The Spearfish formation crops out in the southern and northeastern parts
of the Devils Tower National Monument along the valley of the Belle
Fourche River and its tributaries and forms conspicuous brownish-red to
maroon cliffs that border the Belle Fourche valley for several miles in
the Devils Tower region. The formation is 450 to 600 feet thick in the
northern Black Hills area (Darton, 1909, p. 28); however, only the
uppermost 100 feet are exposed within the National Monument.
The Spearfish formation consists of red to maroon siltstone and
sandstone interbedded with mudstone or shale. Locally, greenish-blue
shale partings are found in the siltstone and sandstone. The formation
is poorly cemented and weathers very easily forming, for the most part,
gentle slopes, as on the northeast and southwest sides of the monument.
Where it does form cliffs, as south of the Tower, the cliffs are cut by
many sharp gullies.
No fossils have been found in the Spearfish formation in the Devils
Tower region, but elsewhere in Wyoming, stratigraphically equivalent
rocks contain land vertebrates of Triassic age.