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The Origin of Comb Ridge
Robert Fillmore, Western State College of Colorado in Gunnison, CO
(An excerpt and images from his new book: Geological Evolution of the Colorado Plateau)
Comb Ridge is a lofty sinuous spine of red sandstone that stretch­es over 80 miles across northern Arizona and southeast Utah. This monocline, as these structures are called, begins near Kayenta and snakes northward to fade away near the west flank of the Abajo Mountains. Monoclines are a peculiar component of the Colorado Plateau, with their long ridges of steeply tilted strata in a region otherwise known for its miles of flat-lying sedimentary rocks. They are hard to miss. Although not confined to the Colorado Plateau, their concentration here is unique. Similar structures make up the San Rafael Swell, Capitol reef, and Colorado National monument near Grand Junction. All are closely related in origin and timing.
ramp of Comb Ridge. Another notable result of this uplift is the ensuing deep incision into the uplift by energized rivers as their runoff seeks a path to lower elevations. The deep narrow canyons of Cedar Mesa owe their existence to Monument Upwarp.
Monoclines are a peculiar component
of the Colorado Plateau, with their long ridges
of steeply tilted strata in a region
otherwise known for its miles of
flat-lying sedimentary rocks.
They are hard to miss.
The term monocline refers to a single-limbed fold; in simple geometric terms, a gargantuan ramp. The ramp of steeply tilted strata separates uplifted regions from those that have dropped downwards, relatively speaking. At Comb Ridge the uplifted re­gion lies to the west in the form of Monument Upwarp, which in­cludes Cedar Mesa, the San Juan River Canyon, and Monument Valley. The net result is older Paleozoic rocks to the west separat­ed from younger Mesozoic rocks immediately east by the colossal
The monoclines formed at the same time as the jagged Rocky Mountains of Colorado and Wyoming. This mountain-building svent is the result of a widespread pulse of compression that rip­pled across western North America 65 to 50 million years ago. The stronger crust of the Colorado Plateau resisted the large scale uplift of alpine mountains but did not escape unscathed.
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Geological Evolution of the Colorado Plateau By Robert Fillmore
Robert Fillmore's clear, easy-to-read text documents spectacular features of the eastern Colorado Plateau, one of the most interesting and scenic geologic regions in the world. The area covered in detail stretches from the Book Cliffs to the deep canyons of the San Juan River area. The events that shaped this vast region are clearly described and include the most recent interpretations of ongoing geologic forces. The book also includes mile-by-mile road logs with explanations of the various features for most of the scenic roads in the region, including Arches National Park, Canyonlands National Park, and the Natural Bridges area.
Robert Fillmore is professor of geology at Western State College of Colorado in Gunnison. He is the author of Geology of the Parks, Monuments, and Wildlands of Southern Utah.





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