Mill Creek Canyon

By William Case

Mill Creek Canyon contains Mississippian- to Triassic-age marine and shoreline marine rocks and Jurassic-age sand-dune rocks. The following descriptions begin with the oldest rocks.

The oldest rocks in Mill Creek Canyon are visible from the road only by looking through the trees toward the south ridge skyline. These rocks are part of the Mississippian- and Pennsylvanian-age formations including Deseret and Round Valley Limestones and Humbug and Doughnut Formations.

The Pennsylvanian Weber Quartzite, originally a sandy marine beach, is common in the canyon particularly at its western end and mouth. Locally, the brown quartzite was dramatically folded and crushed by thrust faulting during the Sevier Orogeny about 85 million years ago.

The Permian Park City Formation is a dark gray limestone that contains fossil shells (brachiopods) in Rattlesnake Gulch at mile 0.7. The best exposure of the Park City Formation is in a road cut at mile 4.8, near the White Bridge Picnic Area.

The Triassic Woodside Shale is a reddish siltstone and fine-grained sandstone deposited in layers up to several inches thick. The Woodside Shale is exposed in road cuts partly covered by vegetation near mile 6.8 and the Clover Springs Picnic Area.

The Triassic Thaynes Formation contains abundant marine fossils such as corals, shells, and other marine animal parts on trails north of Camp Tracy scout camp. The most visible feature of this gray limestone is a massive limestone ridge that juts above vegetation on the north side of the canyon. The massive limestone meets the road at mile 5.5 where the road makes a sharp turn to the southeast.

The Triassic Ankareh Formation and Jurassic Nugget Sandstone are the youngest bedrock units in this canyon. They are seen near the northern-most ridge skyline of the canyon, and red Nugget Sandstone boulders are in debris-flow gravel near mile 4.8.

During the recent Ice Age, glaciers carved some of the upper Mill Creek tributaries and deposited moraines, such as the one seen at mile 7.1. Glaciers did not flow down the main canyon, thus, the canyon maintains the characteristic “V-shape” caused by stream erosion.