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Geology

Canyon Geol­ogy

Con­tent pro­vided cour­tesy of Brigham Young Uni­ver­sity Geol­ogy Department

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.