rock, and this was so hard that it partly dammed the great valley. Right here, too, Whetstone creek flows into it and much sediment has accumulated in the old glacial channel. This "boss" of granite, as the geologist calls it, and the alluvial fan built by Whetstone creek, caused Big Stone lake to be formed. Up the valley about thirty miles the Minnesota river flows into the ancient glacial channel, and when the flow of the glacial River Warren was checked by the granite barrier it could not wash down the sediment deposited by the Minnesota river, so another alluvial fan or delta dammed up the channel enough to form Lake Traverse.

Mountains Filled with Gems and Ore. The Black Hills region comprises an area of about 5,000 square miles, or about the area of Connecticut, extending into Wyoming. This region gets its name from the abundance of pine

and spruce which cover the hillsides and from a distance make them look dark. While called "hills," they are mountains, higher than any of those in the eastern portion of the United States.

The highest point is Harney Peak, 7,244 feet above

sea level.* The highest peak in the eastern portion of the United States is Mt. Mitchell, North Carolina, 6,711 feet above sea level. Lead, the principal city of this region, has about the same altitude as Denver, one mile above sea level. The minerals found in this region are discussed in Chapter IV.

The great plains are made up of a series of thick layers of sedimentary rocks, limestone, sandstone, clays and shale. Under them is the granite core of the earth. Some powerful internal force upheaved this portion of the plain, making a huge dome. The action of the weather and running water has worn down this dome and exposed the granite core in the central part. Deep ravines and canyons have been cut into the sides and edges by the streams which flow out in all directions. Around the central granite portion are arranged upturned and overlapping layers of rock. The edges of the harder layers, such as limestone and sandstone, have been worn into ridges and hills, and in the soft layers broad valleys have been carved out.

The hills, as a rule, have short, steep slopes toward the central portion, with long, gentle slopes away toward the plains. This is shown in the cut representing a portion of Deadwood. The separate ridges are sometimes called "hogbacks." Two of the overlapping layers need special mention, the red beds and the Dakota sandstone.

The Red Valley. One of the rock layers, turned up all the way around the hills near the inner granite area, 1S made of soft sandstone and clay and is usually of a dull red color. Being soft, the materials have worn away and a broad red valley completely encircles the hills. For this reason the Indians called it the "race course." In

This is the latest determination from vertical angles by the United States Geological Survey, being furnished the author by the Chief Geographer under date of December 13, 1910.

Called sedimentary because most of the material was deposited as sediment in the ancient seas which once covered this region.

this red valley are located several of the cities of this region, notably Hot Springs, Sturgis and Spearfish. The name of this red sandstone is the Spearfish formation. It contains valuable beds of gypsum, used in making plaster.

The Dakota Sandstone. Outside of the Spearfish red beds are some rock layers called Dakota sandstone. In places they are several hundred feet thick and extend to the surface all the way around the Hills. They are also turned up to the surface in many places along the foothills of the Rockies. This stone in many places is splendid for building, the beautiful government soldiers' hospital at Hot Springs being built of it. Its principal interest for us, however, is its untold value as the main source of our artesian water supply.

Artesian Wells. A study of the figure showing the

Fig. 18.

Cross section of Southern South Dakota.

Showing the course

of the artesian water supply. G.-Granite rock. D. S.-Dakota sandstone, bearing water. C. S.-Clay and shale.

source of our great artesian water flow, shows how the water, which soaks into the Dakota sandstone about the Hills (and along the foothills of the Rocky mountains), works its way down the slope of these porous layers and rises as artesian* or self-flowing wells. Above and below these water bearing sandstone layers are thick beds of shale, which are not porous and do not permit much leakage. It should be remembered that ravines and can

The name is derived from the French province of Artois, where they were first well known.

yons extend from all directions into the Black Hills and into the Rockies, and the mountain streams which drain this large area carry their sparkling waters across the upturned layers of the Dakota sandstone. The Missouri

Fig. 18a. Eastern Limit of Artesian Basin.

river and the Cheyenne and their mountain stream branches cannot flow over to the great plains without crossing the porous sandstone. Thus great quantities of water are absorbed and carried out under the plains. Now, it is well known that "water seeks its level," so if

pipes are sunk down into the Dakota sandstone in the James river valley, carved down as it is, hundreds of feet below the level of the great plains, the water gushes upward under great pressure.

The Pressure. The water pressure varies greatly in different portions of the state. The city of Aberdeen sunk an eight-inch well in 1911 that gives a pressure of 250 pounds to the square inch at the well, probably the greatest recorded artesian pressure in the world. It supplies about 2,000 gallons of water a minute. At a depth of about 900 feet soft artesian water is found, having a pressure of about 40 pounds to the square inch. The high pressure waters are "hard," due to limestone, and are found at a depth of nearly 1,300 feet (at Aberdeen). The water is freely used for drinking and household purposes. It is perfectly free from any danger of disease, as no organic matter is ever found in it. In portions of the state where the surface elevation is higher, or where there is apparently some leakage from the sandstone to other layers, the pressure is less. In the eastern portion of the state the Dakota sandstone layers thin out and disappear. In a few places in the southeastern part of the state these rock layers come to the surface and their waters leak away. In some places there are shallower artesian wells which get their waters from other porous rock layers (notably the Benton sandstones and shales), or from dipping or slanting layers of glacial drift.

A Warning. Professor Ellwood C. Perisho, state geologist, has repeatedly warned the people of the state against the wastes of the artesian water. Much water escapes around the artesian pipes into porous rock layers. The artesian waters are as permanent as the "eternal snows" of the mountains from whence they come. Nevertheless, only a limited amount soaks into the sponge-like sandstone and hence it should not be wasted.