Making Wyoming

In the Paleozoic Era there was no California, and more importantly, no mountain range in California, between Wyoming and the Pacific Ocean. This very strongly influenced the climate, by allowing the moisture from the Pacific storms to reach the area, and thus the geology, of Wyoming. Wyoming was covered by warm, fairly shallow, tropical, marine seas. From these seas were deposited thick layers of limestone, from the shells of the marine animals, and shales from muds. At that time, Wyoming lay near the equator, part of a broad, shallow continental shelf that ran up the Western margin of the continent all the way to the Yukon. Marine waters intruded and receded periodically throughout this period, depositing layers of sandstone, limestone and shale. While in the eastern part of Wyoming these layers are 3000 feet thick, this thickness increases rapidly as you go west across the state, reaching a depth of greater than 20,000 feet at the western edge of the state.

At the close of the Paleozoic Era, all of the world's continental masses met to form a single supercontinent named Panagea. The northern part of this supercontinent was made up of North America and Europe, called Laurasia. The south part, known as Gondwanaland, was composed off South America, Africa, Antarctica, India and Australia. The process of plate techtonic has moved the continental masses into their present positions.

Following the Paleozoic, the Mesozoic Era deposits are the red Triassic sandstones and shales. The Paleozoic deposits are gray. The reason for this difference is found in the different conditions under which the materials were deposited. In the Paleozoic, the deposits were made at the bottom of a sea, under oxygen deprived conditions. The red Mesozoic deposits were made in the floodplains of rivers, where oxidation could proceed, accounting for the red color of the rock layers. During Cretaceous time marine conditions returned to Wyoming, not from the west, but from a huge inland sea that extended from the Gulf Coast to the Arctic Ocean, with land on the east and the west. The Rocky Mountain states, from New Mexico northward, were covered by this sea which deposited sandstone and the thick, black organic-rich shales that are the souce of the oil found in this region.

Cenozoic time brought an era of mountain building. Marine waters have not returned to Wyoming after the end of the Cretaceous. The Rocky Mountins were created in an event called the Laramide orogeny. The mountains were uplifted from the basement Precambrian rocks, through the overlying sedimentary layers. This took place over a period of 20 - 30 million years from the end of the Cretaceous into early Eocene time. This happened because the area was compressed because of the subduction of the ocean crust on the western side of the continental shelf. This compression caused faulting that is seen today throughout Wyoming. These faults define the uplifts in the Wind River, Big Horn and Laramie ranges.

The highlands proceeded to erode, filling the basins between the ranges with sediments. During the Paleocene era, the weather was much wetter than that today in Wyoming. The thick forests that grew here at that time, have created the coal beds of the Fort Union formation, more than 100 feet thick in places.

During the Eocene time the oil shale beds in the Green River Formation of southwest Wyoming and adjacent regions of Utah and Colorado were laid down. Innumerable fossils of fish skeletons are found in the Green River Formation. The Ogilocene through Miocene epochs saw continued erosion of the overlying sedimentary layers in the high areas, supplemented by enormous deposits of volcanic ash from volcanos in Nevada, Utah, perhaps Arizona, and later the Yellowstone area. These deposits filled the basins to overflowing. The streams that flowed in this landscape were slow and meandering over the level ground. Their location was totally without relation to the mountain ranges buried below. They slowly carried the basin fill away to the oceans; the speed of this erosion increased as the gradient down which the rivers flowed increased. This erosion continued as the area was again uplifted at the end of the Cenozioc era, further increasing the gradient down which the rivers carried the sediments. The Teton range was also lifted and the Yellowstone area became fully developed by the movement of the mantle hot spot through the Snake River floodplain.

However, this history of the Wyoming landscape has left the mountain ranges with a feature peculiar to them, the subsummit surface. The wide level plateaus at the 10 - 12,000 foot level in the mountain ranges are composed of the uneroded fill that covered the mountain ranges, and still does in places!

The deposits found in the delta of the Mississippi delta are not nearly large enough to account for all of the material that had to be removed in order to create today's landscape. The rest of this material had to be removed by wind erosion. The wind is a constant force in Wyoming. There are several lake basins in Wyoming that do not have any association with rivers. They are blowout basins, created by wind erosion. Wind is a constant erosion force in drylands.

The rivers that were carrying much of the deposited sediment away began to cut down through the mountain ranges, in places entirely unrelated to the geometry of the mountain ranges themselves. Instead of the usual pattern of dendritic streams feeding drainage basins from the mountains, these rivers were downcut through the mountains ranges from above. This accounts for the river that is named the Big Horn River in the Big Horn Basin and the Wind River after it emerges from the Owl Creek Mountains into the Wind River Basin. Today Wyoming continues to be excavated by the wind and the water.

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© 1995 Karen M. Strom