Pequea Creek originates on the Welsh Mountain southeast flank and flows in a southeast direction before turning to flow in a west and southwest direction to eventually join the Susquehanna River. The West Branch Brandywine Creek also originates along the Welsh Mountain southeast flank, although east of the Pequea Creek headwaters, and flows in a southeast and south direction with water eventually reaching the Delaware River. The drainage divide between Pequea Creek and the West Branch Brandywine Creek is also the drainage divide between the Susquehanna River and Delaware River and provides significant clues as the origins of the two larger river drainage basins.
Figure 1: West Branch Brandywine Creek-Pequea Creek drainage divide area between Welsh Mountain and the Baron Hills. United States Geological Survey map digitally presented using National Geographic TOPO software.
Figure 1 illustrates the drainage divide area between Pequea Creek and West Branch Brandywine Creek south of Welsh Mountain. Pequea Creek flows in a southeast and south direction near the figure 1 west edge and south of figure 1 turns to flow in a west and southwest direction. The West Branch Brandywine Creek flows in a southeast direction across figure 1 northeast quadrant. Welsh Mountain is a quartzite ridge with elevations generally exceeding 900 feet and in places exceeding 1000 feet. Hills south of location 3 are also a quartzite ridge and reach an elevation of more than 900 feet (just south of location 3) and are included in the Baron Hills. Elevations along the drainage divide between Pequea Creek and the West Branch Brandywine Creek between Welsh Mountain and the Baron Hills are less than 800 feet and drop to less than 710 feet (just south of Cambridge). Bedrock between Welsh Mountain and the Baron Hills is shown on the Pennsylvania Geologic Survey web applications map as being primarily various types of gneisses, which are also usually erosion-resistant rocks. If so, then what eroded the 200-foot deep through valley between Welsh Mountain and the Baron Hills?
At one time the entire figure 1 map region was at least as high or higher than the present day Welsh Mountain crest (more than 1000 feet, although the region probably had a different elevation related to sea level at that time). At that time there was no 200-foot deep or deeper valley between the present day Welsh Mountain and the Baron Hills. Instead massive and prolonged southwest oriented floods from north and east of the present day Schuylkill and Delaware River valleys moved across the region to what at that time was the actively eroding and deep west and southwest oriented Pequea Creek valley. The Pequea Creek valley eroded headward from what was at that time probably a newly eroded south-oriented Susquehanna River valley and had the advantage of eroding headward across relatively easy to erode carbonate bedrock located west of figure 1.
Why massive and prolonged southwest oriented floods? Local drainage routes would not be capable of removing the volume of erosion resistant material that once occupied the region between Welsh Mountain and the Baron Hills. Not only was that material removed, the entire Pequea Creek drainage basin south and west of figure 1 was eroded. Further, similar erosion was occurring north and west of Welsh Mountain and south and east of the Baron Hills. In other words volumes of southwest oriented flow had to be great enough to deeply erode the entire region surrounding what are today high ridges of the region’s most erosion resistant bedrock. Such water movements suggest floodwaters moved in a large-scale anastomosing channel complex that was being captured by headward erosion of the deep south-oriented Susquehanna River valley to the west.
Looking at figure 1 there is evidence that multiple southwest-oriented flood flow channels crossed the present day drainage divide between West Branch Brandywine Creek and Pequea Creek. The deepest channel was located south of Cambridge as already mentioned. Another deep channel was located at location 3. Shallower channels were present at locations 1 and 2 and evidence for still shallower channels also exists. Probably as deeper channels were eroded they captured flow from the shallower channels in what was a constantly changing complex of diverging and converging flood flow channels. The southeast-oriented Pequea Creek headwaters valley seen in figure 1 was eroded headward across the southwest oriented flood flow channels and southwest oriented tributaries provide evidence of the capture of multiple southwest oriented flow channels.
Headward erosion of the West Branch Brandywine Creek valley across the southwest oriented flood flow captured the flow and diverted floodwaters to what at that time was probably a newly eroded Delaware River valley and also ended flood flow to what had been the actively eroding Pequea Creek drainage basin. The southwest, south-southeast, and southeast oriented West Brandywine Creek tributary valley located just east of Cambridge was probably eroded by floodwaters during the West Brandywine Creek valley capture event. Headward erosion of the East Branch Brandywine Creek valley (east of figure 1) subsequently captured the southwest oriented flood flow and diverted floodwaters more directly to the Brandywine Creek valley. Still later headward erosion of the southeast oriented Schuylkill River valley captured the flood flow and diverted the floodwaters even more directly to the newly eroded Delaware River. Floodwaters were subsequently captured by headward erosion of the Wissahickon Creek valley and later by the Neshaminy Creek valley before the Delaware River valley eroded headward along the present Pennsylvania-New Jersey border to capture all of the flood flow.