Ice Age

The blue shows the scope of the most recent Ice Age, the Wisconsinan (17,000 - 22,000 yrs.).

The green shows the edge of the Late Illinoian (132,000 - 198,000 yrs.) ice cap.

The orange shows the edge of the of the Pre-Illinoian (>770,00 yrs) ice cap.

The yellow shows the stratified drift, the outflow resulting form the melting of the last two Ice Ages.

The position of the ice cap can be determined by the till—the debris resulting from the grinding of the surface rock as the massive ice shelf moved over the it. From the maps, it is clear that the most recent Ice Ages never reached the Wissahickon Gorge.

Source