Fishlake National Forest
The dark, somber hues of the rocks comprising Monroe Mountain, also known as the Sevier Plateau, stand in stark contrast to the vivid colors of the Colorado Plateau to the east. This does not mean, however, that the plateau has had a bland history. To the contrary, the mountain has been racked by violent eruptions, earthquakes, and giant landslides.
Beginning about 25 million years ago the area was a scene of intense, explosive volcanic activity. Within what is now called Monroe Mountain there were two main centers, one located around Signal Peak and the other around Langdon Mountain. Eruptions of these volcanoes were so violent that they created their own weather. Clouds of volcanic dust and gasses rose so high that they condensed, forming their own thunderstorms. The resulting torrential floods washed boulders and other debris down the newly formed sides of the volcanoes. Examples of these flood deposits can be seen along the walls of Pole Canyon.
As the eruptions proceeded a tremendous amount of material was removed from the pool of molten lava deep within the earth creating a large void. The overlying rocks subsided into this void much as a piston slides down a cylinder. The resulting depression is called a caldera. Monroe Peak Caldera is tremendous, measuring about 14 miles east-west and 11 miles north-south, from Poverty Flat on the west to Koosharem on the east, and from Tenderfoot Ridge on the north to Big Flat on the south.
This period of violent activity was followed by one of quiet. Erosion reduced the land to an area of low relief where streams slowly meandered. Between the low hills there were swamps and lakes. Gravels and sands were deposited along the stream courses while sands and muds were deposited in the swamps and lakes. This period lasted from about 14 to 7 million years ago and the sediments are known as the Sevier River Formation.
About five million years ago the period of quiescence ended. Forces within the earth lifted and bent large blocks. One such block is Monroe Mountain. On the east side, the rocks dip off to the east and are broken by several small faults that drop the east side downward. The west side, however, is bounded by a truly spectacular fault system. Stretching from about Gunnison to the Grand Canyon, this fault system has dropped the west side relative to the high standing east side. In places there is about two miles of displacement between the two sides. All this movement along the fault did not occur at one time. Only about 20 feet or less normally happens during one earthquake. Over several million years, with the earthquakes happening about once every 400 years, the distance built up.
As the relief between the mountain block and the surrounding valleys built up, the steep mountain slopes became more unstable. Repeated shakings by earthquakes loosened slope material and sent it crashing down as some of the larger landslides in the state of Utah. Thompson Creek Landslide near Monroe (this one is supposed to be the largest in Utah), Elbow Landslide near Marysvale, and The Brink near Burrville are the largest. There are numerous other landslides ringing Monroe Mountain. Thus, the gently rolling top of Monroe Mountain stands in stark contrast to the steep slopes surrounding it.
Richfield Ranger District
115 East 900 North
Richfield, Utah 84701