Geology of Wyoming

When visitors first see Wyoming's vast prairies and mountain ranges, their interest is normally attracted to the spectacular views, the open space, the accompanying wildlife, and the sparse vegetation. Often, little if any thought is given to the geologic history that produced much of what they are seeing. The state's present landscape resulted from a long series of events reaching back in geologic time almost to the inception of the Earth as a planet and continuing to the present.

Scientists can determine the ages of rocks by measuring the radioactive decay of particular elements in the rocks. In Wyoming, the oldest dated rocks are certain granites and granite gneisses which occur in the cores of the larger mountain ranges and are at least 2.8 billion years old. Relatively little, however, is known about these most ancient rocks.

The better documented geologic history of Wyoming dates back about 570 million years, to a time when sedimentary layers covered the ancient granite. The relationship of sediments overlying crystalline rocks is clearly evident around the margins of the mountains where erosion has exposed the interface between these rock units. The sedimentary layers are distinguishable from each other because they have different color, texture, and mineral composition, and because they often contain different assemblages or fossils (the preserved remains of animals and plants that lived during the time these rocks were deposited).

To understand the origin of the landforms in Wyoming, one must accept the fact that the Earth's crust, which is some 22 miles thick and composed of dense, crystalline igneous and metamorphic rocks, subsided relative to sea level for a long period of time. While Wyoming was below sea level, several thousand feet of relatively flat-lying sedimentary rock accumulated below the oceanic waters or along the shoreline areas.

Using the erosional surface that separates underlying Precambrian rocks from overlying sedimentary rocks as a datum plane, the following events can be deduced. The greatest subsidence below sea level occurred in western Wyoming, as evidenced by the 30,000 feet of sediment that accumulated in a north-south-trending trough since the beginning or Cambrian time. A broad area east of this trough on a shelf adjacent to deeper water received much less sediment, perhaps 10,000 feet, during the same time interval. Throughout this long period of time, there were minor upward and downward oscillations of the crust, but the principal activity in Wyoming was subsidence and accumulation of sediments. Wyoming was last at or near sea level during the close of the Cretaceous Period, approximately 66 million years ago.

Structural cross section of Wyoming today.



Physiographic provinces of Wyoming.
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