System Series Formation and member; Thickness, in feet; Columnar section; Description JURASSIC Upper Jurassic Sundance formation Redwater shale member; 100+; Gray and gray-green shale. Thin fine-grained sandstones in lower part; thin fossiliferous limestones in upper part Lak member; 40-65; Yellow soft fine-grained calcareous sandstone Hulett sandstone member; 60-70; Yellow massive fine-grained calcareous sandstone Stockade Beaver shale member; 85-100; Gray and gray-green shales with thin calcareous sandstones UNCONFORMITY Middle Jurassic Gypsum Spring formation; 15-35; White massive gypsum interbedded with thin red mudstone UNCONFORMITY TRIASSIC Spearfish formation; 100; Red to maroon siltstone and sandstone interbedded with some thin shale SEDIMENTARY ROCKS The sedimentary rocks that surround Devils Tower have a total exposed thickness of about 400 feet. They are divided, from oldest to youngest, into the Spearfish formation of Triassic age, the Gypsum Spring formation of Middle Jurassic age, and the Sundance formation of Late Jurassic age (fig. 54). SPEARFISH FORMATION 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. GYPSUM SPRING FORMATION The Gypsum Spring formation is exposed in a thin but almost continuous band around the Tower on the southwest to northeast sides. It also crops out near the top of the small hill at the eastern boundary of the National Monument, a few hundred feet north of the Registration Building. This formation is composed mostly of white gypsum, which stands out conspicuously between the red beds of the underlying Spearfish formation and beds of gray-green shale at the base of the overlying Sundance formation.