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Geology HAPPENINGS February 2018

What’s in a Name? The Science Behind Rock Names
By Allyson Mathis

People who live or spend any time in Moab, even for a quick visit to canyon country, know the names. Navajo Sandstone. Entrada Sandstone. Chinle Formation. Organ Rock Shale. Trail descriptions in guidebooks refer to the rock layers. For example, the Moab Rim Trail follows ledges in the Kayenta Formation. And the White Rim Road in Canyonlands National Park literally sits atop the White Rim Sandstone.

Many of the most-loved areas in southeastern Utah take their defining characteristics from the rocks exposed there. Arches National Park is best known for its natural arches carved into the Entrada Sandstone. The Slickrock Bike Trail exists only because of the distinctive Navajo Sandstone. The Island in the Sky is held up by the Wingate Sandstone and Kayanta Formation.

Informally, we refer to these rock units as “rock layers,” and in many ways this terminology is very applicable. Most of the rocks found in the Moab area are sedimentary (e.g., they were deposited as sediment: sand, silt, mud, etc.) and were deposited in mostly flat layers. In fact, Moab can be described as having “layer cake geology” as the rock layers can be easily seen as broad sheets.

In geology, rock layers are known as “formations.” As with anything scientific, there is a full technical definition for the term. According to the North American Commission on Stratigraphic Nomenclature, the body responsible for overseeing the naming and classification of rock units, a formation is:
A body of rock identified by rock characteristics and stratigraphic position that is mappable at the earth’s surface or traceable in the subsurface. A formation may be defined on a single rock type, or repetitions of two or more rock types. The formation is the fundamental unit in lithostratigraphic classifications.

Utah Geological Survey. Used by permission.

In toto, this definition reads like something out of a law school textbook, but if it is broken into its components, it becomes not only comprehensible, but meaningful and helpful to people who appreciate the geology of southeastern Utah.

Let us examine the main parts of this definition to gain a clearer understanding of specifically what a formation is.

1. A body of rock identified by rock characteristics. Formations are defined based on their rock type(s), or lithology. Some formations consist of only one rock type, such as sandstone, limestone or granite. In these cases, the formation’s “last name” is that rock type. Prominent Moab area examples include the Wingate Sandstone and Mancos Shale. When a rock layer includes more than one lithology, it is termed a formation. The Moenkopi Formation consists of layers of siltstone and mudstone, and the Morrison Formation includes conglomerates, sandstones, siltstones and mudstones.

2. Stratigraphic position. Stratigraphy is the branch of geology that studies the order and relative position of rock layers, also known as strata. Rock layers are deposited through time sequentially and are always found in the same order. The Chinle Formation was deposited before the Wingate Sandstone and is always found below the Wingate cliffs. The Kayenta Sandstone sits on top of the Wingate and is in turn overlain by the Navajo Sandstone.

3. Mappable at the earth’s surface or traceable in the subsurface. Geologic maps show what rock layers are exposed on the surface along with related geologic information using color codes and symbols. The Navajo Sandstone is represented on the Geologic Map of the Moab Quadrangle using a light green color and the symbol “Jn,” which indicates Jurassic-aged Navajo Sandstone.

The sand dune environment when the Navajo Sandstone was deposited. Illustration from Geological Evolution of the Colorado Plateau by Robert Fillmore. Used by permission of the author.

4. Fundamental unit in lithostratigraphic classification. Formations, just like species in biology, have a formal system of naming. (Thankfully, unlike in biology, there are not formal and informal names in geology as they are one and the same.) The name is always taken from the geographic location where geologists first formally studied it. There, they define a “type section” along with a detailed description of the formation from bottom to top.

In biology, each species that has been scientifically-named has a type specimen archived in a museum. Because it is not possible to house an entire outcrop of a rock layer in a museum, type sections serve the same role that biological type specimens do. Type sections for rocks found near Moab include Entrada Point in the San Rafael Swell and near Kayenta, Arizona.

The Navajo Sandstone, which is exposed all around Moab (in Arches and Canyonlands national parks, in Sand Flats and Behind the Rocks), as well as in Zion and Capitol Reef, provides an excellent example of what a formation is. The rock type is sandstone, made up of sand grains that have been cemented together after deposition in large dunes in a sand sea (or erg). It was first described based on exposures of the sandstone in Navajo Nation. The sand dune field was located in what is now Utah and northern Arizona when the climate was even drier, winds were more constant and stronger, there was a large supply of sand and dinosaurs roamed the earth.

Each formation is a record of the time and place in which it was deposited in addition to making up characteristic aspects of today’s canyon country. We will learn more about the Navajo Sandstone and other formations found in and around Moab in future Geology Happenings columns.




The Chinle Formation, Wingate Sandstone, Kayenta Formation and Navajo Sandstone at the Island in the Sky. The dotted lines represent the approximate location of the contacts, or boundaries between the rock layers.

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