Geology of Cedar Breaks National Monument, Utah
The oldest rocks in the monument date from the late-Cretaceous period, when southwestern Utah was a shoreline of a shallow sea to the east. A bygone mountain range towered to the west. Caught in-between, the area was buried in thousands of feet of sediment shed from the disintegrating mountains and deposited along coastal rivers, lakes, and swamps.
The amphitheater, located near the west end of the Colorado Plateau, covers the west side of the Markagunt Plateau, the same plateau that forms parts of Zion National Park. Uplift and erosion formed the canyon over millions of years, raising and then wearing away the shale, limestone, and sandstone that were deposited at the bottom of an ancient lake 70 by 250 miles, known as Lake Claron, about 60 million years ago. It continues to erode at a pace of about 2 inches every 5 years. Atop the plateau, much of the area is covered by volcanic rock known as tuff, formed during cataclysmic eruptions around 35 million years ago.
The rocks of the eroded canyon contain iron and manganese in various combinations, providing brilliant colors that led Indians to call it the Circle of Painted Cliffs. Iron oxides provide the reds, oranges and yellows, while manganese oxides provide shades of purple. The color of the rock is soft and subtle compared to the hoodoos at Bryce Canyon.
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