unlike the sedimentary formations upon which it lies. Anyone who will thoughtfully consider the appearance of the surface of nearly all the eastern part of North Dakota will be impressed by the fact that some widely operative and powerful agency has, within a comparatively recent geological period, been shaping its features and accumulating, mingling and distributing the immense amount of unconsolidated material which covers to a considerable thickness the earlier stratified formations.

The embedding material is usually blue and yellow clay in thick sheets, sometimes alternating with beds of sand and gravel, in both of which are scattered large blocks of various kinds of rocks, sometimes weighing several thousand pounds. These boulders are frequently smoothed and scored with fine parallel scratches. A knowledge of the character of these rock masses and a familiarity with some of the rocks outcropping farther north, in Canada, leads to the belief that the debris was transported from northern regions. Much of the limestone found in the drift in the northern part of the state was undoubtedly taken from the beds which outcrop about Lake Winnipeg. A study of well excavations and of channels of stream erosion shows that this drift material was spread over an old land surface. In some places in the Red river valley drift and alluvial deposits reach a depth of 300 to 350 feet. In the northern and central part of the state the thickness is commonly from thirty to 100 feet.

The character of these various drift deposits indicates that a very large area in North Dakota was, at a late geological period, covered by a great, slowly moving ice sheet, similar to that which now covers a large part of Greenland, which stretched far away to the north into Canada. This vast ice sheet, which in its northern portion at least must have been very deep, tore away exposed rock ledges, and enveloped and bore along with it the loose material with which it came in contact. This debris was

frozen into the ice, and under the enormous weight above it became a mighty grinding power. As the ice sheet moved onward from the north, slowly but irresistibly, the enclosed rock masses were worn away to smaller fragments, pebbles, sand and clay, and all mixed with the sand and clay and soils of the surface over which the ice moved. Thus was formed, during the centuries of the glacial period, an enormous amount of this rock refuse, which, with the return of a warmer climate and the melting of the ice sheet, was intermingled and spread far and wide.

This drift material, by reason of its variety of composition and depth of deposit, was well calculated to become the formation of the rich soil so characteristic of the eastern and central parts of North Dakota.

With a change in climatic conditions the ice began to melt along its southern border, and the water formed by the melting, being banked on the north by the great ice barrier, gradually accumulated in a glacial lake along the southern boundary of the sheet. As the glacier continued its retreat to the north, the area and depth of the lake increased, and the water, spreading out over the Red river valley and finding no other outlet, at last overflowed the height of land near Lake Traverse, making its way through that lake and Big Stone lake into the Minnesota river. and thence into the Mississippi river. When the ice had melted far enough toward the north, a natural outlet was opened through Lake Winnipeg and Hudson bay, and the present valley of the Red river was begun. The total area covered by this great lake, known as Lake Agassiz, has been estimated by Warren Upham* at 110,000 square miles, over which the water often reached depth of 500 to 700 feet. At last the ice sheet had retreated so far that it left open the Nelson river, thus affording an outlet through this river and Lake Winnipeg into Hudson bay. From this time Lake Agassiz was rapidly drained. In the lowland of the Winnipeg basin, however, a large body of water was left, a portion of which forms the present Lake Winnipeg.

a

This great body of water has recorded its presence in three way: By lacustrine sediments, by extensive alluvial and delta deposits, and by corresponding extensive erosion. The fine silt and clay which are characteristic of the Red river valley were deposited by the waters of Lake Agassiz and the many glacial rivers which brought debris into this basin from the surrounding higher lands. The water of the glacial Red river gradually narrowed, but in the central portion of the valley, being much deeper, it remained a longer time, and thus gave opportunity for a thicker deposit of sediment than is found along the old lake margin. Warren Upham has traced a series of beaches marking clearly the extent of Lake Agassiz at its various stages.

The streams which flow through the lacustrine sediments usually have narrow and shallow banks, but the valleys of those *The Glacial Lake Agassiz, by Warren Upham; Mon. U. S. Geol. Survey, No. XXV, 1895.

streams which flow into the basin of Lake Agassiz are commonly deep and wide, showing much erosion. This is particularly noticeable of the streams flowing from the Cretaceous highlands on the west, for example, Park river, Tongue river, Little Pembina and the Pembina rivers. Along the eastern escarpment of the Pembina mountains the erosive action of the old lake is clearly seen in the almost cliff-like ascent of the Cretaceous tableland.

Lake Agassiz was not the only glacial lake by which the surface of the level prairies of North Dakota was modified. In the central part of the state there were probably several lakes at various periods following the glacial epoch which were formed from the melting of arms of the ice sheet. One of the most important of these was glacial Lake Souris. Devils lake. and its immediate drainage basin is doubtless a remnant of one of these lakes. The Sheyenne and James rivers probably were started and the high bluffs along the western portion of these streams washed out during the time when districts to the north, about Devils lake, and to the west, being flooded by the melting ice, were drained of great quantities of water by these rivers. All through the eastern and central portion of the state the ice sheet, the lakes and the river torrents formed by melting ice, exerted a powerful influence in giving fertility to the soil and final shape to the surface of our North Dakota prairies.

DEVILS LAKE

The most characteristic feature of the region under discussion is Devils lake, which occupies a basin formed largely by morainic ridges. As shown on the map, it lies along Ramsey and Benson counties, with its length extending east and west, as illustrated in plate XXXVIII. The length of the lake, including the arms which properly belong to its present stage, though some of them are nearly dry or separated by portions of land, is about twenty-four miles, and the width averages perhaps between four and seven miles. Its shore line is exceedingly long, owing to the numerous bays and other irregularities, and it is probable that it will reach several hundred miles.

The southern shore of the lake, which is often very thickly strewn with large boulders, rises rapidly into a high rolling country, whose surface is broken by numerous steep knobs, some of them 200 to 275 feet above water level.

The western