Geology and Grand County (MP-Q)

By: H. H. Doelling and others

Grand County is located in the heart of the Colorado Plateaus physiographic province, ranks 9th in land area with 3,692 square miles with a population of only 8241, 20th out of 29 Utah counties (1980 census). Perhaps more than in any county in Utah, the financial base is linked to the geology. Mining (extracting minerals), tourism (seeing the geologic wonders), and land management and exploration provide many jobs for its people.

Limestone, salt, and shale were deposited in an ocean basin that covered Grand County in Pennsylvanian time. From then until Upper Triassic time, the ancestral Rocky Mountains (Uncompahgre Uplift) rose near the present Colorado line as a mountainous peninsula or island. The erosional products of the mountain range were deposited in huge coalescing alluvial fans that extended across Grand County as a coastal plain. The salt anticlines formed during this time.

During the remainder of Triassic time and extending well into the Tertiary Period, the area subsided and received sediments. Savannah-type landscapes, floodplains, tidal flats, and sandy deserts prevailed into which the dinosaurs and other primitive life forms wandered. During the Cretaceous Period the area was again inundated by an ocean at the bottom of which the drab gray Mancos shales were deposited. Sharks, oysters, and cephalopods inhabited this sea. As the seas withdrew, sandy beaches, marshy backswamps and lagoons appeared. The swamp vegetation was preserved and coalfield to form the coal beds of the Book Cliffs.

In mid-Tertiary time Grand County experienced crustal deformation and igneous activity. The rock formations were folded and faulted. Deep rocks were melted and forcefully intruded into overlying strata in the La Sal Mountain area. About 10 million years ago Grand County was elevated and subjected to erosion. Since then, the Colorado River and its tributaries have cut magnificent canyons, exposed the granitic rocks of the La Sal Mountains, exhumed the salt anticlines, and formed the beautiful landscapes we enjoy today.

Other Information:
Published: 1987
Pages: 16