Cleland Rock is a ledge of Kittanning Sandstone which forms a scenic overlook 400 feet above the deepest part of the 12-mile long Slippery Rock Gorge. It is also the highest part of the rim of the gorge.
If you were standing here 140,000 years ago, you would have seen a ridge extending across what is now the Slippery Rock valley here. The ridge divided the south-flowing "Wurtemburg Run" from the north-flowing "McConnells Run."
Lake Prouty lay in the valley to the right (north) of this divide. Overflow from Lake Prouty, assisted by meltwater and sediment released from the nearby glacier, spilled over the divide at Cleland Rock from right to left (north to south) into "Wurtemburg Run." This overflow eventually eroded a channel through the divide at Cleland Rock deep enough to join "McConnells Run" and "Wurtemburg Run" to form the ancestral south-flowing Slippery Rock Creek, and drain Lake Prouty.
After Lake Prouty drained, the new Slippery Rock Gorge was probably no more than half of its present depth. From this beginning, it has been eroded to greater than its present depth by catastrophic flooding from the draining of glacial Lakes Watts and Edmund, further upstream, during the last glaciation, 23,000 years ago. Rocks that have fallen into the gorge from the gorge walls have reduced the depth of the gorge by about 40 feet.
Erosion by the present Slippery Rock Creek has continued since the last glaciation, but compared to the days when water roared through the valley, there has been little change.