The Schuylkill River between Phoenixville and Norristown makes several remarkable direction changes including two complete U-turns just upstream from Valley Forge National Historical Park. Other interesting Schuylkill River U-turns or perhaps incised meanders are found between Pottstown and Phoenixville, although this essay will focus on the U-turns between Phoenixville and Valley Forge National Historical Park. Specifically the Schuylkill River flows in a southeast direction from Phoenixville to the mouth of east and north oriented Pickering Creek and then turns to flow in an east-northeast direction before turning in a southeast and south direction to be joined by south and southwest oriented Perkiomen Creek. After Perkiomen Creek joins it the Schuylkill River then flows in a southwest direction before turning in a south-southeast direction to the mouth of north oriented Valley Creek where the Schuylkill River then turns to flow in an east-northeast direction through and adjacent to Valley Forge National Historical Park.
Figure 1: Schuylkill River elbows of capture upstream from Valley Forge National Historical Park. See text for details. United States Geological Survey topographic map digitally presented using National Geographic TOPO software.
Figure 1 illustrates the Schuylkill River U-turns upstream from Valley Forge National Historical Park. Phoenixville is in the map northwest corner and the Schuylkill River flows in a southeast direction from Phoenixville to the mouth of east and north oriented Pickering Creek (location 1) and then turns to flow in an east-northeast direction to location 2. At location 2 the Schuylkill River turns to flow in a southeast and south direction to location 3 where it is joined by south and southwest oriented Perkiomen Creek. From location 3 the Schuylkill River flows in a southwest direction and then in a south-southeast direction to location 4 where it is joined by north oriented Valley Creek, which has eroded the water gap between Mount Misery (west) and Mount Joy (east) found in Valley Forge National Historical Park. From location 4 the Schuylkill River flows in an east-northeast direction to the figure 1 east edge and east of figure 1 it turns to flow primarily in a southeast direction to reach the Delaware River in Philadelphia.
To understand how this S-shaped section of the Schuylkill River valley was eroded we need to reconstruct how the region looked before the erosion of 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 the southwest oriented segments of the newly eroded Delaware River valley to capture southwest oriented flood flow moving north of the newly eroded and deep southwest-oriented Delaware River valley.
Important to the erosion of the Schuylkill River valley S-shaped segment, but not seen in figure 1, was headward erosion of the south oriented Brandywine Creek valley from what was the actively eroding Delaware River valley head. Brandywine Creek valley (and its tributary valley) headward erosion was primarily across southwest oriented flood flow and captured the southwest oriented flood flow so as to divert the floodwaters to the newly eroded Delaware River valley. Brandywine Creek and its East Branch (and other Brandywine Creek tributary valleys) eroded headward across what is now the Chester Valley, which is just south of figure 1. The Chester Valley is today a 1-3 mile wide east-northeast oriented valley underlain by easily eroded carbonate rocks and surrounded by more erosion resistant quartzite, gneiss, schist, and phyllite ridges and hills, although at that time the Chester Valley did not exist as a topographic valley. Mount Misery and Mount Joy are located at the east-northeast end of the North Valley Hills, which today forms the Chester Valley north margin, although the Pennsylvania Geologic Survey web applications map suggests the two are composed of slightly different bedrock, suggesting the possibility of a more easily eroded zone where the two bedrock units come together.
The deep southwest-oriented Delaware River valley eroded headward in a northeast direction from the Wilmington, Delaware area to the Philadelphia area and then to the Trenton, New Jersey area along major southwest oriented flood flow channels, meaning headward erosion of the deep south-oriented Brandywine Creek valley began before the actively eroding southwest-oriented oriented Delaware River valley reached Philadelphia where headward erosion of the deep southeast-oriented Schuylkill River valley began. In other words headward erosion of deep Brandywine Creek tributary valleys reached and crossed the present day Chester Valley before headward erosion of the deep southeast-oriented Schuylkill River valley reached and crossed the Chester Valley.
Up until the time the deep south-oriented Brandywine Creek tributary valleys reached the Chester Valley the southwest oriented floodwaters were unable to erode the easily eroded Chester Valley bedrock 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 east-northeast Chester Valley orientation being determined by carbonate bedrock orientation and the orientation of more erosion resistant bedrock units that today underlie the bordering North and South Valley Hills. In other words, the Chester Valley south of figure 1 was eroded headward in an east-northeast direction by southwest oriented flood flow that was moving to the newly eroded and deep south-oriented East Branch Brandywine Creek valley (west of figure 1).
Headward erosion of the Chester Valley significantly lowered base level south of figure 1 and south-oriented valleys eroded headward from the newly eroded Chester Valley to capture southwest oriented flood flow channels located in the figure 1 map area (although on an upland surface that no longer exists). The present day north oriented Valley Creek valley between Mount Misery and Mount Joy (south of location 4) was eroded headward as a south oriented valley to capture a southwest oriented flood flow channel on the present day east-northeast oriented Schuylkill River alignment downstream from location 4. That south oriented valley continued to erode headward in a north direction along the present day south-southeast oriented Schuylkill River valley alignment upstream from location 4 and captured another southwest oriented flood flow channel on the present day southwest oriented Perkiomen Creek and Schuylkill River alignment at location 3. The southeast and south oriented Schuylkill River valley segment between locations 2 and 3 was probably eroded headward to capture a southwest oriented flood flow channel on the present day alignment of the northeast oriented Schuylkill River valley segment between locations 1 and 2. Floodwaters from these captured flood flow channels moved in a south direction between Mount Misery and Mount Joy to the Chester Valley and then in a west-southwest direction to the south oriented East Branch Brandywine Creek valley.
While the south-oriented Brandywine Creek-East Branch Brandywine Creek valley was eroding headward into and along the Chester Valley and then between Mount Misery and Mount Joy to capture southwest oriented flood flow channels at locations 4, 3, and 2 the southwest-oriented Delaware River valley was eroding headward to Philadelphia, which enabled the southeast-oriented Schuylkill River valley to begin to erode headward toward the Valley Forge National Historical Park area. Shortly after the East Branch Brandywine Creek captured the southwest oriented flood flow channels at locations 4, 3, and 2 headward erosion of the much deeper southeast-oriented Schuylkill River valley beheaded the southwest-oriented flood flow channel on the alignment of Schuylkill River valley segment downstream from location 4, which had been captured by headward erosion of the south-oriented East Branch Brandywine Creek via the water gap between Mount Misery and Mont Joy. Floodwaters on the east-northeast end of the beheaded flood flow channel reversed their flow direction to move to the much deeper Schuylkill River valley.
The reversal of flow also reversed flow in the water gap between Mount Misery and Mount Joy and on a segment of the west-southwest oriented flood flow channel in the Chester Valley and created what is today east-northeast and north oriented Valley Creek, which joins the Schuylkill River at location 4. Needless to say the reversal of flow also enabled the deep Schuylkill River valley to erode headward in a north direction along the captured south oriented valley from location 4 to locations 3, 2, and 1, with headward erosion between locations 2 and 1 also being along a reversed flood flow channel. The east and north oriented Pickering Creek drainage route was also developed by that same reversal of flow and for a time headward erosion of the deep east-oriented Pickering Creek valley captured southwest oriented flood flow moving in channels north and west of the Schuylkill River valley. However headward erosion of the present day southeast-oriented Schuylkill River valley upstream from location 1 eventually beheaded all southwest oriented flood flow routes to the newly eroded Pickering Creek valley.