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Geology of the Southwest- the Sandstone and Fossils of Southeast Utah


A Brief Introduction to Millions of Years of Geology

It is safe to say that southern Utah and the Four Corners region offers textbook examples- literally- of geology. Between sandstone escarpments as high as 1500 feet, the water-carved canyons of the San Juan, Colorado, Green and Escalante rivers, their bowknots or goosenecks, the sandstone spires of Monument Valley and the Valley of the Gods, the limestones and assciated fossils of ancient seaways, and volcanic dikes an sills, the area offers some of the most perfect examples of geologic processes in the world. Pick up any geology textbook and you will find photographs and discussions of these features.

What is a Desert?

A simple and rather broad definition of a desert is an arid or semi-arid land, receiving limited, irregular rainfall (less than 20 inches anually), where evaporation rates exceed rainfall. Temperatures across and within desert regions are highly variable, soils are typically nutrient deficient, unstable or sterile in the case of dunes. Nearly 33 percent of the earth's surface is covered by deserts; the general consensus is that this amount is increasing. That definition leaves us with a rather dreary picture of deserts, maybe a picture of a bleak wasteland devoid of all life. For those with knowledge of the southwest, we know this couldn't be further from the truth. The desert southwest is full of life and activity. There is water, there are plants and animals that have adapted and thrive under desert conditions. However you see the desert, it is the processes at work in the desert- the wind, rain, snow and flash floods- that have formed and are still forming the landscape of the southern Utah, the Four Corners region, and the desert Southwest.

Desert Rocks

In southeast Utah you will find exposed sedimentary rock as old as the Pennsylvanian period, about 300 million years ago. Most of what you will see is Mesozoic, from 245 to 66 million years ago. If you visit this area you will have a chance to see the exposed, colorful bedding planes of many millions of years of shifting sands, deposition and erosion, and of transgressing and regressing seas. The bedding planes include the obvious sandstones, as well as limestone, siltstone, mudstone,shale, and even some coal.

You do not have to search far to find exposed traces of ancient geologic processes. You might see exposed, crossbedded planes of ancient dunes with windblown barchans, star dunes, or transverse dunes- different types of dune formations based on the strength and prevailing directions of winds. Or you might find signs of ancient, near shore environments with fossils of brachiopods, echinoderms, bryozoa, and petrified wood.

See Halka Chronic's Roadside Geology series for some really great, easy to follow texts on local geology.




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