Laramie Formation

The Laramie Formation is a geologic formation of Cretaceous age, named by Clarence King in 1876 for exposures in northeastern Colorado, in the United States.

The formation is exposed around the edges of the Denver Basin and ranges from 400–500 feet (120–150 m) on the western side of the basin and 200–300 feet (60–90 m) thick on the eastern side. The Laramie conformably overlies the Fox Hills Sandstone and unconformably underlies the Arapahoe Conglomerate. The formation can be divided into a lower unnamed member containing bedded sandstone, clay, and coal and an upper unnamed member composed predominately of 90 to 190 m of drab-colored mudstone, some sandstone, and thin coal beds. Nodular ironstone concretions occur in the mudstones that contain plant remains. The coal and clay were once economically important. The Laramie Formation was deposited on a coastal plain containing coastal swamp. Some of the material in the sandstones originated from silicic volcanoes far to the west.