as more and more sand is carried away and the up-rooted plants are swept on with the gale, the embryonic blow-out comes rapidly into existence. At this early stage it appears as an area of bare sand a few feet or yards across, over which the wind sweeps and continues to eat its way deeper and deeper into the sand. During this early stage the deep seated roots of woody plants frequently appear strewn over the surface of the shallow depressions until the wind has finally eaten its way far below the point of penetration of the deepest rooted plants. At last the whole rounded or conical hill top is blown away and a deep crater is developed in its stead.
The two chief factors that enable the wind to begin this work of destruction are fire and over-grazing. Both factors frequently result in reducing the vegetation to a point below effective wind resistance and as soon as this is done, if the exposure be right, wind erosion begins. Nothing is quite so terrible as a prairie fire in paving the way for shifting sands and the development of blow-outs, since in such cases absolutely everything above the surface is destroyed. And so if cattle are allowed to run for too long a time over a given range the grasses are seriously reduced and the soil is tramped bare of plants for considerable distances, making it very readily possible for the wind to strike at the open sands. The effects of over-grazing are contrasted to a striking degree in the Sand Hills, where a fence separates the over-grazed pasture from the ungrazed range. Such sights have resulted in the enactment of grazing laws which naturally do not in all cases please the cattlemen, but they do usually protect the range and make it more stable.
When the young blow-out is no more than a foot in depth the sand begins to slide into the depression from the sides. This sand is blown