Much of the Triassic area is covered superficially by glacial drift and alluvium of the Trent.
The disturbances among the underlying rocks of Ohio have been slight, and originally the surface was a plain only slightly undulating; stream dissection changed the region to one of numberless hills and valleys; glacial drift then filled up the valleys over large broken areas, forming the remarkably level till plains of northwestern Ohio; but at the same time other areas were broken by the uneven distribution of the drift, and south-eastern Ohio, which was unglaciated, retains its rugged hilly character, gradually merging with the typical plateau country farther S.E.
Slates and fine-grained sandstones appear here freely through the glacial drift.
West of the Missouri river the sheet of glacial drift is absent, and the lands everywhere show evidence of extensive stream erosion.
The glacial drift east of the Missouri river, unlike that of the New England states, is remarkably free from boulders and gravel, except in a few morainic belts.
