The Schuylkill River Gorge, as defined here, extends in a southeast direction for about eight miles from Spring Mill (east of Consohocken) to the Roosevelt Avenue Bridge Elbow of Capture in the City of Philadelphia. While longer than the seven-mile long Wissahickon Creek Gorge the Schuylkill River Gorge is much wider and unlike the Wissahickon Gorge is today a major highway and rail transportation route. Further urban development has encroached on some Schuylkill River Gorge valley walls and also extends into the Schuylkill Gorge valley. For these reasons some observers might question whether the Schuylkill River Gorge should be compared with the Wissahickon Creek Gorge, yet both gorges have been eroded across the same metamorphic and igneous rock complex and were eroded at the same time under the same conditions.
Figure 1: The Schuylkill River Gorge extends from location 1 to location 2. The Wissahickon Gorge extends from north of location 3 to the Schuylkill Gorge. United States Geological Survey map digitally presented using National Geographic TOPO software.
Figure 1 illustrates the Schuylkill River Gorge, which extends from the Spring Mill Elbow of Capture at location 1 to the Roosevelt Avenue Bridge Elbow of Capture at location 2, and also the Wissahickon Creek Gorge, which extends in a south-southeast direction from location 3 before making an abrupt turn to the southwest to reach the Schuylkill River Gorge a short distance upstream from location 2. High points on the upland surface surrounding the two gorges rise to more than 400 feet while the Schuylkill River crosses the 40-foot contour line about one mile downstream from the Spring Mill Elbow of Capture and Wissahickon Creek crosses the 100-foot contour line near location 3. Bedrock underlying the upland surface and found exposed in outcrops along walls of both gorges is a west-southwest to east-northeast oriented band of erosion resistant metamorphic and igneous material known as the Wissahickon Formation.
Both gorges were eroded headward across diverging and converging flow channels such as might be found in a large-scale anastomosing channel complex. At that time all regional elevations were at least as high or higher than the highest elevations on the upland surface today. The deep Schuylkill River valley eroded headward (in a northwest direction) from the Roosevelt Avenue Bridge Elbow of Capture at location 2 across the southwest oriented flood flow and captured the southwest oriented flood flow.
Southwest oriented flood flow moving into the newly eroded and deep southeast oriented Schuylkill River Gorge eroded southwest oriented tributary valleys seen in figure 1. At the same time flood flow on northeast ends of beheaded flood flow channels reversed flow direction to create northeast oriented tributary streams flowing into the newly eroded Schuylkill River Gorge. Because flood flow channels were anastomosing (diverging and converging) reversed flow in newly beheaded flow channels could capture flow from yet to be beheaded flow channels further to the northwest. Such captures provided the water to erode northeast-oriented tributary valleys seen in figure 1. The Spring Mill Elbow of Capture was created when the actively eroding Schuylkill River valley eroded headward in a southwest direction along a beheaded flood flow channel to create the northeast oriented Schuylkill River valley segment upstream from location 1.
Headward erosion of the Wissahickon Creek Gorge began as soon as headward erosion of the deep Schuylkill River valley reached the present day Wissahickon Creek mouth. However the Wissahickon Creek valley first eroded headward in a northeast direction along one of the southwest oriented flood flow channels before turning to erode headward in a north-northwest direction to create the present day south-southeast oriented Wissahickon Gorge segment. As the south-southeast oriented Wissahickon Gorge segment eroded headward it captured southwest oriented flood flow moving to the newly eroded Schuylkill River Gorge and the southwest oriented flood flow eroded southwest oriented tributary valleys into the wall of the newly eroded Wissahickon Gorge as seen in figure 1. Because the Wissahickon Creek valley had initially eroded headward in a northeast direction to create the southwest oriented Wissahickon Gorge segment headward erosion of the Wissahickon Creek valley was sufficiently delayed that it never beheaded flood flow channels to the actively eroding Schuylkill River valley head.
Leave a Reply