Oct 07 2014
Amazing as it seems none of the hills in the Pittsburgh Low Plateau were ever mountains. They were all made by water cutting rock.
When dinosaurs roamed the earth, Pittsburgh was at the shore of a shallow inland sea to our west. At the time our soil was composed of sandy beaches, mudflats and swamps. The sand became sandstone, the mud became shale, and the decayed swamp plants became coal.
No geologic events deformed our rocks. To our east the Appalachians slammed into the edge of the plateau and pushed up the Allegheny Mountains. To our north the retreat of the glaciers made the land rise in a bounce-back from the pressure release. Our plateau remained essentially flat, tilting slightly to the south and west. Indeed the rock layers in our area are horizontal. You can see this at highway road cuts.
But Pittsburgh doesn’t look flat.
Water transformed us, cutting dendritic paths in the landscape as it drained to the inland sea (now the Gulf of Mexico). The paths became deep valleys. When you stand in a valley you see hills.
Vegetation now hides what happened to the land. You see the hills but none of the water cuts as shown in this view of the Kiskiminetas (Kiski) River at the mouth of Roaring Run.
Take a short hike up Roaring Run and see what water can do.
Here the creek is carving a notch in weak shale. The valley walls are steep and narrow. The notch is big enough to wade in.
Eventually Roaring Run may look like the Kiski with tree-covered hills.
It all comes from water cutting rock.
(photos by Kate St. John)