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Geology HAPPENINGS July 2018

The Moenkopi Formation: Red Beds and Ripple Rock
By Allyson Mathis
NPS photo Neal Herbert

Red is the dominant color of the canyon country landscape. Most of the rock layers around Moab have one of an array of red hues, ranging from orange red to rust red to burgundy to reddish brown. The Moenkopi Formation is perhaps the reddest of Moab’s red rocks, reaching a deep chocolate red color.

Geologists have a technical term for these red layers of rock: Red beds. Bed is, of course, the geological word for an individual layer or stratum of rock. A bed can be of almost any thickness, from less than an inch thick to many feet thick, although many of the beds in the Moenkopi Formation are thin, usually no more than a few inches thick. The red color comes from iron oxides (hematite or related minerals), either as coatings on sand grains or impregnated in clay minerals.

The Moenkopi has other defining characteristics in addition to its chocolate brown color, and these features help geologists reconstruct the environments in which the unit was deposited. The Moenkopi’s signature feature is the ripple marks that are common throughout its sandstone layers. Ripple marks form when wind or water currents stack sediment into low ridges. Some of the ripple marks in the unit are steeper on one side than the other, indicating that they were formed by river currents, which flow only one direction. Other ripple marks show that currents flowed in two directions, such as what happens as tides go in and go out.

Also found in the layer are mud cracks that formed as wet sediment (usually clays) dried out and cracked. Together, the ripple marks, mud cracks and other features tell the story of what conditions were like when the sediments were being deposited. These features (called sedimentary structures) are clues to geologists, much like evidence in a crime scene that allow detectives to decipher what happened in the past.

The Moenkopi red beds were deposited approximately 240 million years ago in a variety of environments near the coastline of the supercontinent Pangaea. The climate then was hot and arid like it is today, but otherwise the landscape was completely unlike today’s Utah canyon country. In the Moab area, sluggish rivers flowed across a low, nearly flat landscape of tidal flats, mudflats, floodplains and river deltas, and the nearest ocean swept these shores close by to the southwest.

The Moenkopi is usually found in the lower part of slopes that are found throughout canyon country. Rock layers (known as formations to geologists) are always found in order representing the sequence in which they deposited. The oldest rocks are at the bottom, and, as sediments are deposited, layers get younger progressively towards the top. A sequence of rocks exposed along the edge of a canyon can be viewed like a stack of pancakes; the oldest is at the bottom, and the youngest is at the top.

The Moenkopi Formation is the oldest in a set of four rock layers that are seen all around Moab and throughout much of southeastern Utah. Together these four rock layers form slopes capped by vertical cliffs that together rise more than a thousand feet high. The slopes are made up of the Moenkopi and overlying Chinle Formations, because both of these units are relatively soft and are not strong enough to hold up vertical cliffs like the rocks above them. Above these two are the Wingate Sandstone and the Kayenta Sandstone. Much of the characteristic scenery along the Colorado River, in Canyonlands National Park, on the Moab Rim, and in Lockhart Basin, is the formed by these four rock layers. (There are other rock layers in the area that are older or younger than this foursome, but these four rock layers make up the classic slope-and-cliff combo seen throughout canyon country).

While the Moenkopi Formation is present all around Moab, sometimes it may be hard to see because it is often covered by rockfall debris from the layers above. It is easier to see on the slopes below Dead Horse Point State Park or below the Island in the Sky in Canyonlands, especially where it sits on top of the White Rim. Perhaps the best place to see the Moenkopi Formation is in Capitol Reef National Park, where thinly-bedded slopes of Moenkopi Formation rise above the Scenic Drive and form the base below landmarks such as Chimney Rock.

Each rock unit around Moab has its own “personality,” if you will. The slickrock of the Entrada Sandstone in Arches National Park, the tan and gray badlands of the Mancos Shale in the Book Cliffs, and the vertical cliffs of the Wingate Sandstone are a few examples. The Moenkopi sometimes may not be as distinctive as some of the other formations in the area, but it is the only unit that contains abundant ripple marks and has characteristic thin chocolate brown beds. And it records an important chapter in the geologic history of what is now southeastern Utah.


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