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Wyoming Geology: A Brief Overview

by Wyoming Livestock Roundup

By Chris Carlsen, Natural Resources Conservation Service Engineering Geologist

What do you see when you look around Wyoming? The mountain ranges collect snow during the winter months and, in late spring, the snow begins to melt. This melting snow is carried to basins by drainage systems carved out of the mountains and foothills by the melting snow and heavy rain events. This charges the stream systems in the basins and is diverted for irrigation by man. The vast majority of irrigated crops in Wyoming are located in the basins and foothills of Wyoming, such as the Powder River Basin and the Green River Basin.

Many ranchers in western Wyoming start to hay in late July and early August because of their dependence on the snow melts. They get only one cutting, while folks in Sheridan get two and those in Wheatland get about three, and that’s due to the water cycle they have learned to work with as a result of Wyoming’s landscape.  

The landscape today is shaped by what happened between 570 million to 266 million years ago.

Wyoming is situated in the western United States, within the Rocky Mountains. Until 66 million years ago Wyoming was mainly at or below sea level. During this period several thousand feet of thick layers of sediment were deposited on a flat surface – the Precambrian granites and granite gneisses rock dated at approximately 2.8 billion years old today.

When the sediments were deposited in the sea and along the coast they formed a nearly flat feature. As tectonics occurred along the western coast of the U.S., compression forced caused the sediments to compress and uplift to form mountains. These sediments were folded and compressed and then eroded into the features we see today. This erosion exposed much of the sediments that were deposited prior to 66 million years ago.

Today Wyoming is broken down into physiologic providences. These providences are based on their structure and orientation on the landscape. The basins are filling with the sediments eroded from the mountains that border the basins and the sediments that were deposited during the transgressive and regressive series of sea levels from 570 to 66 million years ago. As the mountains are eroded away, the core complex of the mountain ranges becomes exposed. These exposed cores are Precambrian granites and granite gneisses rock dated at approximately 2.8 billion years of age. Though we know very little about the formation of the basement rocks, they are identified and dated with little history.

The majority of the mountains in Wyoming were elevated about 60 million years ago, since the oldest rocks in the cores of these mountains are about 2.8 billion years old and have been eroded down to their current elevations. The Teton Range is a medium-sized but distinct mountain range, and the eastern slopes, which extend above the glacial lakes at the base, resulted from intermittent but major movement of more than 20,000 feet on a steeply inclined fracture plane that slopes to the east. The rugged mountain peaks have been carved from the elevated segment of the crust. Glacial processes have produced the Matterhorns and U-shaped valleys that are the characteristic landforms.

The Tetons are young by geologic standards, having reached their current height less than 10 million years ago, and this places them among the youngest range in the Rocky Mountains. Numerous minor earthquakes within historic time in Jackson Hole attest to the fact that these mountains may still be growing.

Although the high Yellowstone Plateau and Absaroka Range of northwest Wyoming present a mountainous terrain, these areas are remnants of a plateau that coincided with the top of a vast pile of nearly horizontal sheets of rock materials derived from nearby volcanic vents in the Yellowstone area. Once the pile of volcanic debris had accumulated, the region was subjected to the ever-present forces of erosion, which cut deep valleys. The mountains are those of erosion – the deep dissection of relatively flat lying rock layers – and are a classic example of this type of development.

The Yellowstone Plateau is a seismically active area with intruding molten rock several thousand feet below the surface in some places. Another indication of the thermal activity are the geysers found in the park beneath the plateau, and researchers have documented recent uplift of portions of the Yellowstone Caldera (remnants of the older volcano), rising at rates up to approximately a half inch per year. The most recent volcanic activity on the plateau, however, has been dated around 600,000 years before the present time.

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