My grandparents farm was just east of the line, in southern Nebraska. You should have seen all the smooth rocks, of various sorts of composition/materials, including flint, quartz, and other stuff, that the farmers kept finding in their fields. Frost heave pushed them up, but the glaciers smoothed them out. Some were nearly spherical, but most were more ovoid. The flint tended to be a very flattened ovoid.
