THE GREAT PLATEAUS.—Although the Prairies merge gradually into the Great Plateaus, yet between the two there are considerable differ ences in topography, climate, vegetation, and economic develop ment. The latter, which are much more undulating in character, at one time, no doubt, constituted a true plain, but the rivers which descend from the Rocky Mountains have dissected the greater part of it, and converted it into a series of plateaus, which differ greatly from one another according to the nature of the land in each, and the way in which each has been affected by climate and the processes of erosion. The Bad Lands of South Dakota and of parts of Montana consist of unconsolidated clays and soft sandstones, and have been minutely dissected by wet weather streams, while in western Nebraska the precipitation is just sufficient to allow of the growth of a thick sod which has prevented any erosion of the ancient plain.
Over the whole region the range of temperature is very great and the rainfall low. In Montana, notwithstanding the modifying influence of the Chinook winds, the mean winter temperature is always below, and sometimes considerably below, freezing point, while during the blizzards, which here have their full development, the thermometer may fall to –60° F. The mean summer temperature is between 65° F. and 70° F., though occasionally the thermometer rises to 110° F. In the south the mean winter temperature is generally over 40° F. and the summer temperature between 70° F. and 80° F. Over the whole region the rainfall is as a rule between 10 and 20 inches.
Grass is the typical vegetation of this part of the United States ; but, where the ancient plain still survives, in an irregular belt stretch ing from north to south about midway across the long eastward slope from the Rocky Mountains to the prairies, it is found growing in close formation and is specially suitable both for cattle and sheep.
Over the remainder of the region bunch grass prevails, and, although stock can be raised upon it, a large area per head is necessary. Millions of cattle wander about at all times of the year upon the open ranges, and, the snowfall being light, they are always able to obtain their food. Severe losses occur during the blizzards, and, partly for this reason, Texas, where climatic conditions are more favourable, has become a great ranching state, though, in the north, winter shelter is now being provided. From the whole region, cattle are forwarded to the North Central States. Owing to the low rainfall, very little arable farming is possible on the Great Plateaus without the aid of irrigation ; and, as the rivers usually flow at some depth below the level of the land, irrigation is costly except in the valley bottoms, where it is chiefly carried on at present. There is no extensive system in operation, but river and underground water are both used for the growth of crops to supply local needs, for fodder, and for stock, and it is probable that considerable development may take place in the near future.
The mineral wealth of the Great Plateaus is not great. Lignite underlies considerable areas in North Dakota, and from the Black Hills of South Dakota some gold and small quantities of silver and lead are obtained. But the region is essentially a ranching one. The population is small, and is almost entirely confined to the valley bottoms, where live the ranchmen, whose herds graze upon the plateaus. The few towns which exist are situated, like Denver, at the meeting-places of lines of communication.