The majority of the state is of Tertiary (Cenozoic) age and terrigenous origin. A few slivers of Oligocene rocks crop out in the west, as do larger areas of Miocene (pale tan), but most is of Pliocene age (yellow). The Oligocene and Miocene rocks are freshwater lake beds ranging from limestone to sandstone, the sediment derived from the rising Rockies to the west. They include large volcanic ash beds from eruptions in present-day Nevada and Idaho. The Pliocene rocks are sandy and limy deposits; the Sand Hills in the west-central part of the state derive from these.
The thick green lines in the east mark the western limit of the great Pleistocene glaciers. In these areas glacial till overlies the old rock: blue clay, then thick beds of loose gravel and boulders, with occasional buried soils where once forests grew.
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