Page:Popular Science Monthly Volume 80.djvu/223

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219
THE GREAT AMERICAN DESERT

green patches from ten to seventy feet across contrast very greatly with the surrounding bunch-grass vegetation. Bessey's sand cherry is one of the most ubiquitous plants of the whole region. It is found in almost every site of the uplands and with its low, short twigs with tufts of glossy green leaves is seen springing from the sand on practically every side of every hill. Very frequently it forms extensive communities. The prairie clovers seldom form well-defined communities, but they occur as more or less scattered individuals, especially on the lower slopes of the hills adjacent to the larger valleys.

The most striking habitats of the uplands are the "blow-outs." Blow-outs are conical or rounded depressions of varying depth and diameter formed by the blowing of the sand and vegetation from certain spots on the upper slopes and crests of the hills. The rim of the more or less conical depression is sometimes almost circular but it is usually irregular with a general circular outline. Since the prevailing winds of the region are from the west, and since "blow-outs" are the direct products of wind action, these peculiar structures are mostly confined to the west sides of the hills. The greatest number occur on the northwest-facing slope, but they range in position from northwest to southwest, depending somewhat upon the shape of the hill concerned and its relation to the adjacent hills. Blow-outs do not occur on all hills, nor does a single hill show more than a single blow-out, as a rule.

On an exposed upper slope when the vegetation becomes broken or seriously depleted from any cause, the wind as it sweeps up the slope catches the sand and carries it over the crest of the hill a few yards farther away and deposits it upon the lee face of the hill. In this way

PSM V80 D223 Blowout from the east with sand blown out from the crater by western winds.png
Fig. 6. a Blow-out from the East. The sand has been blown out of the crater on the other side. Bunch-grass in the foreground.