Echo Canyon Conglomerate : Witches Bluffs EarthCache

Near the town of Echo in northern Utah is a cluster of reddish-brown natural monuments called The Witches (also known as Witch Rocks, Witches Rocks, Witch Bluffs, or Witches Bluffs), composed of the Echo Canyon Conglomerate.

In 1858, army Captain Albert Tracy described them in his journal as “witch-like” and “so singularly like figures in kirtles [long skirts] and steeple-hats, or bonnets that they have received the appellation [Witch Rocks]”. By using your imagination (and perhaps squinting a bit), you can picture a coven of witches in long robes and witches’ hats standing on the hillside.

Nearby Echo Canyon has long been used as a main thoroughfare between southern Wyoming and northern Utah, first by Native Americans, fur trappers, and explorers, then by wagon trains on their way to Salt Lake City or other points west. Before the interstate highway, passengers on the Overland Stage and then the Union Pacific Railroad also made their way through the canyon.

At the town of Echo, the canyon opens into the Henefer Valley where most of these travelers rested and marveled at the unusual rock formations, some even drawing sketches or taking photographs of The Witches.

Geologic Information:

Between 170 and 40 million years ago, western North America experienced a mountain-building period called the Sevier orogeny. Dense oceanic crust beneath the Pacific Ocean collided with and moved under the lighter continental crust of North America.

This convergence generated compressional forces that produced low-angle thrust faults within the crust. Transported tens of miles eastward along these thrust faults, rock formations were pushed up and over adjacent rock layers, forming the Sevier mountain belt.

The Echo Canyon Conglomerate was deposited about 90 to 85 million years ago when Sevier thrust faulting in Utah had reached its peak. The conglomerate is composed of sandstone and quartzite pebbles, cobbles, and boulders eroded from the mountainous areas to the west and northwest. Streams then carried these sediments off of the highlands, eventually depositing the heavier material in large alluvial fans (fan-shaped stream and debris-flow deposits).

In more recent times, the elements have eroded areas of the conglomerate near Echo and within Echo Canyon into the fantastic shapes we see today; in addition to The Witches, there are rock structures named Pulpit Rock, Castle Rock, Devils War Club, Sphinx, and Sentinel Rock, to name a few.

The upper “caps,” or witches’ hats, are formed of a lighter-colored conglomerate layer cemented into a harder mass than the softer, underlying conglomerate layer that forms the witches’ “robes.” The harder cap rock erodes more slowly and helps protect the rock underneath.

But as a witch becomes more and more slender, her hat eventually falls off and can no longer protect her, and thus, like the infamous Wicked Witch of the West, she succumbs to the effects of water and gradually melts to the level of the surrounding landscape.

The location of the eastern edge of the Sevier orogeny was determined by conglomerates largely made up of boulders that would have been shed from the eastern and steepest edge of the rising mountains. Such conglomerates can be seen throughout Utah in Echo Canyon, the Red Narrows in Spanish Fork Canyon, and in Leamington Canyon near Delta, Utah. Today Sevier faults at the surface have been broken up and tilted steeply from their original gently dipping positions due to the extension of the Basin and Range faulting. The earliest thrusts of the Sevier are located furthest west with each newer thrust cutting the older thrust. This pattern caused the older thrusts to ride on top of the younger thrusts as they moved eastward. The Paris-Willard thrust in Utah was determined to be the oldest thrust in the series using this pattern. The youngest thrust is the Hogback in Wyoming.

The Sevier thrust belt in Utah can be divided in two, north of Salt Lake City and South of Salt Lake City. The thrusts to the north are much better understood because oil and gas are often associated with them. The northern portion runs through present day Utah, Idaho, and Wyoming. The southern portion stops around Las Vegas. The total crustal shortening of the northern portion was roughly 60 miles.

While continental margins are typically the most deformed in orogenic events, the interior of continental plates can also deform. In the Sevier-Laramide orogenic events evidence for interior plate deformation includes folds, cleavage and joint fabrics, distorted fossils, persistent faulting, and calcite twinning.