Geology -- Fillmore Ranger District

Pahvant MountainsPahvant Range and Canyon Mountains

The Pahvant Range runs from Clear Creek Canyon at the south to Scipio Pass on the north. From there, the Canyon Mountains extend northward to the Sevier River at Leamington Canyon, the lowest point in the Fishlake National Forest. The Pahvant Range truly has a split personality, as the rocks on the west side are over twice as old as those on the east.

The eastern front of an ancient mountain range runs down the center of the Pahvant Range of today. West of this front, the rocks are limestones, quartzites, sandstones, and shales that were deposited below sea level in a gradually deepening ocean basin called a geosyncline. Compressive forces of continental collision thrust huge sheets of this sedimentary sequence up and over other sheets to form high mountain ranges. East of this front, the rocks are conglomerates, sandstones, and shales shed off these ancient mountains and deposited at their base. Because this debris was deposited on land, much of it is colored red and yellow, in contrast to the drab grays and tans of the marine deposits on the west side of the Pahvant Range.

A second difference between the two sides of the range is that the rocks on the west side are contorted and stand vertically in many places due to over thrusting. On the east side, the rocks are flat lying or dip slightly to the east, due to subsequent uplift. Thirdly, the Paleozoic rocks, on the west side, are well indurated so they form numerous high cliffs and steep slopes while the Tertiary rocks to the east are generally not as well indurated so the slopes are generally more rounded. Shortly after the deposition of these Tertiary rocks, the Pahvants were uplifted along block faults to form the range we see today and to start the present period of erosion. Since this mountain block is lighter than the blocks on either side, it is continuing to rise and the bounding faults are still active.

The Canyon Mountains were formed from the same material and in the same manner as the Pahvant Range. The main difference is that the bulk of these mountains are composed of the overthrust Paleozoic sequence. Because they lie west of the Pahvant, only their northeast corner is composed of debris shed off the mountain front. The Church Hills area, at the south end of the Canyon Mountains, is also underlain by mountain debris, but this material is much younger and was deposited as basin fill after the area had been uplifted and closed basins formed.

Written by Andrew E. Godfrey, Geologist
 

 

 

 

 



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