KIESTER MORAINE.

Where is this moraine?

LAKES TRAVERSE AND BIG STONE. Where was glacial Lake Agassiz? What was River Warren? How were Lakes Traverse and Big Stone formed? What river is the outlet of each? MOUNTAINS FILLED WITH GEMS AND ORE. What is the area of the Black Hills region? Why so named? What is the height of the Black Hills as compared with the Appalachians? What is the highest peak and its altitude? Explain how the Black Hills were formed. Describe the hills around the granite center. In Figure 20 the great plains lie off to the left; in Figure 26 to the right. Compare these with Figure 17.

THE RED VALLEY. Describe this valley. What did the Indians call it? Why? What cities are located in this valley? What is the name of this rock formation? What valuable mineral is found in it? THE DAKOTA SANDSTONE. Where is this rock layer upturned to the surface? What use is made of it? Why is it especially interesting to us?

ARTESIAN WELLS. Make a diagram like Figure 18, marking the Dakota sandstone. How is the water which is in these layers kept in? From whence does the water come? Why does it come to the surface in wells out on the plains?

THE PRESSURE. Suppose the well pipe at Aberdeen were projected up in the air seven hundred feet. What would be the pressure at the top then? The top of the pipe would then be level with the plain to the west. (The altitude of Aberdeen is 1,300 feet; of Bowdle, 1,995 feet.) Why is the water pressure less on the higher plains than in the James river valley?

A WARNING. What is the warning of the state geologist?

THE BADLANDS. What is said of the fertility of the soil? Where is the largest badland region in the state? What bulletin has been published on this subject? Describe the appearance of this region. The rock layers. What conditions produce these surface features?

CHAPTER JII

CLIMATE

Seasons. South Dakota has a continental climate, which differs from oceanic climate by having greater changes in temperature. The usual seasons of the temperate zone-spring, summer, fall and winter-are found here. Spring and fall are somewhat shorter than in the eastern portion of the United States, March often being a winter month and summer coming quickly in May. Winter sometimes sets in at about Thanksgiving time. Occasionally, however, we have an early spring, with seeding in March, and frequently there is little winter weather until Christmas time.

Temperatures. The average temperature for the entire state is about 45° for the year. The western half of

Cities of other states are given in some of the tables for comparison. All records are those of the United States Weather Bureau. Temperatures are of the Fahrenheit scale.

the state is slightly warmer than the eastern half. The average is 45.6° in the west and 45.5° in the east. The following table gives a good idea of the temperature as recorded by the United States Weather Bureau:

January is the coldest month, though the coldest days. are sometimes in December or in February. The average temperature for the state for January is 16.4°. The lowest temperature average —33°, although occasionally it gets as low as -40°, though such temperatures are rare and remain so low for only a few hours, when the air is very still.

July is the warmest month, the average temperature for the state being 72°. The highest temperatures are often over 100°, although such temperatures occur only on one or two days, and then for only a few hours in the afternoon. It is very rare that the evenings of even the hottest days are uncomfortably warm; usually summer nights are delightfully cool.

City

Aberdeen

Huron

....

AVERAGE TEMPERATURES.

|Jan Feb Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec

10.9 11.1 24.9 44.5 55.8 65.6 71.2 69.0 59.2 45.6 28.5 16.9 19.5 12.9 26.7 44.6 57.3 66.6 71.5 69.1 59.6 44.7 27.4 15.7 17.9 18.1 30.8 47.8 58.1 68.2 74.8 73.4 63.6 50.7 33.3 23.8 Rapid City... 22.9 22.2 31.1 45.6 54.0 63.7 70.7 70.0 61.2 49.2 35.4 29.4

Pierre

Yankton

St. Paul.

16.4 19.2 31.1 47.3 59.2 68.6 73.7 72.2 63.1 50.6 33.9 23.5 11.9 15.4 28.2 45.8 57.7 67.2 72.0 69.7 60.5 48.4 31.0 18.8

Winds. South Dakota lies in the zone of prevailing westerlies. Usually the surface air lying over the state is that which comes from the Rocky mountains, and hence has the dryness and crispness of mountain air. The moisture comes principally from the Gulf of Mexico and Atlantic ocean in the winds which "back in" to the prevailing drift of air from west to east across the United States.

Cyclones and Anticyclones. As explained in any physical geography, there are two kinds of general disturbances in the air of this zone-low pressure storms of warm, damp, rising air, or cyclones, and high pressure storms of cool, dry, descending air, or anticyclones. These are huge eddies in the atmosphere, sometimes covering one-half of the United States, and they always travel toward the east. A glance at any daily weather map of the United States will usually show several such cyclones and anticyclones, the center of the cyclone being marked "low" and that of the anticyclone "high." These storms follow each other with considerable regularity, each one taking three or four days to cross over a given place. A few days of warm weather occur, with occasional clouds and showers, while a cyclone is passing over the state, followed by three or four days of the colder, clearer air of the anticyclone. In New England these follow each other more rapidly, making the climate much more changeable than it is here. An occasional long, hot spell in summer is due to a slow movement of a

cyclone or to the influence of a second one closely following.

Tornadoes. The cyclone should not be confused with the tornado, the correct term for the small, violent and destructive storm which sometimes occurs within the area covered by a cyclone. A tornado is a small part of a cyclone and is due to the presence of very moist air and intense heat. Owing to the dryness of South Dakota air, tornadoes are very rare here.

Chinook Winds. These are frequent in the western. part of the state, especially in winter. They are hot,† dry winds from the eastern slope of the Rockies. These winds usually occur often enough to keep the western grazing plains free from snow, for the nutritious Buffalo grass makes splendid grazing the year round. The rise in temperature when the Chinook winds blow is sometimes very rapid and snow disappears in a few hours.

"First a puff of heat, summer-like in comparison with what had existed for two weeks, and we run to our instrument shelter to observe the temperature. Up goes the mercury, 34° in seven minutes. Now the cattle stop traveling, and with muzzles turned toward the wind low with satisfaction. Weary with two weeks' standing on their feet they lie down in the snow, for they know that their salvation has come; that now their bodies will not freeze to the ground. The temperature has risen to 38°, the great expanse of snow is becoming damp and honeycombed by the hot winds, and we retire satisfied that the Chinook is a genuine and lasting one."—A. B. Coe, Monthly Weather Review, November, 1896.

Blizzards. The blizzards (snow accompanied by high northerly wind), which sometimes sweep over the Dakotas, Minnesota, Iowa and many other states, are severe storms which may occur to the east of an anticyclone.

* Pronounced shee-nook', the name of a tribe of Indians in Oregon. Whenever air ascends it expands, and expansion of the air cools it one degree for every 183 feet of ascent. This is the principal cause of rain. Now when a low pressure storm passes along the eastern slope of the Rockies the air flows over them and through the passes from the Pacific coast. As the air flows down the eastern slope it is warmed by compression just as it was cooled by expansion on the western slope. When vapor changes to liquid, heat is always released. The Chinook wind thus gets its heat in these two ways (a) by compression, and (b) by the release of heat in the upper air during the rain on the western slope.