Read it through once

The very rocks themselves would be scratched, polished, and rounded off. When the glaciers melted away and left the surface free, it would consist of these rounded rocks alternating with clay-filled hollows. The trap-rock below would be covered by a subsoil due to particles of trap, of Highland and other mud, with remains of the miocene vegetation. Upon this surface, frost, wind, sunshine, and rain would again begin to perform their work.