Figure 1: Chester Valley segment between Valley Creek and East Brandywine Creek showing East Brandy Creek and Valley Creek valleys in the South Valley Hills. United States Geological Survey map digitally presented using National Geographic TOPO software.
One the Philadelphia region’s most easily recognized landforms is the Chester Valley, which extends in an east-northeast direction through Coatesville and Dowington to the King of Prussia area and further. Figure 1 illustrates a short Chester Valley segment near Downington and also illustrates a puzzling drainage history problem presented at several Chester Valley locations. The East Branch Brandywine Creek originates in the hills north of the Chester Valley and flows in a south direction to enter the Chester Valley at location 1 and then flows across the Chester Valley at location 2 to enter a narrow valley cut into South Valley Hills at location 3 and then to flow to location 4 where it is joined by south oriented Valley Creek. Valley Creek originates in the Chester Valley north and east of figure 1 and flows in a southwest direction to locations 5 and 6 before turning to flow in a narrow valley carved in South Valley Hills before joining East Brandywine Creek at location 4. To make the situation even more puzzling a Valley Creek tributary originates in the North Valley Hills and flows in a south direction to enter the Chester Valley at location 7 and then flows across the Chester Valley to location 6 where it joins Valley Creek and enters the narrow valley carved into the South Valley Hills.
What makes this drainage pattern puzzling is the drainage divide between Valley Creek and East Branch Brandywine Creek in the Chester Valley is much lower in elevation than elevations of the South Valley Hills through which both East Brandywine Creek and Valley Creek have carved narrow valleys. Further, the Chester Valley provides low elevation routes to other drainage systems both east and west of figure 1. Even more puzzling is the fact the Chester Valley is underlain by easily eroded limestone and dolomite bedrock units while less easily eroded bedrock underlies the South Valley Hills. Why did the East Branch Brandywine Creek and Valley Creek both ignore the Chester Valley low elevation routes and carve narrow valleys across the South Valley Hills?
The solution to this puzzling problem requires going back to a time when the Chester Valley, the East Brandywine Creek valley and the Valley Creek valley did not exist. At that time the entire region was at least as high or higher than the highest South Valley Hills elevations today and massive and prolonged southwest oriented floods were flowing across that high level surface. Floodwaters flowed initially in shallow diverging and converging channels with water spilling freely between the channels. Because there was no lower base level nearby floodwaters moving over the easily eroded Chester Valley bedrock were not able to erode a deep valley and were flowing at approximately the same elevation as floodwaters that were flowing across the South Valley Hills.
Deep erosion began when the deep East Brandywine Creek valley eroded headward into the region to capture the southwest oriented flood flow. Headward erosion of the deep East Brandywine Creek valley first captured southwest oriented flood flow channels crossing the South Valley Hills. Evidence of such a capture can be seen just south of Harmony Hill where a southwest-to-northeast dry valley crosses the Valley Creek-East Brandywine Creek drainage divide. Projecting that dry valley’s alignment in a southwest direction leads to a northeast oriented East Brandywine Creek tributary valley. The northeast oriented tributary was created by a reversal of flood flow on the northeast end of the beheaded flood flow channel. Because flood flow channels diverged and converged and because floodwaters moved freely between flood flow channels the reversed flow was able to capture significant yet to be beheaded flood flow from north of the actively eroding East Brandywine Creek valley head and that captured flood flow helped erode the deep northeast oriented tributary valley seen today. Remember at the time the channel south of Harmony Hill was captured the actively eroding and deep East Brandywine Creek valley had yet to erode headward into what is now the Chester Valley.
When the deep East Brandywine Creek valley eroded headward into the present day Chester Valley it captured west-southwest oriented flood flow that had begun to erode a deep west-southwest oriented channel headward from the deep West Brandywine Creek valley (which had previously eroded headward across the Chester Valley west of figure 1). Floodwaters on the east-northeast end of the beheaded flood flow channel reversed their direction and flowed to the deep East Brandywine Creek valley and created an east-northeast oriented stream (Beaver Creek and its east-northeast tributary Valley Run) flowing to south oriented East Brandywine Creek at Dowingtown as a barbed tributary. At the same time southwest oriented floodwaters were flowing across the North Valley Hills and the deep East Brandywine Creek valley was able to erode its valley headward into the North Valley Hills to capture that flood flow (the East Brandywine Creek valley in the North Hills may have been initiated by floodwaters flowing to the west-southwest oriented flood flow channel eroded headward from the West Brandywine Creek valley prior to that channel’s capture by East Brandywine Creek valley headward erosion).
While the deep East Brandywine Creek valley was eroding headward into and across the present day Chester Valley the south oriented Valley Creek valley was eroding headward from location 4 to capture southwest oriented floodwaters moving to the newly eroded East Brandywine Creek valley. Headward erosion of the deep Valley Creek valley into what is now the Chester Valley captured southwest oriented flood flow moving to the newly eroded East Brandywine Creek valley and created the present day drainage divide between Valley Creek and East Brandywine Creek in the Chester Valley. Valley Creek valley headward erosion also captured flow from south oriented valleys that had eroded headward into the North Valley Hills to capture southwest oriented floodwaters north of the Chester Valley location. West-southwest oriented flood flow in the Chester Valley by this time had eroded a deep valley, but was soon beheaded and reversed by headward erosion of the deep Schuylkill River valley east of figure 1 (ending all flood flow to the newly eroded Valley Creek valley).