Nature has been kind to the State of Colorado and has provided her with examples of many, many types of geological phenomena and land features. The State has exposures of a diverse variety of rocks, fossils of all phyla, and geomorphic features that almost defy description. Because of the large amount of public land in Colorado, virtually all attractions are available for public viewing and examination.
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Devils Tower National
Monument, Wyoming. Devils Tower, viewed from the west, showing distinctive
jointing. 1953. Courtesy of U. S. Geological Survey.
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Two of the thought-provoking structural features of intense interest to many people are the fractures or cracks in rocks that geologists classify as: 1) faults; or 2) joints. The latter are the most common of these features and are found in virtually all outcrops. They are fractures or cracks where no lateral or vertical movements are apparent along the breakage plane. Joints generally occur in sets and are often the result of rocks being “stretched”. What “stretches” the rocks---tectonic deformation is one common cause. A very distinctive type of joint unrelated to deformation is known as columnar jointing. These fractures occur when igneous lava cools rapidly and causes shrinkage cracks to form, commonly in a polygonal pattern. A great example is found at Devil’s Tower in northeastern Wyoming where the joints are vertical and the columns hexagonal in cross section. Closer to home, the basalt capping North and South Table Mountains near Golden displays columnar jointing.
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Vertical joints in the
Wingate Sandstone (Jurassic), Colorado National Monument.
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Faults are classified as either: 1) dip-slip; or 2) strike-slip. The former have relative movement with a vertical component while lateral displacement defines the latter. In addition, movement of the footwall and hanging wall relative to each other further defines dip-slip faults. Early miners chasing ore bodies along a fault could “walk up” the footwall while needing to “hang off” the hanging wall. In a normal fault the hanging wall has slipped down relative to the footwall. Normal faults are commonly caused by tensional stress and/or crustal extension----the layers are pulled apart. A small normal fault is easily visible near the parking lot for Red Rock Canyon Open Space off U. S. 24. One can easily trace the fault plane and notice the directional movement.
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Sketch to illustrate terminology for dip-slip faults: A) hanging wall; B) footwall. Courtesy of U. S. geological Survey. |
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Sketch to explain
dip-slip (Normal and Reverse) and strike-slip faults. Courtesy of U. S. Geological Survey.
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An interesting type of normal faulting is the activity related to horst and graben structures. Perhaps the best example of these structures may be found in the Basin and Range Province in Nevada where the entire area was subjected to crustal extension, tensional stress, and broke into numerous mountain ranges (horsts) and down-dropped basins (grabens). Closer to home the Rio Grande Rift System extends from near El Paso, TX to Leadville, CO. The Arkansas River Valley north of Salida is composed of a series of grabens with bounding normal faults along the mountain ranges. The tectonic forces producing these Colorado structures are related to the Basin and Range activities to the West.
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Sketch showing horst and
graben features. Courtesy of U. S.
Geological Survey.
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Another type of dip-slip fault is termed a reverse fault—the hanging wall has moved up relative to the footwall. Reverse faults are related to compressional stress and are seen at several places in the Garden of the Gods. These small faults are related to the much larger Ute Pass/Rampart Range Fault Zone, a series of very large reverse faults with perhaps 10,000 feet of displacement (U. S. Geological Survey, 2004).
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Reverse fault at North Gateway Rock, Garden
of the Gods. The younger Lyons Formation
is thrust over the older Fountain Formation.
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A special type of reverse fault is termed a thrust fault, essentially a low angled fault, perhaps less than 30 degrees to almost horizontal. Thrust faults can be quite large and may move large volumes of rocks for several miles. Geologists believe that the Cheyenne Mountain Thrust Fault (perhaps a zone) moved portions of the Precambrian Pikes Peak Granite (now Cheyenne Mountain) eastward over the Cretaceous Pierre Shale---a remarkable feat (Rowley and others, 2003). The force for this great movement is probably related to the tectonic plate collision far to the west. In addition, the erosion of sedimentary rocks off the granite as the mountain was being forced upward, and outward, allowed for a rebound effect. Unfortunately, the Cheyenne Mountain Thrust Fault is difficult for the causal geologist to observe.
In Grand County a few miles north of Kremmling and just east of U. S. 40, is Wolford Mountain, a vista observed by hundreds of passing motorists on a daily basis. Very few of these drivers realize the significance of the geologic activities so vividly displayed on the Mountain. It is easy to observe that coniferous trees grow on the upper part of the mountain but are absent on the lower section. However, the demarcation zone between these two zones is the location of the Williams Range Thrust Fault, a large fault that has moved Precambrian rocks westward on top of the Cretaceous rocks. The rock section is “wrong”—older rocks overlie younger rocks (but is explained by the fault)! This is an amazing site and easy to view so don’t miss it on your next trip to Steamboat Springs.
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Williams Range Thrust Fault north of Kremmling. |
Strike-slip faults are fractures with lateral movement, rather than vertical, along the breakage plain. By far the most famous strike-slip fault in the world is the 800-mile long San Andreas Fault in California where pundits insist that part of western California (Pacific Plate) is sliding away (northwest) from the rest of the state (North American Plate). Sudden movement along strike-slip faults can release tremendous amounts of energy with resulting earthquakes and ground shifts. Most strike-slip faults are located along tectonic plate boundaries and therefore Colorado may be devoid of these faults (Matthews and others, 2003).
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Stream offset in Carrizo Plains, California. Stream has been offset one-fourth mile by the San Andreas fault. San Luis Obispo County, California. 1965. Courtesy of U. S. Geological Survey. |