The Andorra Wind Gap links an east-northeast oriented tributary to the Wissahickon Gorge north entrance with a west-southwest oriented Schuylkill River tributary and has an elevation of 266 feet. To the south Schuylkill River-Wissahickon Creek drainage divide elevations rise to 420 feet before decreasing until Wissahickon Creek joins the Schuylkill River. To the north a low ridge separates the wind gap from even lower regions including the Marble Hall Through Valley with a floor elevation of between 210 and 220 feet. The change in elevations is related to underlying bedrock. To the south both the Schuylkill River and Wissahickon Creek have eroded 300-foot deep gorges across a wide band of erosion resistant bedrock. To the north easily eroded bedrock underlies a low land region including the Marble Hall Through Valley and the Whitemarsh Through Valley (further to the north). The question posed by the Andorra Wind Gap is why would a shallow wind gap be eroded into a ridge surrounded by much deeper valleys?
Figure 1: The Andorra Wind Gap is found at location 1. The east-northeast valley beginning at the Andorra Wind Gap drains to the Wissahickon Gorge north entrance at location 3. The west-southwest oriented valley draining to location 4 joins the Schuylkill just west of the map at Spring Mill. Spring Mill Creek can be seen flowing in southwest and south directions in the map northwest corner. The Marble Hall Through Valley at location 2 links the Spring Mill Creek valley with the south oriented Wissahickon Creek valley. United States Geological Survey map digitally presented using National Geographic TOPO software.
The Andorra Wind Gap was eroded prior to headward erosion of the south oriented Wissahickon Gorge and also prior to erosion of the low lands to the north (including the Marble Hall and Whitemarsh Through Valleys). A west-southwest oriented flow channel moving water to what at that time was the actively eroding Schuylkill River valley head eroded the Andorra Wind Gap. The flow channel was one of many diverging and converging flow channels in a flood formed southwest oriented anastomosing channel complex that crossed the entire region. At that time the south oriented Wissahickon Creek valley head had not yet eroded headward to reach what is today the Wissahickon Gorge north entrance (near Chestnut Hill College). Also at that time the southwest and south oriented Spring Mill Creek valley was just beginning to capture southwest oriented flow north of the Andorra Wind Gap flow channel and to erode the deeper Marble Hall Through Valley (subsequently headward erosion of the southwest and south oriented Plymouth Creek valley captured southwest oriented flow and then eroded the still deeper Whitemarsh Through Valley, which is found still further to the north).
West-southwest oriented flood flow eroded the west-southwest oriented valley leading from the Andorra Wind Gap to the Schuylkill River. The east-northeast oriented valley leading from the Andorra Wind Gap to the Wissahickon Gorge north entrance was eroded when headward erosion of the deep south oriented Wissahickon Creek valley beheaded and reversed flow on the east-northeast end of the west-southwest oriented flow channel. Because the flow channels diverged and converged the reversed flow was able to capture southwest oriented flood flow moving north of the actively eroding Wissahickon Creek valley head and that captured flow helped erode the east-northeast oriented valley. Headward erosion of the deep southwest and south oriented Spring Mill Creek valley to the west enabled west-southwest oriented flood flow north of the Andorra Wind Gap to erode the Marble Hall Through Valley. Wissahickon Creek valley headward erosion next beheaded the west-southwest oriented flow channel located in the Marble Hall Through Valley and ended all flood flow to the southwest and south oriented Spring Mill Creek valley.