The Valley Forge National Historical Park is located south of a northeast oriented Schuylkill River segment and at the east-northeast end of the ridge forming the Chester Valley west-northwest margin. Mount Joy in the Valley Forge Park is where the high ridge ends and west of the Mount Joy the ridge segments are named Mount Misery, Diamond Rock Hill, and the North Valley Hills. Valley Creek flows in a north direction through the water gap eroded between Mount Joy and Mount Misery while wind gaps are located between Mount Misery and Diamond Rock Hill and between Diamond Rock Hill and the North Valley Hills. Also of importance in this region are through valleys on the Chester Valley floor linking Valley Creek with northeast oriented Trout Creek, which flows in a northeast direction to the Schuylkill River at the Chester Valley east end.
Figure 1: Schuylkill River valley and Chester Valley in the Valley Forge National Historical Park area. See text for detailed description. United States Geological Survey map digitally presented using National Geographic TOPO software.
Figure 1 illustrates the Valley Forge National Historical Park region and numbers on the map identify several important locations. Mount Joy is located at location 1, Mount Misery at location 2, Diamond Rock Hill at location 3, and the North Valley Hills at location 4. The Valley Creek water gap between Mount Joy and Mount Misery is found between locations 1 and 2 and the deep wind gap between Diamond Rock Hill and the North Valley Hills is found at location 5. Before turning in a north direction to flow between Mount Joy and Mount Misery Valley Creek flows on the Chester Valley floor in an east-northeast direction at location 6. Little Valley Creek at location 7 also flows in an east-northeast direction on the Chester Valley floor before turning in a northeast direction to join Valley Creek and to flow in a north direction between Mount Joy and Mount Misery. Location 8 identifies northeast oriented Trout Creek, which drains the Chester Valley east-northeast end. Evidence seen in figure 1 raises several questions, one of the most important of which is, why does Valley Creek turn to flow in a north direction between Mount Joy and Mount Misery when the Chester Valley extends in an east-northeast direction to the Trout Creek and Schuylkill River valleys?
To understand how the Valley Creek water gap was eroded we first need to understand the Schuylkill River “S” shaped route seen north and west of the Valley Forge National Historical Park. The Schuylkill River flows from the figure 1 north edge in a south, southeast, and east-northeast direction before making a U-turn to flow in a southwest direction and then makes another U-turn to flow in an east-northeast direction, with an east-southeast jog, to reach the figure 1 east edge. East of figure 1 the Schuylkill River turns to flow in a southeast direction to eventually reach Philadelphia and the southwest oriented Delaware River. The south and southwest oriented stream joining the Schuylkill River southwest oriented segment is Perkiomen Creek and the east oriented stream that turns in a northeast and north direction to join the Schuylkill River in the figure 1 north center area is Pickering Creek (Pickering Creek Reservoir floods the lower valley).
We now need to reconstruct how the region looked before erosion of the Chester Valley, the Schuylkill River valley, and the Schuylkill River tributary valleys seen in figure 1. At that time the entire region was at least as high if not higher than the highest points in figure 1 today and the deep valleys seen in figure 1 did not exist. Sea level at that time was probably significantly lower than it is today and massive and prolonged southwest oriented floods were moving across southeast Pennsylvania. The flood source cannot be determined from southeast Pennsylvania evidence although a logical source would be melt water from a large North American continental ice sheet. Whatever the flood water source the deep Delaware River valley began to erode headward across and along flood flow channels as it captured southwest oriented flood flow and deep south and southeast oriented valleys eroded headward from southwest oriented segments of the newly eroded Delaware River valley, also to further capture the southwest oriented flood flow.
Brandywine Creek and its West and East Branches (and other valleys) eroded headward in a west to east progression from the newly eroded southwest oriented Delaware River valley (at Wilmington, Delaware) across what is now the Chester Valley. Even though easily eroded carbonate rocks underlie the Chester Valley up until the time that deep south-oriented valleys eroded across it the southwest oriented floodwaters were unable to erode it because there was no lower regional base level. However, once deep south-oriented valleys began to erode across the Chester Valley the southwest oriented floodwaters could erode the Chester Valley carbonate bedrock and a west-southwest oriented valley began to develop with that Chester Valley orientation being determined by carbonate bedrock and the more erosion resistant bedrock units that today underlie the North and South Valley Hills. In other words, the Chester Valley was eroded in an east-northeast direction by southwest oriented flood flow that was progressively captured in a west to east sequence by headward erosion of deep south-oriented valleys from the actively eroding southwest-oriented Delaware River valley (and previously from the actively eroding southeast-oriented Susquehanna River valley).
As already mentioned the south and southeast oriented Brandywine Creek valley eroded headward when the actively eroding southwest-oriented Delaware River valley reached the present day location of Wilmington, Delaware and Brandywine Creek tributary valleys subsequently eroded headward in sequence from west to east across the present day Chester Valley. The southeast oriented Schuylkill River valley began to erode headward when the actively eroding southwest-oriented Delaware River valley reached Philadelphia and by the time the actively eroding southeast-oriented Schuylkill River valley reached the present day Chester Valley it encountered a moderately deep west-southwest oriented flood flow channel that was already eroding headward from the newly eroded south-oriented West Brandywine Creek valley (west of figure 1).
South oriented valleys had eroded headward from the actively eroding west-southwest oriented Chester Valley flood flow channel to capture southwest oriented floodwaters moving north of the present day Chester Valley. One such south oriented valley was eroded headward along the alignment of the present day Valley Creek water gap between Mount Joy and Mount Misery and that valley captured southwest oriented flood flow moving along the alignment of the present day east-northeastoriented Schuylkill River valley segment north of Valley Forge National Historical Park. Another such south-oriented valley eroded headward eroded from Cedar Hollow between Diamond Rock Hill and the North Valley Hills and captured southwest oriented flood flow on the present day northeast oriented Pickering Creek alignment. Initially the captured flood flow moved in a south direction into the Chester Valley and then in a west-southwest direction to the south oriented West Branch Brandywine Creek valley and then to the Delaware River.
As the deep southeast-oriented Schuylkill River valley eroded headward across the present day Chester Valley it captured west-southwest oriented flood flow from the northeast but was not able to reverse flow in the Chester Valley itself (probably because at that time gradients at the present day Chester Valley east end were too steep to prevent a major flood flow reversal). However, when Schuylkill River valley headward erosion beheaded the southwest oriented flood flow channel that was supplying floodwaters to the Chester Valley on the present day Trout Creek alignment floodwaters on that channel were reversed to flow in a east-northeast direction to the newly eroded and much deeper Schuylkill River valley. The Schuylkill River then eroded headward along that reversed flood flow channel to create the east-northeast oriented Schuylkill River valley segment near the figure 1 edge while flow at the Chester Valley east end was reversed to create the northeast oriented Trout Creek drainage route seen in figure 1.
Next Schuylkill River valley headward erosion beheaded and reversed a southwest oriented flood flow channel that was supplying floodwaters to the south oriented valley between present day Mount Joy and Mount Misery, which had already captured southwest oriented flood flow on what is now the southwest oriented Perkiomen Creek-Schuylkill River segment to the north. The Schuylkill River then eroded headward in a northwest and northeast direction along the captured flood flow route while flood flow in the Chester Valley was reversed and began to flow in a north direction between Mount Joy and Mount Misery to reach the deeper Schuylkill River valley. Headward erosion of deep Schuylkill River valley then beheaded and reversed the southwest oriented flood flow channel supplying floodwaters to the Chester Valley via the wind gap at location 5. The reversal of flow created the northeast oriented Pickering Creek drainage route and ended southwest oriented flood flow to the Chester Valley segment seen in figure 1.