Basin and Range Province

   This is a geographic zone which occupies much of the American West, mostly lying within Nevada, Utah, Arizona, New Mexico, Oregon, west Texas, and southern California.
   Within this area, extension of the Earth's crust has caused great blocks of crustal rock to settle downward due to gravity.
   Between these blocks higher crustal pieces are seen as now-eroded mountain ranges.  Anyone driving long distances in this part of the world is familiar with the pattern of long, wide valleys separated by narrow mountain ranges, mostly in parallel fashion.
   The extension began in western North America during the Tertiary period, around (open to debate) 25 million years ago.  Arizona is in the older portion, so our mountains are more eroded and we have relatively little earthquake activity.  Younger portions, to the north in Utah, and to the west in California, are much more active geologically.
   An excellent book to read if you are interested in this part of the West, is John McPhee's Basin and Range.  Many stories are woven within the context of the geology, and it is written for the average reader, not terribly technical in its language.

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