Spring Mill Creek Drainage Basin Origin and History

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Spring Mill Creek (also known as Spring Mill Run) is a relatively minor Schuylkill River tributary and flows in a southwest, south, west, and south direction to the Schuylkill River at Spring Mill (just east of Conshohocken). The Spring Mill Creek drainage basin is bounded on the north and west by the southwest and south oriented Plymouth Creek drainage basin and on the east by the south oriented Wissahickon Creek drainage basin. Spring Mill Creek joins the Schuylkill River at an elbow of capture (the Spring Mill Elbow of Capture) where the Schuylkill River turns from flowing in an east-northeast direction to flow for eight miles in a deep southeast oriented valley (the Schuylkill River Gorge) carved into an east-northeast oriented erosion resistant bedrock band.

Figure 1: Spring Mill Creek drainage basin region.

Figure 1: Spring Mill Creek drainage basin region.

Figure 1 illustrates the Spring Mill Creek drainage area. Spring Mill Creek originates near location 1 and flows in a southwest, south, west, and south direction to reach the Schuylkill River at location 2. Plymouth Creek flows in a southwest direction at location 3 and west of the map turns to flow in a south direction to join the Schuylkill River. Wissahickon Creek flows in a south direction from the map northeast corner to enter at location 6 its seven-mile long Wissahickon Gorge, which was carved in the same metamorphic and igneous rock complex the Schuylkill Gorge crosses.

Elevations surrounding the Wissahickon Gorge rise to 420 feet. Location 4 identifies the Andorra Wind Gap, which links an east-northeast oriented Wissahickon Creek tributary valley with a west-southwest oriented Spring Mill Creek tributary valley. The Andorra Wind Gap elevation is 266 feet. The Marble Hall Through Valley at location 5 links the west oriented Spring Mill Creek valley segment with the south oriented Wissahickon Creek valley and has an elevation of between 210 and 220 feet. North of location 5 is Lafayette Hill and north of Lafayette Hill is the Whitemarsh Through Valley, which links the Spring Mill Creek headwaters at location 1 with the south-oriented Wissahickon Creek valley near the map northeast corner and which has an elevation of 206 feet.

These elevations pose some interesting questions, why did Wissahickon Creek erode a seven-mile long gorge across a ridge of erosion resistant metamorphic and igneous rock when much lower routes to the Spring Mill Creek valley were available? Or, if the lower routes were not available then how and why were the lower routes eroded if Wissahickon Creek could freely flow in a south direction through the Wissahickon Gorge?

To answer these questions it is necessary to reconstruct the landscape as it existed at the time Schuylkill and Wissahickon Gorge erosion began. At that time elevations across the entire region were as high or higher than the highest elevations surrounding the Schuylkill and Wissahickon Gorges today. There was no upstream Schuylkill River valley and there was no upstream Wissahickon Creek valley. Instead massive and prolonged southwest oriented floods were flowing across that high-level surface. The deep southeast oriented Schuylkill Gorge valley eroded headward to capture the southwest oriented floodwaters and to divert the flow to what at that time was probably a newly eroded Delaware River valley (where the Schuylkill joins the Delaware today).

The south oriented Wissahickon Gorge eroded headward from the newly eroded Schuylkill Gorge to also capture the southwest oriented flow and to divert the floodwaters more directly to the newly eroded Schuylkill Gorge. Schuylkill Gorge headward erosion proceeded fast enough that Wissahickon Gorge headward erosion did not behead southwest oriented flow routes that were moving water to the actively eroding Schuylkill Gorge valley head and the headward erosion of the Schuylkill Gorge reached location 2 before Wissahickon Gorge headward erosion reached location 6. When Schuylkill Gorge headward erosion reached location 2 it captured the west-southwest oriented flood flow channel that eroded the Andorra Wind Gap at location 4. Water on the east-northeast end of the beheaded flow route reversed flow direction and the Schuylkill River valley eroded headward along that reversed flow route to create what is today the east-northeast Schuylkill River valley segment upstream from location 2.

At the same time as when the Schuylkill River valley was eroding headward along the reversed flow route upstream from location 2 the south oriented Spring Mill Creek valley eroded headward from the newly eroded and deep Schuylkill River valley at location 2 to capture west-southwest oriented flood flow moving along the alignment of what is today the Marble Hall Through Valley at location 5. Headward erosion of what is today the Spring Mill Creek headwaters valley to location 1 next captured west-southwest oriented flood flow moving along the Whitemarsh Through Valley route.

Remember at the time of these captures the flood flow was moving on a high-level surface as high as the surface through which the deep Wissahickon Gorge was being eroded. Because rocks along the Andorra Wind Gap, Marble Hall Through Valley, and Whitemarsh Through Valley routes were more easily eroded than the rock through which the Wissahickon Gorge was being eroded the west-southwest oriented flood flow to the newly eroded and Spring Mill Creek valley began to erode headward along those routes with elevations being significantly lowered in that region.

However, southwest oriented flood flow still reached the actively eroding Wissahickon Gorge valley head and eventually Wissahickon Creek valley headward erosion crossed the erosion resistant metamorphic and igneous rock complex and captured the west-southwest oriented flood flow channel moving along the Andorra Wind Gap route and water on the east-northeast end of the beheaded flow route reversed flow direction to create the present day east-northeast Wissahickon Creek tributary (the reversed flow channel probably captured flow from yet to beheaded flow channels, which provided the water necessary to erode the east-northeast oriented tributary valley). Next Wissahickon Creek valley headward erosion captured flow that had been moving along the Marble Hall Through Valley route and there probably was another flow reversal on the east end of the beheaded flow channel. And the process was repeated as the Wissahickon Creek valley eroded headward to capture flow on the Whitemarsh Through Valley route.

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