With at least 18 different rock layers (formations to geologists) exposed in the canyons, cliffs, and mesas of southeastern Utah, there is a lot of geology for amateur enthusiasts, tourists, and recreationalists to know. Each layer has its own distinctive features which in turn create different defining characteristics of canyon country. For example, four-wheel-drive roads frequently follow ledgy sandstones in the Kayenta Formation and mountain bikers love riding the Navajo Sandstone’s slickrock domes.
Even though each layer records a distinct part of the rock record and was deposited at a specific time in geologic history, some of layers look alike, especially viewed in passing while on a scenic drive or on the trail. And some have names which make them easily confused with other layers. Together, there are at least eight pairs of rock layers where two formations may get confused with one another.
This Geology Happenings column is the first of two on this topic. Both articles will provide a quick guide on how to tell these pairs apart as well as some tips that are useful in identifying rock layers found near Moab. Part 2 will be in the March issue of Moab Happenings.
Sources of Confusion
In general, there are three main reasons that rock layers in canyon country get confused with other layers: they have similar names, they look similar because they were deposited in similar environments, and they look like layers immediately above or below them in the rock record. This article will cover rock layers with similar names and begin discussing layers that look similar to one another. The March column will continue with similar-looking layers, and address the layers that look like ones below or above them.
Rock Layers with Similar Names
Most of easily confused rock layers that have similar names do not look alike one another. The Cedar Mesa Sandstone and the Cedar Mountain Formation have nearly identical names, but would never be confused based on their lithology (rock type).
The Cedar Mesa is a eolian in origin, meaning that it was deposited by wind in an ancient sand dune environment. The Cedar Mesa makes the rock spires of the Needles District of Canyonlands National Park. The Cedar Mountain Formation is found north and west of Arches National Park and consists of pastel-colored slopes and thin drab-looking sandstone beds. The Cedar Mountain is probably best known for its diverse dinosaur fauna, but the Cedar Mesa doesn’t contain any dinosaur fossils since it was deposited before the time when dinosaurs lived. If these two rock layers had names that were as dissimilar as their rock types and fossil records, they would never get mistaken for one another.
The Carmel and Curtis Formations are another pair of layers that can get mixed up because of their names. Personally, I frequently have think to make sure that I am referring to the correct layer, although near Moab these two layers are mostly known by their members (unique subdivisions) that have distinctive names making them less likely to be mixed up: the Dewey Bridge Member of the Carmel and the Moab Tongue Member of the Curtis Formation. These members also do not look alike: the Dewey Bridge is deep maroon in color, and the Moab Tongue is a nearly white eolian sandstone.
Rock Layers that Look Similar
Sediments that accumulate in similar depositional environments have similar characteristics. For example, dunes are almost always made up of sand, and rivers may deposit sand and gravel in their channels, and clays and silt in their floodplains. In turn, most eolian sandstones look somewhat like other eolian sandstones as do most fluvial deposits.
Rocks formed on floodplains usually consist of siltstones and mudstones with ancient soil horizons (paleosols). The Chinle and Morrison Formations both contain extensive floodplain deposits as well as thin river channel sandstones. The candy-striped mudstone slopes in both layers look a lot alike.
The key in telling these two layers apart is identifying the rock layers above and below them; in other words, pay attention to stratigraphy. (Stratigraphy is the science of rock layers.) Rock layers are a historical record, and like all history, there is a set sequence of events. In stratigraphy, there is a set sequence of formations for any given area.
The Chinle and Morrison Formations may look alike, but they are in unique places in the stratigraphic column. The fact that the sedimentary sequences in southeastern Utah mostly have not been folded, faulted, or tilted means that is it relatively easy to track where a layer is within the overall stratigraphic column. The Chinle is found below the prominent cliff-forming Wingate Sandstone. The Morrison is much younger than the Chinle, and is below another soft layer, the Cedar Mountain Formation.
Another difference is that the Morrison Formation mostly contains sandstone beds in its lower half (in the Salt Wash Member), and channel sandstones are found interspersed with the mudstones in the Chinle.