The Chester Valley is a narrow lowland extending from near the southeast oriented Susquehanna River valley in an east-northeast direction almost to the southeast oriented Schuylkill River valley. The Pennsylvania Geologic Survey web applications map shows the Chester Valley to be underlain by easily eroded carbonate rocks. The Chester Valley north rim (North Valley Hills) rises 200-300 feet above the valley floor and is composed of erosion resistant quartzite with erosion resistant gneiss located further to the north. The south rim (South Valley Hills) also rises 200-300 feet above the Chester Valley floor and is underlain by more erosion resistant phyllite, schist, gneiss, and granite bedrock. The Chester Valley lowland is today a through valley and crosses multiple drainage divides including the drainage divides between the Susquehanna and Delaware Rivers and the drainage divide between the Delaware and Schuylkill Rivers.
Figure 1: Chester Valley segment containing the drainage divide between the Delaware and Schuylkill Rivers. Number 1s show drainage routes flowing to east-northeast oriented Valley Creek and the Schuylkill River. Number 2s dhow drainage routes flowing to west-southwest oriented Valley Creek and the Delaware River. United States Geological Survey map digitally presented using National Geographic TOPO software.
Figure 1 illustrates the Chester Valley segment crossing the drainage divide between the Delaware and the Schuylkill Rivers. Exton is located near the figure 1 southeast corner and Malvern is located in the South Valley Hills near the figure 1 east edge. An east-northeast oriented Valley Creek (headwaters are marked with the number 1) drains the east half of this Chester Valley segment and water eventually reaches the Schuylkill River. A west-southwest oriented Valley Creek (headwaters are marked with the number 2) drains the west half of this Chester Valley segment and water eventually reaches the Delaware River. As seen in figure 1 both Valley Creeks have headwaters adjacent to each other in the South Valley Hills that flow in north directions to enter the Chester Valley where they diverge to flow in different directions. The low point on the drainage divide between the two opposing Valley Creeks has an elevation of between 370 and 380 feet while the North Valley Hills and South Valley Hills are 200 feet or more higher.
Water eroded the Chester Valley and the question can be asked, how did water erode the Chester Valley so as to create a drainage divide between two major rivers? To answer that question it is necessary to visualize the figure 1 region as it was just before the Chester Valley erosion began. At that time all elevations in the figure 1 map region were as high or higher than highest elevations in figure 1 today (the region also probably stood higher above sea level than it does today) and the Chester Valley did not exist and the Delaware River and the Schuylkill River valleys did not exist either. At that time massive and prolonged southwest oriented floods (perhaps melt water flowing from a large continental ice sheet) flowed across what is today southeast Pennsylvania and the deep Delaware River valley began to erode headward along a southwest oriented flood flow channel from what is today Delaware Bay to the Philadelphia region. Deep south-oriented tributary valleys then erode headward from the newly eroded southwest oriented Delaware River valley and captured southwest oriented flood flow moving north and west of the actively eroding Delaware River valley head.
West of the figure 1 map area the south oriented East Branch Brandywine Creek valley eroded headward across what is now the Chester Valley and into what are now the North Valley Hills. This newly eroded and deep East Branch Brandywine Creek valley provided a significantly lower base level, which meant the southwest oriented flood flow could then erode deeper valleys along easily eroded bedrock units. The most easily eroded regional bedrock units were located where the Chester Valley is now located and the southwest oriented floodwaters were able to significantly lower the Chester Valley floor (other south oriented Susquehanna and Delaware River tributary valleys eroded headward across the present day Chester Valley further to the west prior to the East Branch Brandywine Creek headward erosion and had probably already begun the Chester Valley erosion process). At the same time floodwaters were able to create shallower west-southwest oriented channels between ridges in the North and South Valley Hills and floodwater from some of those parallel channels spilled into the deeper Chester Valley and eroded what are today water gaps used by minor streams entering the Chester Valley.
As the deep Delaware River valley eroded headward to where Philadelphia now is located the deep southeast oriented Schuylkill River valley eroded headward from the actively eroding southwest-oriented Delaware River valley to capture more of the southwest oriented flood flow. Headward erosion of that deep and southeast-oriented Schuylkill River valley eventually captured the southwest oriented flood flow moving into the newly eroded and west-southwest oriented Chester Valley. The Schuylkill River valley was much deeper than the west-southwest oriented Chester Valley flood flow channel and water on the east-northeast end of the Chester Valley flood flow channel reversed direction to flow into the deeper Schuylkill River valley (there are some additional complications not seen in figure 1 and not addressed here). The reversal of flood flow in the Chester Valley created the drainage divide between the west-southwest oriented Valley Creek and the east-northeast oriented Valley Creek seen in figure 1.