Glacial Lakes and the Slippery Rock Gorge
Slippery Rock Creek flows in a deep, winding gorge cut into flat-lying layers of bedrock. The gorge cuts through an old drainage divide, the remnants of a ridge that once separated streams flowing to the north and to the south. The gorge is deepest and narrowest near Cleland Rock (Site 6) on the divide and becomes progressively shallower and wider both to the north and to the south. This is not a typical pattern because the valleys of streams in areas of flat-lying rocks typically become progressively shallower and wider toward their outlets.
Why does Slippery Rock Creek have this configuration? Diagrams illustrate the geologic history of the gorge.
Pre-glacial Slippery Rock and Muddy Creeks joined and flowed northwestward from near the present location of Kennedy Mill (C). From the divide at Cleland Rock, "McConnells Run," flowed north past McConnells Mill. "Wurtemburg Run," flowed south from the divide into Connoquenessing Creek, 6 miles away.
About two million years ago, thick masses of continental ice began to periodically accumulate in central and northeast Canada and spread southward. At least four of these ice sheets, many hundreds or thousands of feet thick, eventually reached northwestern Pennsylvania and modified the landscape and stream patterns. As the ice front that moved southeastward into the area about 140,000 years ago, it dammed northwest-flowing Slippery Rock-Muddy Creek.
At the ice-advance maximum, separate lakes formed in the Slippery Rock (Lake Edmund) and Muddy Creek (Lake Watts) lowlands. A third, much smaller lake, (Lake Prouty) formed in the "McConnells Run" valley when it was dammed south of Muddy Creek. Lakes Edmund and Watts drained through outlets at their eastern ends (D and E), far from the glacier dams, into Connoquenessing Creek.
Lake Prouty spilled over a low point in the old divide near Cleland Rock into "Wurtemburg Run." It sufficiently eroded the old divide to form the ancestor of today's south-flowing Slippery Rock Creek.
As the glacier melted back to the northwest, Lake Watts began to drain westward through a series of temporary outlets, each lower than the preceding. The first of the outlets is Alpha Pass (F). After later drainage through Beta Pass (G), the remnant of Lake Watts was still slightly larger than present-day Lake Arthur. Finally, Gamma Pass (H), the present Muddy Creek channel (I), drained Lake Watts. When the ice receded far enough, Lake Edmund drained past Kennedy Mill (C) in a single event.
As each pass opened, a short-term vast flood surged into the gorge. These floods deepened the gorge and eroded the soft shale beneath the hard Homewood Sandstone, leaving the sandstone as an overhanging cliff. Because the sandstone is cut by many intersecting cracks, unsupported large blocks fell from the rim. Some of these large blocks moved only a few feet downslope, but others now fill the bottom of the gorge. This has widened the gorge, but left it about 45 feet less deep than originally carved. (See cross-section)
During the last glaciation to reach the area, 23,000 years ago, ice-dammed lakes again formed in the Slippery Rock and Muddy Creek basins. Drainage from these lakes further deepened and widened the Slippery Rock Gorge.
Today, Slippery Rock Creek still has a steep gradient through the gorge (28 ft per mile compared to 8 feet per mile north of Kennedy Mill). The rushing water has carved potholes in many of the large blocks and has scoured out tubes through the remaining debris. The combination of rapids, waterfalls, and eddy currents within the channel makes for a very treacherous stream course. These conditions also provided the waterpower to run both McConnells and Kennedy Mills.