Geology of Cedar Breaks

Cedar Breaks

The rocks of Cedar Breaks National Monument reveal the powerful forces of geologic change that have created the canvas upon which today’s remarkable landscape is painted. Standing at the rim of Cedar Breaks amphitheater, you gaze into a high-altitude wonderland of colorful cliffs and pinnacles. Yet the rocks tell stories of ancient seas, violent volcanoes, and a time when a visitor to Cedar Breaks would have found themselves afloat in a body of water the size of modern-day Lake Erie.

Late Cretaceous- 90 Million years ago

Ancient Mountains & Seas Hidden in the forested Ashdown Gorge lie the oldest rocks in the monument, relics from a time when Cedar Breaks would have been beach front property! In the late-Cretaceous period (~90 million years ago), southwestern Utah was a shoreline. To the east was a shallow sea. 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. These sediments became the fossil-rich brown sandstones and shales of the Straight Cliffs and Wahweap Formations, which are very prominent on the drive to Cedar City via Highway 14.

Eocene- 50 Million years ago

Lake Claron Utah’s First Great Lake: By the end of the Cretaceous, the sea had retreated and the Rocky Mountains were beginning to rise to the east. Now surrounded by mountains on all sides, Southwest Utah became a closed basin home to ancient Lake Claron. By about 60 million years ago, streams were bringing sand, silt and mud into Lake Claron, where it settled to the lake bottom. Small organisms like snails fed in the muddy ooze, adding their calcareous skeletons to the detritus upon their death. Trace amounts of iron in the sediment would combine with oxygen and water, “rusting” many of the layers into warm red, orange, and pink hues. These processes continued for millions of years, gradually filling the basin. During wet periods, the lake level would rise. During dry periods, the lake level would fall. Ancient soils preserved between the rock layers suggest that at times the lake would dry up entirely. This constantly (but gradually) changing climate over ~25 million years created the many intricate and vibrantly colored layers of the Claron Formation, the most prominent rock layer at Cedar Breaks and nearby Bryce Canyon. 

Early Miocene- 20 Million years ago

An Explosive Landscape: A suite of volcanic rocks above the rim of the amphitheater point to the arrival of violent and turbulent times, just as the days of the tranquil Lake Claron were ending about 35 million years ago. Soft grey rock near North View and on the lower slopes of Brian Head Peak belongs to the Brian Head Formation. This layer contains material erupted from volcanoes to the west near the Utah/Nevada border (more than 60 miles away) as well as sediment that settled to the bottom of the dwindling lake. These volcanic eruptions, among the largest in Earth’s history, sent pyroclastic flows (hot clouds of ash, volcanic gasses, and molten rock fragments) racing across the landscape. These flows form a volcanic rock called tuff; good examples are the Leach Canyon and Isom Formations found near the summit of Brian Head Peak.