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The Geology of Rock Canyon

The earliest pages of geologic history recorded in Rock Canyon began about 800-900 million years ago and continue through several cycles of deposition, deformation, and erosion until the present time.

The history of Rock Canyon can be divided into four main phases. These phases do not include the entire span of earth history because the geologic history recorded in Rock Canyon is incomplete. It is like a book with some of the pages missing. In order to fill in the gaps in the geologic record, the missing pages must be found or at least references to them in other books. These references are found in the rock outcrops in other nearby areas. This guide will focus on only those geologic events that can be interpreted from the rocks that are found throughout the canyon.

Phase I began 800-900 million years ago and extends up to 200 million years ago. During this phase layers of sedimentary rock accumulated in mostly shallow marine environments near the edge of the continental shelf. It was, tectonically, a rather quiet time. The rocks were not deformed by plate motion. However, an examination of the ages of rocks found in the canyon shows that Ordovician and Silurian age rocks are missing. This break in the sequence of rock ages might indicate a pulse of tectonic activity during this early phase.

Phase II of Rock Canyon’s history began 200 million years ago and extends up to nearly 25 million years ago. During this period of time, The North American continent was actively colliding with numerous tectonic plates to the west. This collision caused the flat-lying rocks, deposited in shallow marine conditions during Phase I, to be uplifted, folded, and thrust eastward. This activity formed a mountain belt. This mountain building event is called the Sevier Orogeny. Erosion of these mountains left thick deposits of sandstone, conglomerate, and shale in central and eastern Utah. None of this erosional debris is visible in Rock Canyon, but these deposits can be seen in Spanish Fork Canyon a few miles to the south. What is visible are the folds, faults, and fractures created when the Sevier mountains formed during the orogeny.

Phase III began 25 million years ago as western North America began to extend and pull apart. This extension of the west continues up to the present day and is responsible for the formation of the Basin and Range province. In the Rock Canyon area, this extension is responsible for the uplift and formation of the Wasatch Mountains. The uplift has occurred along a major fault zone called the Wasatch fault.

Phase IV began about 1 million years ago and continues up to the present. This phase overlaps the last part of Phase III, but is generally separated out as a separate phase because it has left some very distinctive features in this area. This phase is often called the "Great Ice Age." However, the Great Ice Age has not been a single period of ice advance and cooler temperatures; evidence suggests that during the last 1 million years several glacial periods have occurred with intervening times (interglacial periods) of ice retreat and warmer temperatures. In Utah, during the last ice advance, a large lake, Lake Bonneville, covered much of the state. This lake formed as the climate allowed for more rain, and less evaporation. Utah’s higher mountain valleys, including those in Rock Canyon, were filled with large glaciers. Additionally, at the terminal edge of the glaciers, large amounts of glacial melt filled the valley and surrounding area with enormous amounts of water. It was much like filling a bath tub with water. Following this last glacial advance, the Earth went into an interglacial period which drastically reduced the size of Lake Bonneville. Today, the Earth remains in this interglacial period.