Colorado Plateau Geology
A geologic history of the Colorado Plateau
By John Weisheit
John Wesley Powell, a geologist who explored the Green and Colorado rivers in 1869, said that geology "is the noblest of the sciences," because he understood the connection geology has with almost every other scientific discipline. Many of America's finest geologists have made significant contributions to this science after completing comprehensive studies on the Colorado Plateau; it is a very famous place in the world of academia. Grove K. Gilbert, probably America's most famous geologist, noted that geologic study for most landscapes is very difficult, because the rocks are covered by water, soil, plants, or even ice and snow. But not so for the Colorado Plateau; probably no other landscape on earth has this much bedrock exposed for the beholder to marvel upon. Another famous geologist, Clarence Dutton, who beheld many a magnificent view in the Plateau country, used a wonderful and powerful phrase to describe this landscape: "The Great Denudation." For me, those three words tell the story of the Colorado Plateau better than any other. Physical geology is what these men taught and it is a fascinating natural history which requires no special tools; just keen observation. For their story is that of a landscape created and then a landscape destroyed; destroyed by the erosive power of a river. The chapter that you are now enjoying, is the part about the landscape being destroyed by a river-the Colorado River. Aren't you glad you came into the story at the right time? Who would ever think that destruction, or denudation, could be so pretty?
The geology of the earth is divided into four distinct lengths of time. These time zones are called the Precambrian (before life), the Paleozoic (early life), the Mesozoic (middle life) and the Cenozoic (late life). These divisions of time were designated according to the fossils found within the rocks; life forms from the simple to the complex. For example: Precambrian bacteria, Paleozoic fish, Mesozoic dinosaurs, and Cenozoic mammals. These time capsules are all represented on the Colorado Plateau and can be examined very closely with your keen powers of observation. Getting to the exposures is what is difficult and it requires a commitment of time to walk about the landscape. One of the easiest ways to view the geology of the Colorado Plateau is the way that John Wesley Powell did it in 1869-by boat. If you were to float down the rivers of the Colorado Plateau, as Powell did in 1869, you would first observe the youngest of rocks (Cenozoic), then the middle-aged rocks (Mesozoic), the old rocks (Paleozoic), and finally the ancient rocks (Precambrian).
This formula of time travel is logical when considering the Colorado River as the newcomer which is now actively eroding down into a bedrock that is progressively and immensely older. I like to think of the Colorado Plateau as a big, multi-layered, multi-tiered wedding cake that is being sliced, piece by piece, and then consumed until it is gone-all gone. We are not yet at that particular point of time, but it is important to realize that about one-half of the Colorado Plateau has already been consumed by the Colorado River; transported grain by grain to the basins of the ocean. If this process were to continue unhampered and given another 20 million more years of time, the Colorado Plateau will gone-reduced to sea level.
Cenozoic rocks on the Colorado Plateau are mostly deposits formed in large, continental fresh water lakes. These rocks erode into buttresses and castle-like towers. Desolation Canyon on the Green River is where these rocks are exposed.
Mesozoic rocks (seen in photo 1 on right) are mostly terrestrial rocks comprised of soft, colorful sandstones and shales that erode into slopes, mounds, ledges, and very pronounced escarpments. Approaches from the Green River and the Colorado River into Canyonlands National Park are where these rocks are exposed.
Paleozoic rocks (seen in photo 2 on right) are usually hard limestones that formed in marine environments. They erode into sharp, hard and resistant ledges and cliffs. Cataract Canyon in Canyonlands National Park is the place to find these rocks.
Precambrian rock (seen in photo 3 on right) is the basement rock of the Colorado River and is very old-1.8 billion years old-and very thick-25 miles thick. They consist of hard, black metamorphic rocks called gneiss and schist that were intruded by veins of molten rock (igneous materials). These rocks are relatively rare and found only on one Colorado River trip in Utah: Westwater Canyon above Moab, Utah.
I hope you will think seriously about coming to Utah to run some rivers with us. Besides being a lot of fun, it is also very educational. This place has produced a lot of famous geologists; maybe it will produce a few more. ♦
John Weisheit is a professional river guide who works for Tag-A-Long Expeditions in Moab, Utah. He is the editor of The Confluence, the journal of Colorado Plateau River Guides, an association of professional river guides that advocate education of the natural and human history of the Colorado Plateau.
Information courtesy of Utah Guides & Outfitters Association.
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