Mill Creek originates just north of the Villanova University campus and flows in an east direction along the north sides of the Rosemont College and Bryn Mawr campuses until it turns to flow in an east-southeast and a south-southeast direction before making an abrupt turn to flow in a northeast direction to join the southeast oriented Schuylkill River as a barbed tributary. The Schuylkill River-Mill Creek drainage divide provides clues to the Mill Creek and Schuylkill River valley origins and is the subject of this brief essay.

Figure 1: Schuylkill River-Mill Creek drainage divide area north and east of Bryn Mawr. United States Geological Survey map digitally presented using National Geographic TOPO software. 

Figure 1 illustrates the Mill Creek drainage basin east of the Bryn Mawr College campus (straddling the west edge of the figure 1 southwest quadrant). Mill Creek is labeled and flows in an east, east-southeast, and south-southeast direction from just north of the Bryn Mawr College campus to location 4 where it turns to flow in a northeast direction to join the southeast oriented Schuylkill River at location 8. Location 1 identifies a shorter northeast oriented Schuylkill River tributary while location 2 identifies a south oriented Mill Creek tributary. Locations 3, 5, 6, and 7 identify saddles located along the Schuylkill River-Mill Creek drainage divide.

The saddles found along Schuylkill River-Mill Creek drainage divide at locations 3, 5, 6, and 7 provide evidence of multiple shallow valleys eroded at the time the much deeper Schuylkill River and Mill Creek valleys were being eroded. At that time massive and prolonged southwest oriented floods moved across the region on a surface at least as high as the present day Schuylkill River-Mill Creek drainage divide. Floodwaters eroded shallow complexes of anastomosing channels into that high-level surface while the deep southeast-oriented Schuylkill River valley eroded headward across the southwest oriented flood flow channels to capture the massive flood flow and to divert the floodwaters to the what at that time was the actively eroding Delaware River valley.

The northeast oriented Mill Creek segment between locations 4 and 8 originated as a southwest oriented flood flow channel (possibly along a zone of geologic weakness, which permitted more rapid erosion) and was beheaded and reversed by headward erosion of the much deeper southeast oriented Schuylkill River valley (prior to headward erosion of the deep Mill Creek valley floodwaters were moving to what at that time was the actively eroding Cobbs Creek valley located south and west of figure 1).

Reversed flow on the former southwest oriented flood flow channel captured yet to be beheaded (by Schuylkill River valley headward erosion) floodwaters flowing in a southwest direction along the present day route of the northeast oriented Schuylkill River tributary at location 1. At that time the deep northeast oriented valley at location 1 did not exist and the floodwaters flowed at a level at least as high as the saddles at locations 3, 5, 6, and 7 (and west of figure 1). The captured floodwaters were diverted in a southeast and then northeast direction to the much deeper and newly eroded Schuylkill River valley and eroded the deep Mill Creek valley seen in figure 1. Subsequently Schuylkill River valley headward erosion beheaded and reversed flow on the northeast oriented tributary alignment at location 1. Reversed flow on that newly beheaded and reversed flood flow channel captured yet to be beheaded flood flow from further to the northwest and that captured flood flow then eroded the deep northeast oriented valley seen in figure 1.