The Mill Creek-Southampton Creek through valley (illustrated in figure 1) links south-southwest oriented Southampton Creek (location 2), which flows to Pennypack Creek (seen in the figure 1 southwest corner), with southeast and east-northeast oriented Mill Creek (location 3), which flows to Neshaminy Creek (located east of figure 1). The deepest notch (location 1) has a floor elevation of between 240 and 250 feet. Elevations to the north rise to more than 350 feet (at location 4) while to the south the drainage divide elevation rises to more than 340 feet (at location 5).
Figure 1: The Mill Creek-Southampton Creek Through valley extends between Southampton Creek (location 2) and Mill Creek (location 3). The through valley deepest notch is at location 1 and has an elevation of between 240 and 250 feet. High points bounding the through valley at locations 4 and 5 have elevations exceeding 340 feet. United States Geological Survey map digitally presented using National Geographic TOPO software.
This broad through valley crosses the Neshaminy Creek-Pennypack Creek drainage divide and was eroded by massive and prolonged west oriented floods moving water to what at that time was an actively eroding south oriented Pennypack Creek valley. The Pennypack Creek valley eroded headward from what at that time was the actively eroding Delaware River valley segment (in northeast Philadelphia) to capture southwest oriented flood flow and to divert that water to the much deeper Delaware River valley.
Prior to Pennypack Creek valley headward erosion into the figure 1 region all regional elevations were at least as high as the highest elevations today (approximately 350 feet in terms of today’s elevations, although at that time sea level probably was significantly lower than it is today). West oriented flood flow moving across what is now the Neshaminy Creek valley (east of figure 1) and the southeast oriented Mill Creek valley (seen in figure 1) eroded the deep Southampton Creek valley headward into the figure 1 region. The Southampton Creek valley eroded headward in the region west of location 4 to capture flood flow moving across the high-level surface to the north of figure 1.
As the actively eroding southwest oriented Delaware River valley head eroded headward in a northeast direction along a major southwest oriented flood flow channel the south-oriented Neshaminy Creek valley began to erode headward (in a north direction) to capture the southwest and west oriented flood flow that at time was moving to the newly eroded Pennypack Creek valley. Headward erosion of the deep Neshaminy Creek valley beheaded a major west-southwest oriented flood flow route (not seen in figure 1) and floodwaters on the east-northeast end of the flow route reversed flow direction to permit a deep east-northeast oriented Mill Creek valley to eroded headward towards figure 1.
The southeast oriented Mill Creek valley seen in figure 1 then eroded headward across the west oriented flood flow route to the newly eroded Southampton Creek valley and by doing so beheaded and reversed the flood flow. The reversed flow probably captured some of the south oriented flood flow moving in the Southampton Creek valley to location 2 and diverted that water across the deepest Mill Creek-Southampton Creek through valley notch (at location 1) to the newly eroded southeast oriented Mill Creek valley at location 3, although most south oriented water in the Southampton Creek valley continued to flow to the Pennypack Creek valley. Headward erosion of other Neshaminy Creek tributary valleys (east and north of figure 1) subsequently beheaded all flood flow routes to the Southampton and Mill Creek valleys.