flow in a southwest direction for the final mile to reach the southeast oriented Schuylkill River. The gorge is approximately 300 feet deep near its northern entrance although that depth decreases in a downstream direction. Throughout the Wissahickon Gorge outcrops of the erosion resistant bedrock can be seen along valley walls, on the Wissahickon Creek channel floor, and in valleys of tributary streams entering the gorge. Tributary valleys in the gorge almost always originate on the high ridge crest. Tributaries from the east are almost always oriented in a southwest direction while some, but not all tributaries from west are oriented in northeast directions and join Wissahickon Creek as barbed tributaries. Tributaries to the southeast oriented Schuylkill River Gorge, which was cut through the same erosion resistant rock, also show a pronounced northeast to southwest orientation and many join the Schuylkill River as barbed tributaries.
Perhaps the most important question that needs to be answered is how was Wissahickon Creek, which today has a relatively small drainage basin, able to carve a seven-mile long gorge in erosion resistant rock when lower routes exist to the east and much lower and easier to erode routes exist to the west? To answer this question we need to recognize that Wissahickon Creek drainage basin erosion began with the Wissahickon Gorge because it is eroded across some of the highest elevations and into some of the most erosion resistant bedrock found in the Wissahickon Creek drainage basin. As seen in figure 1 elevations on either side of the gorge north entrance exceed 420 feet, which is higher than many elevations along drainage divides separating the Wissahickon Creek drainage basin from drainage basins further to the west and also higher than some elevations along drainage divides to the east. The high elevations surrounding the Wissahickon Gorge entrance mean gorge erosion must have occurred when southwest oriented flood flow was moving across a high-level surface at least as high as the highest elevation surrounding the Wissahickon Gorge today.
The southwest and northeast oriented tributaries flowing to both Wissahickon Creek and the Schuylkill River (see figure 1) provide evidence that when the Wissahickon and Schuylkill gorges were eroded large volumes water were moving in multiple southwest oriented anastomosing channels across the upland now bounding both the Wissahickon Gorge and the Schuylkill Gorge. Schuylkill River valley headward erosion first captured the southwest flow with water on northeast ends of the beheaded channels reversing flow direction to create northeast oriented tributaries. Since some northeast oriented tributary valleys are not trivial the reversed flow in them captured flow from yet to beheaded channels north and west of the eroding Schuylkill River valley head, which suggests the Schuylkill River valley and the Wissahickon Creek valley were being eroded headward across a large (and probably prolonged) southwest oriented floods. If so, the Schuylkill River Gorge erosion occurred slightly in advance of the Wissahickon Creek Gorge headward erosion, which beheaded southwest oriented flow channels moving water to the Schuylkill River Gorge. Schuylkill River Gorge headward erosion proceeded fast enough to progressively capture new sources of southwest oriented flow more rapidly than Wissahickon Creek valley headward erosion beheaded the flow channels.
Once headward erosion of the Schuylkill River Gorge reached the easily eroded bedrock north and west of the erosion resistant metamorphic and igneous rock material south oriented tributary valleys eroded headward from the newly eroded Schuylkill River valley to capture southwest oriented flood flow that was moving across the present day Whitemarsh Valley area (although on a high-level surface that no longer exists). These captures caused the southwest oriented flood flow to erode in sequence what are described on this web site as the Andorra Wind Gap, the Marble Hall Through Valley, and the Whitemarsh Through Valley. These through valleys were eroded as the Wissahickon Creek valley head broke through the erosion resistant bedrock barrier and began to enter the present day Whitemarsh Valley region. Headward erosion of the deeper Wissahickon Creek valley head then captured in sequence from south to north the southwest and west oriented flow moving through the newly eroded Andorra Wind Gap, the Marble Hall Through Valley, and the Whitemarsh Through Valley. Similar captures of southwest oriented flow channels occurred as the Wissahickon Creek valley continued to erode headward in a north direction. Flood flow to the Wissahickon Creek drainage basin ended when headward erosion of the Neshaminy Creek valley from what at that time was a newly eroded southwest oriented Delaware River valley segment captured all southwest oriented flow channels moving water to what had been the actively eroding Wissahickon Creek drainage basin.