After flowing in a southwest, south, south-southeast, and east direction Wissahickon Creek turns at its Gynedd Valley Elbow of Capture to flow in a south direction through its Gynedd Valley Water Gap. Southwest and west-southwest oriented Trewellyn Creek joins the east oriented Wissahickon Creek segment at the Gynedd Valley Elbow of Capture. The Gynedd Valley Water Gap is not as spectacular as many water gaps elsewhere in Pennsylvania, although it has served as an important transportation route since the mid 19th century and today the SEPTA Lansdale Regional Rail line follows Wissahickon Creek through the Gynedd Valley Water Gap.
Figure 1: Wissahickon Creek turns from flowing in an east direction to flowing in a south direction at the Gynedd Valley Elbow of Capture and the south oriented valley is located in a water gap that cuts across a west-southwest to east-northeast oriented ridge. United States Geological Survey map digitally presented using National Geographic TOPO software.
The east-oriented Wissahickon Creek valley segment, the west-southwest oriented Trewellyn Creek valley, and the southwest oriented Willow Run valley (which drains to Wissahickon Creek at Penllyn just south of the water gap) help define the ridge through which the Gynedd Valley Water Gap has been eroded. A 20-40 foot deep through valley links the east oriented Wissahickon Creek valley segment north of the ridge with a southwest oriented Stony Creek valley segment and also helps define the ridge. Stony Creek flows in a southwest and south direction to join the southeast oriented Schuylkill River at Norristown. A southwest oriented Stony Creek tributary valley originating near Montgomery County Community College helps define the ridge southwest flank.
Figure 2: Wissahickon Creek flows in a south direction at location 1 and then turns to flow in an east direction at location 4. A through valley at location 2 links the southwest oriented Stony Creek valley with the Wissahickon Creek valley and the valley of a southwest oriented Wissahickon Creek tributary. Note ridge at Franklinville. United States Geological Survey map digitally presented using National Geographic TOPO software.
Wissahickon Creek crosses the 240-foot contour line as it flows through the water gap. To the west the ridge rises to more than 360 feet in the Franklinville area and to the east the ridge rises to more than 340 feet near Gynedd Valley, although further to the east the ridge rises to more than 380 feet. The ridge is probably composed of more erosion resistant bedrock than bedrock underlying the adjacent valleys, although the southwest oriented valleys also provide evidence of what were once diverging and converging channels in a southwest oriented flood formed anastomosing channel complex that crossed the region prior to Wissahickon Creek valley headward erosion. Just prior to Wissahickon Creek valley headward erosion into the Gynedd Valley region headward erosion of the Stony Creek valley to the west captured the southwest oriented flow and diverted water in more of a south direction to the newly eroded Schuylkill River valley. Wissahickon Creek valley headward erosion then captured the southwest oriented flow and diverted the water more directly to the newly eroded Schuylkill River.
The question might be asked, if the southeast oriented Schuylkill River valley was eroding headward across southwest oriented flood flow and the Wissahickon Creek and Stony Creek valleys eroded headward from that actively eroding southeast oriented Schuylkill River valley to capture that same southwest oriented flow and divert the water more directly to the Schuylkill River valley then how was the Stony Creek valley able to reach the Gynedd Valley region before the Wissahickon Creek valley, which would have begun eroding headward first? The answer is the Wissahickon Creek valley head had to erode a seven mile long gorge through erosion resistant bedrock while the Stony Creek valley originated north of the erosion resistant bedrock and was able to erode headward much faster. However, being east of the Stony Creek valley the Wissahickon Creek valley headward erosion was able to capture all of the southwest oriented flow that had been moving to the newly eroded Stony Creek valley.