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Geology of Southwest Arizona Arizona's dry climate and varied topography make it a geologic wonderland. From desert lowland to barren mountaintop about 2 billion years of geologic happenings have left their traces for us to piece together a wonderful history of the various geological areas. For our purposes we will look at the bottom region of the state containing the Basin and Range deserts of southern and western Arizona. In the Basin and Range deserts each rocky mountain range at first seems different, out of character with its neighbor, the differences between them buried deep beneath debris-filled desert valleys. Many of the ranges - particularly those with smoothly doomed summits - are recognized now as having a common basic structural pattern. Many others display certain rocks in common, suggesting a single ancient mountain-building episode that created long-gone mountains to match today's Sierras. In yet others, older and much distorted rock exhibits a definite fabric a northeast-southwest trend of cracking and jointing and mineral alignment as well as rock types, telling us of even earlier, even greater ranges of folded rock, mountains of Himalayan magnitude, crushed and forced upward by collision between continents. Arizona's southern deserts lie in what has been for the last75 million years a particularly mobile region, where collisions and separations have frequently changed the tenor of the land. Geological topics of special interest in Arizona are water and copper. Here, water is in short supply, at least as far as man is concerned. Copper is of special interest because copper ores are abundant here; Arizona produces more than half of the copper produced in the United States, with the economic benefits and environmental detriments that go with mining of almost any kind. Arizona's southern and western deserts arc from the southeast corner to the far northwestern edge of the state. Light rainfall is soaked up by the desert and foes to nourish sparse, highly adapted vegetation. Most desert streams - washes in the Southwest - flow only after rare torrential rains. Within a few moments a tree shaded, sand floored picnic spot may become a roiling, muddy cataract quite capable of pushing and rolling cows and cars downstream. Rainwater that sheets across the desert during such cloudbursts augment the floods and nourishes the desert's plants. A week after the heavy soaking the dessert wears a carpet of green; in two weeks the carpet may be spangled with wildflowers. A more persistent performer when it comes to desert erosion is wind. On almost any warm day, tall whirling columns of dust - the dust devils familiar to all Southwesterners - spiral against the sky. Larger dust storms are less frequent thought they sweep north from Mexico to the Phoenix area an average of 3.5 times per year. The dust storms seldom last long, but may be followed by rain. The billowing storms are caused by a combination of high surface temperatures and downdrafts from decaying thunderstorms. This provides a short background of the southern area of Arizona including the cities of Arizona City, Eloy and Casa Grande. Over the next several months we will be adding to this information, featuring geological formations and information for the "rock hounds" along the major highways across the southern part of Arizona. So, stop back often and travel along the highways of southern Arizona. Information for this subject taken from the Roadside Geology of Arizona by Halka Chronic.
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Copyright © 2004
Sunland Visitor's Center
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