Page:A Brief History of South Dakota.djvu/17

Rh present James River valley, and from this river sloped up to the top of the coteau at the east line of the state.

By this time several agencies were at work which resulted in a great change in the climate of the region. The uplifting of the Black Hills and the Rocky Mountains had cut off the warm breezes from the Pacific Ocean, and in the far north vast heaps of ice were being piled up by the almost continual freezing of the frigid climate. These heaps of ice had become so deep that they could not support their own weight, and so began to run or spread out as you may have seen a large lump of dough spread when turned from the kneading pan to the table. When we examine a piece of ice, it seems to be so hard and brittle that it does not seem possible for ice to spread in this way; nevertheless, scientific men have shown beyond doubt that ice does spread when placed under a great weight.

The spreading of this ice sent it down from the north-east until it had run far down into the South Dakota country. It was so thick and heavy that it completely dammed up the valley of the great river, so that its waters became a great lake, lying north of the ice and extending far back into the Rocky Mountains. The ice pushed along until its western edge had traveled as far as the line now occupied by the Missouri River, when it began to melt away. The waters which were dammed up in the upper part of the great valley began to seep about the western edge of the ice, until they ran entirely around it and reached the old bed of the stream below Yankton.

Thus the ice quite changed the surface of South Dakota. Before it came the Grand River extended east from its