




Above the Spearfish formation is a thin band of white gypsum, called the Gypsum Springs Formation. This layer of gypsum was deposited during the Jurassic period, 195 to 136 million years ago.


During the Paleogene Period, around 50 to 60 million years ago, the Rocky Mountains and the Black Hills were uplifted. Magma rose through the crust, intruding into the existing sedimentary rock layers.

Columnar joints at Devil's Tower in Wyoming. This is all that remains of an ancient volcano - the "plug." The magma formed pencil shaped columns as it very slowly cooled.