The Tropic Shale was deposited in the Sevier Foreland Basin during Cenomanian time, as part of the Western Interior Seaway. Ammonites are found in exposures of this formation in what is now Grand Staircase-Escalante National Monument and around the towns of Orderville and Alton.

References: White, C. A., 1877, Report upon the Invertebrate fossils collected in portions of Nevada, Utah, Colorado, New Mexico and Arizona, by parties of the expeditions of 1871, 1872, 1873, and 1874, U. S. Geographical and Geological Explorations and Surveys West of the 100th Meridian Report (Wheeler), vol. 4, pt. 1; Stanton, T. W., 1893, The Colorado Formation and it's Invertebrate Fauna, U.S.G.S. Bulletin 106; Hyatt, A., 1903, Pseudoceratites of the Cretaceous: edited by T. W. Stanton, U.S.G.S. Monograph 44; Swenson, A. J., 1962, Anisoceratidae and Hamitidae (Ammonoidea) from the Cretaceous of Texas and Utah, B.Y.U. Geology Studies Vol. 9, pt. 2