Geology
The Sierra Madre Occidental is a high plateau of volcanic rock that is eroded in areas to reveal a basement composed of plutonic and sedimentary rocks underlying the two groups of extrusive volcanics. The lower of these groups is a series of volcanic rocks formed by lava flows. The surface of the plateau is made up of a second series of volcanic ignimbrites, covered mostly by clay rich organic soils. Some intrusions in the area have deposited ore veins that are commercially viable.
The landscape was shaped after rock placement. Rivers cut valleys into the plateau, following folds in the rocks. Rock have been broken off of the slopes, forming rocky sides. The bedrock is covered by the clay rich soils, except in eroded areas, where water carries away deposits leaving a stony surface.
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