Geology Happenings

All five national parks in Utah are geoparks in that they were set aside for their geologic heritage. Four of the five parks have a lot in common: they are dominated by a singular rock type (sandstone) and share many of the same rock layers (formations to geologists). For example, the Navajo Sandstone that makes up the Petrified Dunes in Arches and Mesa Arch in Canyonlands National Park. It also forms Capitol Dome in Capitol Reef and the cliffs of Zion. Visiting national parks in Utah during the summer usually means experiencing the intense heat of the desert with the spring wildflower season long over.

But one national park in Utah is different. Bryce Canyon National Park has much cooler temperatures and is resplendent with blooms in the summer. And it features a distinctive rock layer: the Claron Formation. Although Bryce is about 225 miles from Moab, Bryce Canyon is a destination for both Utah residents and visitors from all around the world. And therefore, it makes sense to try to get to know the rock layer that makes Bryce the wonderland of geology that it is.
Arches, Canyonlands, Capitol Reef, and Zion all feature big, bold, strong, solid, and sheer cliffs of rock. Sometimes these cliffs host natural arches or alcoves with dripping springs, but they are robust and sturdy. Bryce Canyon projects geologic delicacy and intricateness. Rocks at Bryce are finely layered and eroded into fantastical spires known as hoodoos. Even the colors of the rocks at Bryce are more delicate; they seem soft in their pastel hues of pink and ginger that seem almost ethereal.

The softness of the rocks at Bryce is also literal, resulting in an incredibly fast erosion rate of 2 to 4 feet (!!) per hundred years. With the exception of volcanic eruptions, and geologically sudden events like big earthquakes, landslides, and major floods, such changes in landscape are typically not something that people can directly see.

Hoodoos develop because of slight differences in the hardnesses of the different rock types found in the Claron Formation. The Claron is mostly made up of thin layers of sandy limestone, lime-rich mudstone, and siltstone. All of these rock types are more erosive than the sandstones of Utah’s other national parks, but slight differences in the hardnesses of adjacent layers in the Claron cause hoodoos to form through differential erosion. Harder layers form the intervals that are wider than the rest of the hoodoos and softer layers make recesses or indentations. Once a hoodoo forms, it continues to erode rapidly from all sides as well as from the top, eventually either collapsing completely or simply just disintegrating more gradually.

Vertical fractures called joints also play a role in the formation of hoodoos. The fractures serve as conduits for groundwater which speeds up erosion and weathering, and can leave free-standing pillars between them.

This process of differential erosion on a grand scale is responsible for the variations between massive cliffs and the long gentle slopes beneath them that are found throughout canyon country. At Bryce, differential erosion operates on a fine scale between individual sedimentary beds that are sometimes even less than a foot thick.

The Claron Formation is also substantially younger than the rock layers found in the other national parks in Utah. Most of the rocks the other parks date from the Mesozoic Era (more popularly known as the Age of Dinosaurs) and are largely between about 270 million years and about 70 million years old. The Claron Formation dates to between about 56 and 40 million years ago, during the Cenozoic era, also known as the Age of Mammals (although fossils are very rare in this rock layer). Geologists had thought that the Claron had long been deposited in lake environments, but more recent research has revealed that most of the formation was deposited by sluggish rivers and on their floodplains, with only some of the beds being deposited in lakes.

The pink and orange colors in the Claron Formation are due to the presence of small amounts of iron oxides that formed during soil development when the sediments were being deposited.

Although Bryce is a park apart compared to the other national parks in Utah, it does share geology (and scenery) with spots on the Powell Plateau north of Bryce as well as with Cedar Breaks National Monument. Cedar Breaks is located northwest of Bryce and is located above Cedar City. It also features the colorful Claron Formation and its intricately carved hoodoos. Each national park, monument, or other scenic spot has its own unique set of characteristics that sets them apart from other places, but at least in Utah, so much the scenery derives directly from bedrock. Hence, the story of Bryce Canyon National Park is the story of those colorful thin beds of limestone and siltstone of the Claron Formation.

A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology