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Boyce Thompson
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In the Tucson Mountains near Saguaro National Monument, we undertook our first mountain climb. Although waning daylight curtailed our climb, we got a fantastic view of the adjacent landscape, and had close encounters with Sonoran biome flora. In the foreground, characteristic Sonoran biome plants, saguaro (tall cactus), ocotillo (with long slender stems), teddy-bear cholla (fuzzy looking), jumping cholla (foremost), and palo verde (tree in background) are visible.

The region southwest of Tucson typifies the Basin and Range geologic province. In it, roughly parallel faults divide blocks of rock that have subsided in elevation (grabens) from blocks that have not (horsts). Over time, the elevated blocks have eroded into a series of small, parallel mountain ranges while the sediment they have shed has filled the adjacent graben valleys. The Tucson mountains are a horst block, separated from the similar Sierrita Mountains, visible to the southwest, by a graben valley.