Chris: Perhaps the most memorable place we visit on the trip to the Gorge Metropark is Mary Campbell Cave, a dramatic natural overhang formed from the Sharon Conglomerate. You have to delve right into the depths of the overhang to find the really interesting part: a contact with an underlying shale unit. The fact that the contact clearly undulates across the width of the cave is strong evidence of an erosional unconformity. Across this contact, a lot of time is missing: enough time for tectonic uplift to change the calm, deep waters recorded by the shale below, to the much shallower and energetic waters that laid down the conglomeratic sandstones above.
In the present day, the transition from permeable sandstones above to impermeable shales below also means that a lot of groundwater is flowing out of the cliff at the contact.