Pennypack Creek as seen in figure 1 flows in a southeast direction from location 1 to location 2 with two noticeable northeast oriented turns and two noticeable southwest oriented segments. Tributaries from the east identified by locations 3, 4, 5, and 6 all flow in southwest directions to join south oriented Pennypack Creek segments at normal angles. Tributaries from the west at locations 7 and 8 however flow in northeast directions to join south oriented Pennypack Creek as barbed tributaries while Rockledge Brook (labeled and located between locations 7 and 8) flows in an east direction. The southwest and northeast orientation of most Pennypack Creek tributaries and the noticeable southwest and northeast orientations of Pennypack Creek segments are an example of aligned drained. The northeast oriented tributary joining Pennypack in Lorimer Park (location 7) is a classic barbed tributary and the northeast oriented tributary at location 8 not only is a barbed tributary, but it drains to an elbow of capture where Pennypack Creek turns from flowing in southwest direction to flow in a northeast direction.
Figure 1: Pennypack Creek valley in Fox Chase region. See text for description. United States Geological Survey map digitally presented using National Geographic TOPO software.
The aligned drainage routes and barbed tributaries, and especially the barbed tributary at location 8, provide important clues as to how the Pennypack Creek valley was eroded. Prior to Pennypack Creek valley erosion all elevations in the present day map region were at least as high or higher than the highest elevations today. The pre-Pennypack Creek valley surface no longer exists although the aligned drainage routes and barbed tributaries provide evidence that the deep Pennypack Creek valley eroded headward into the region across massive and prolonged southwest oriented floods that were flowing across the region. Floodwaters were moving in shallow diverging and converging southwest oriented flow channels in what was a large-scale anastomosing channel complex. Headward erosion of the deep Pennypack Creek valley captured the southwest oriented flood flow channels in sequence from south to north and diverted the water to what at that time was probably a newly eroded southwest oriented Delaware River valley.
The captured southwest oriented flood flow channels eroded the southwest oriented Pennypack Creek tributary valleys. Southwest oriented flow in these captured flow channels was subsequently beheaded as headward erosion of south oriented Delaware River tributary valleys further to the east captured the flow. Water on the northeast ends of southwest oriented flood flow routes beheaded by Pennypack Creek valley headward erosion reversed flow direction to flow in a northeast direction to reach the newly eroded and much deeper Pennypack Creek valley. This reversed flow captured flood flow from southwest flow channels north of the actively eroding Pennypack Creek valley head and the captured water helped erode the northeast oriented tributary valleys.
The elbow of capture formed where the barbed tributary at location 8 joins Pennypack Creek shows where two southwest oriented flood flow channels once converged. Pennypack Creek valley headward erosion first captured the flood flow channel that eroded the southwest oriented valley at location 6. Water on the northeast end of the beheaded flow channel then reversed direction to flow to the newly eroded and deep south-oriented Pennypack Creek valley. Because that flow channel converged with the southwest oriented flood flow on the alignment of the present day northeast oriented tributary at location 8 the reversed flow was able to capture the flow from the location 8 flood flow channel with the result that the deep Pennypack Creek valley first eroded in a west-southwest direction to the point where the two flood flow channels converged and then in a northeast direction along the more northern of the two converging flood flow channels. Headward erosion of the deep Pennypack Creek valley head along this zigzag route beheaded and reversed flood flow moving south and west of the elbow of capture, which created the northeast oriented tributary seen at location 8, and also created the elbow of capture where Pennypack Creek flows in a southwest direction and then abruptly turn to flow in an east-northeast direction.