Chester Valley between Buck Run and West Branch Brandywine Creek

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The Chester Valley between Parkesburg and Coatesville is an east-northeast oriented segment of a much longer through valley and includes the drainage divide between south oriented Buck Run at Pomeroy and south oriented West Branch Brandywine Creek at Coatesville. West Branch Brandywine Creek flows from the 600-700 foot high North Valley Hills (north of the Chester Valley) to enter and cross the 300-400 foot high Chester Valley and then enters a narrow valley carved in the 600-foot and higher South Valley Hills (south of the Chester Valley. Buck Run flows in a southeast direction from the 600-700 foot high North Valley Hills (north of the Chester Valley) to enter and cross the 400-500 foot high Chester Valley and then enter a narrow southeast oriented valley carved in the 600-foot and higher South Valley Hills (south of the Chester Valley). Yet the Buck Run-West Branch Brandywine Creek drainage divide elevation in the Chester Valley is less than 500 feet. Other south oriented streams also cross the deep Chester Valley, although this essay is focused on Buck Run and West Branch Brandywine Creek. Why did these two south oriented streams ignore the lower elevation Chester Valley and erode deep valleys into and across the South Valley Hills?

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The Chester Valley, which extends in an east-northeast direction through Coatesville to the King of Prussia area and further and also in a west-southwest direction from Parkesburg, is underlain by relatively easy to erode limestone and dolomite bedrock while the surrounding North and South Hills are underlain by more erosion resistant rock types. Figure 1 illustrates the Chester Valley segment between Parkesburg and Coatesville and shows where south oriented West Branch of Brandywine Creek flows from the North Valley Hills in the map northeast quadrant to enter and cross the east-northeast oriented Chester Valley at Coatesville before entering a narrow valley carved in the South Valley Hills. Buck Run originates in the map northwest quadrant and flows in a southeast oriented direction to Pomeroy in the Chester Valley and then enters a narrow southeast oriented valley to reach the figure 1 south edge. South and east of figure 1 after flowing in southeast and northeast directions Buck Run eventually joins the West Branch Brandywine Creek, which also flows in southeast and northeast directions. Evidence seen in figure 1 and elsewhere poses a puzzling problem. How could running water erode what are today two south oriented West Branch Brandywine Creek related valleys across the Chester Valley and also erode the east-northeast oriented Chester Valley?

The solution to this puzzling problem requires going back to a time when the Chester Valley, the Buck Run valley, and the West Branch Brandywine Creek valley did not exist. At that time the entire region including the Chester Valley was at least as high or higher than the highest North and South Valley Hills elevations today and massive and prolonged floods flowed across that high level surface. The flood flow direction cannot be easily determined from evidence in figure 1, although evidence from surrounding areas suggests floodwaters were moving in a southwest direction. Floodwaters probably flowed initially in shallow diverging and converging channels with water spilling freely between the channels. Although bedrock underlying the Chester Valley is more easily eroded than bedrock underlying the North and South Valley Hills the floodwaters were not able to erode the Chester Valley because there was no significantly lower base level nearby and flood flow crossing the present day Chester Valley flowed at the same elevation as floodwaters flowing across the North and South Valley Hills.

Deep erosion began when the Buck Run valley eroded headward into the region to capture the southwest oriented flood flow. Headward erosion of the Buck Run valley first captured southwest oriented flood flow channels crossing the South Valley Hills. When the Buck Run valley eroded headward into the present day Chester Valley it captured southwest oriented flood flow that was moving across the more easily eroded bedrock underlying the Chester Valley. By providing a lower base level that captured southwest oriented flood flow was able to erode a deep west-southwest oriented valley headward along the present day Chester Valley alignment. Next headward erosion of the deeper West Branch Brandywine Creek valley beheaded all southwest oriented flood flow channels across the South Valley Hills and eroded headward into and across the Chester Valley. The south oriented West Brandywine Creek valley was significantly deeper than the west-southwest oriented flood flow channel (in the present day Chester Valley) that had been captured by the newly eroded Buck Run valley and the West Brandywine Creek valley captured all flow in the Chester Valley, beheading the west-southwest oriented flow channel to the newly eroded Buck Run valley. Further floodwaters on the east-northeast end of the beheaded flood flow channel reversed direction and flowed to the deep West Brandywine Creek valley and created an east-northeast oriented stream (now Sucker Run) flowing to south oriented West Brandywine Creek at Coatesville as a barbed tributary. The reversed flow was also supplemented by floodwaters spilling out of the Buck Run valley and this additional flood flow helped erode the east-northeast oriented Sucker Run drainage route on the Chester Valley floor.

What is important about the Buck Run-Sucker Run drainage divide at Pomeroy is floodwaters for a time at least moved in a south direction in the Buck Run valley from the North Valley Hills into the Chester Valley and then diverged with some floodwaters flowing in a southeast direction in the Buck Run valley carved in the South Valley Hills while some of the floodwaters flowed in an east-northeast direction in the Chester Valley to reach the deeper West Branch Brandywine Creek valley. The east-northeast oriented flood flow to the deeper West Branch Brandywine Creek valley would have captured all Buck Run flow at Pomeroy except headward erosion of the West Branch Brandywine Creek valley into the North Valley Hills beheaded all southwest oriented flood flow routes to the Buck Run valley head in the North Valley Hills and ended the south oriented flood flow in the Buck Run valley before the capture at Pomeroy could occur.

 

 

 

 

 

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