A shallow, but interesting through valley crosses the Schuylkill River-Delaware River drainage divide at the Main Line community of Strafford and links east oriented Gulph Creek, which flows to the Schuylkill River, with south oriented Darby Creek tributaries, with Darby Creek eventually reaching the Delaware River. The drainage divide elevation at the Strafford through valley is between 420 and 430 feet while elevations to the south rise at least 40 feet higher and to the north elevations rise to more than 530 feet. This through valley is evidence a former water flow route once crossed what is today the drainage divide between two major and different river drainage basins.
Figure 1: Strafford through valley region. United States Geological Survey map digitally presented using National Geographic TOPO software.
Figure 1 illustrates the Strafford through valley area with the through valley extending from location 1 to location 3 and the deepest part of the through valley being between location 1 and location 2. Location 1 is at the head of east oriented Gulp Creek, which east of figure 1 flows in an east-northeast, north, and east-northeast direction to reach the southeast oriented Schuylkill River. Location 2 is at the head of Little Darby Creek, which flows in a south and south-southeast direction to reach Darby Creek just south of figure 1. Darby Creek can be seen in the southwest corner of figure 1 and flows in a southeast and south direction to eventually join the southwest oriented Delaware River. Location 3 is at the head of a southwest and south oriented Darby Creek tributary, which joins Darby Creek at the southwest corner of figure 1. The deeper through valley segment between locations 1 and 2 indicates that through valley segment was still being eroded after flow between location 2 and 3 ended and suggests the through valley was eroded by west oriented flow that had been captured first by headward erosion of the south oriented Darby Creek tributary at location 3 and subsequently by headward erosion of south oriented Little Darby Creek valley.
The east oriented Gulph Creek valley was eroded following a reversal of flow east of location 1 after the Darby Creek drainage basin had been eroded. The depth of the Gulph Creek valley, which was eroded in erosion resistant metamorphic rocks, suggests vast quantities of water were required to erode the valley. Since the Darby Creek and Little Darby Creek valleys were eroded before the flow reversal the water that eroded the Gulph Creek valley could not have been in the present day Darby Creek drainage basin. Nor is it likely that the Gulph Creek drainage basin downstream from location 1 ever generated sufficient water volumes to erode the downstream Gulph Creek valley. The only other water source available would have been to the north of the Gulph Creek headwaters. Evidence for flow from the north can be seen in the form of southeast and south oriented Gulph Creek tributary valleys in figure 1.
Movement of vast quantities of water from the north into the Gulph Creek valley would only be possible if the region was being deeply eroded by massive and prolonged southwest oriented floods that were being captured by headward erosion of the deep Delaware River valley and its tributary Darby Creek and Schuylkill River valleys. Such a situation would result in headward erosion of the Darby Creek valley first followed by headward erosion of the deep Schuylkill River valley (as the Delaware River valley eroded headward), which would have beheaded and reversed flow to the newly eroded Darby Creek valley. Yet to be beheaded floodwaters moving north and west of the actively eroding Schuylkill River valley head could have reached the reversal flow channel on the present day Gulp Creek route before erosion of the Chester Valley (located north of figure 1) and could have eroded the deep Gulph Creek valley as the floodwaters flowed to the much deeper and newly eroded Schuylkill River valley. This process was then repeated to the north of figure 1 as the Chester Valley was eroded so as to behead all flood flow routes to the Gulph Creek valley.
Leave a Reply