Chester Valley between Coatesville and Downingtown

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The Chester Valley between Coatesville and Dowingtown is an east-northeast oriented through valley linking south oriented West Branch Brandywine Creek with south oriented East Branch Brandywine Creek. West Branch Brandywine Creek flows from the 500-600 foot high North Valley Hills (north of the Chester Valley) to enter and cross the less than 350-foot high Chester Valley and then enters a narrow valley carved in the 500-600 foot high South Valley Hills (south of the Chester Valley. East Branch Brandywine Creek flows in a south-southeast and southwest direction from the 500 to 600 foot high North Valley Hills (north of the Chester Valley) to enter and cross the Chester Valley and then enters a narrow valley carved in the 500-600 foot high South Valley Hills (south of the Chester Valley). Yet the West Branch-East Branch drainage divide elevation in the Chester Valley is only slightly higher than 350 feet (at location 3 in figure 1). Why did these two south oriented Brandywine Creek branches ignore the low elevation Chester Valley route and erode deep valleys into and across the South Valley Hills?

Chestervaldet

Figure 1: Chester Valley segment between Coatesville and Downtown. West Branch Brandywine Creek flows across the Chester Valley at location 1 and East Branch Brandywine Creek flows across the Chester Valley at location 2. The West Branch-East Branch drainage divide is near location 3. United States Geological Survey map digitally presented using National Geographic TOPO software. 

The Chester Valley, which extends in an east-northeast direction through Coatesville and Dowington to the King of Prussia area and further, is underlain by relatively easy to erode limestone and dolomite bedrock while the surrounding North and South Hills are underlain by more erosion resistant schist and gneiss. Figure 1 illustrates the Chester Valley segment between Coatesville and Downington and illustrates where the south oriented West and East Branches of Brandywine Creek flow from the North Valley Hills to enter and cross the east-northeast oriented Chester Valley before entering narrow valleys carved in the South Valley Hills. The West Branch Brandywine Creek flows in a generally south oriented direction from near the figure 1 northwest corner to Coatesville in the Chester Valley (at location 1) and then enters a narrow southeast oriented valley to reach the figure 1 south edge. The East Branch Brandywine Creek originates in the hills north of the figure 1 northeast corner and flows in a south direction to reach Dowingtown in the Chester Valley (at location 2) and then enters a narrow south-southeast oriented valley cut into South Valley Hills. South of figure 1 the West and East Branches meet and form south oriented Brandywine Creek. Evidence seen in figure 1 poses a puzzling problem. How could running water erode the two south oriented Brandywine Creek branch valleys 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 East Brandywine Creek 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 South Valley Hills elevations today and massive and prolonged southwest oriented floods flowed across that high level surface. Floodwaters flowed initially in shallow diverging and converging channels with water spilling freely between the channels. Evidence for these diverging and converging southwest oriented flood flow channels can be seen in the southwest-to-northeast oriented valleys crossing drainage divides today (these show up especially well in the southeast quadrant of figure 1). 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 was flowing at the same elevation as floodwaters flowing across the North and South Valley Hills.

Deep erosion began when the deep West Brandywine Creek valley eroded headward into the region to capture the southwest oriented flood flow. Headward erosion of the deep West Brandywine Creek valley first captured southwest oriented flood flow channels crossing the South Valley Hills. When the deep West Brandywine Creek valley eroded headward into the present day Chester Valley it captured west-southwest oriented flood flow that was moving across the more easily eroded bedrock underlying the Chester Valley. By providing a much 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 deep south-southeast oriented East Branch Brandywine Creek valley beheaded all southwest oriented flood flow channels across the South Valley Hills and erode headward into and across the Chester Valley. The south oriented East 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 West Branch Brandywine Creek valley and the East Brandywine Creek valley captured all flow in the Chester Valley. Further floodwaters on the east-northeast end of the beheaded flood flow channel reversed 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. The reversed flow was also supplemented by floodwaters spilling out of the West Branch Brandywine Creek valley and this additional flood flow helped create the east-northeast oriented Valley Run-Beaver Creek drainage route on the Chester Valley floor.

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