As already stated， a plain of this kind， remarkable for its fertility and extent， is drained by the Red River of the North and comprises the eastern Dart of North Dakota and about half of the Province of Manitoba. A somewhat similar plain is found in northern New York， bordering Lake Ontario. This was formed at the time when the outlet of the lake was the Mohawk River， the present outlet then being obstructed by ice. The ice dam has since melted， the lake has been lowered， and a part of its old bed has been exposed.
LAKE PLAIN.
The ice dam in this lake has recently receded.
A change in the amount of rainfall may cause the formation of a lake plain. If not as much water is furnished to the lake as evaporates， the lake dries up and exposes its bed as a flat plain with perhaps a small remnant of the former lake still existing at the lowest part. Such is the region around Great Salt Lake， Utah.
185.River Plains. -Sometimes a river widens its valley enoughso that it swings slowly from one side to the other， and， at high water， floods the valley for a considerable distance on either side of its course. A low， flat plain is thus developed， sometimes terminating near the mouth of the river in a delta.
"BOTTOM LANDS."
These plains are very fertile and are usually called "bottom lands" by the farmers. They are often unhealthy because of floods and poor drainage. Where the water in the river rises rapidly and to a considerable height， it is dangerous to inhabit these plains. Thus it is necessary to build strong levees along the river bank， as in the case of the lower Mississippi and some of its tributaries. But sometimes these plains are so fertile that they are densely populated， as the plain of the Ganges.
186.Prairies of the United States. -North of the Ohio River andextending westward beyond the Mississippi is a region of rolling landwith a deep， rich soil. Early in the last century it began to be rapidly populated on account of its great agricultural advantages. Owing partly to the fineness of the soil， but mostly to the frequent burning over of the region by the Indians， the area was destitute of trees except in some places along the river courses.
Thus the emigrant did not need to go to the trouble and delay of clearing the forests before beginning to farm. Cultivation could begin in earnest with the first spring， and， as a rule， rich harvests could be obtained. The soil here is transported soil； it is deep and unlike that of the underlying rock. In some places it is rather stony and in others very fine and without stones. It is so deep that the underlying rock is only seen in deep cuts.
This soil was probably deposited by the great continental glaciers which once covered the region and was spread out either by the action of the slowly moving ice or by the water from the melting ice. This water flowed over the surface in shallow débris-laden streams， bearing their silt into the still waters of transient ice-dammed lakes. Whatever the original surface of the region， at present it is an irregularly filled plain due to the ancient ice sheet. As the soil is composed of pulverized rock not previously exhausted by vegetable growth it is strong andALFALFA CUTTING ON THE FERTILE PRAIRIES.
enduring， so that this country has， since its settlement， been noted for its productivity.
187.The Great Plains of the United States. -West of the Missis-sippi River， and merging almost imperceptibly into the prairie region on the north and the coastal plain region on the south， there is a broad extent of territory usually called the Great Plains. This region consists of irregular intrenched valleys 50 to 100 feet deep. Sometimes there are hills and mountains， but viewed from an eminence the country ap- pears flat.
A HIGH， DRY PLAIN.
The elevations are either flat topped hills， the strata of which are slightly inclined and correspond in position to those found in the plain beneath， or they are masses of igneous material which appear to have been thrust up through the rock surrounding them. In the former case the elevations are simply remnants of the layers of rocks which once extended over the country， but which have now been eroded away over the larger part of it； in the latter case they are the igneous masses which have withstood erosion. The Great Plains may thus be considered as an example of a plain of erosion.
Here， as in the prairie region， trees are wanting， but their absence is due rather to the lack of the necessary rainfall than to the reasonsassigned for the former region. Although formerly considered almost a desert on account of its small rainfall， this region now supports vast herds of cattle， and by the aid of irrigation will soon possess great agricultural wealth.
188.Life on Plains. -The life conditions on plains are very dif-ferent from those in places where the irregularities of the surface are great. The climate of plains is quite uniform and depends to a large ex-A HERD OF CATTLE ON THE GREAT PLAINS.
tent upon their position on the earth"s surface. Movement is as easy in one direction as in another， and the lines of travel tend to be straight. There is usually no reason for an accumulation of population in any one place， so the population tends to be uniformly distributed.
As movement from place to place is easy， it is not difficult for the inhabitants of a plain to mass themselves together at one point. In case of invasion by a superior enemy there is no place for hiding or safe retreat， and subjection or extermination are the alternatives， unless the plain is so large that the enemy is unable to spread over it. In the case of animals this has been shown in the practical extermination of the American bison and antelope. In the case of men it was shown on the plains of Russia in the thirteenth century when the Tartars conquered the region and threatened to overrun Europe.
Another instance was that of the fatal invasion of Russia by Napoleon.
The Russians， unable to find a strategic place to make a stand， retreatedHERD OF BISON.