The outlines of the broad Williston basin in the west are clear; these rocks (brown and purple) all date from Tertiary times (younger than 65 million years). The rest, starting with the light blue, make up a thick Cretaceous section (140 to 65 million years) covering the eastern half of the state. A narrow strip of Archean basement, billions of years old, with a few stray blobs of much younger Ordovician (pink) and Jurassic (green) rocks, spills across the border from Minnesota.
I have prepared this map in a 1200x1000 pixel version (550 KB) that includes a key to the different rock units. You can also buy a printed 8-1/2 x 11 copy from the state; order publication MM-36.
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