One of the characteristics of such a climate is a wide range of temperature, and to this the State can make good claim. North Dakota has a recorded range from 124° F., registered September 3, 1912 at Medora, to -60° F., recorded February 15, 1936 at Parshall. These temperatures are, of course, unusual, but the mercury often reaches 100° F. during the summer, whereas 30° to 40° F. below zero is not uncommon in winter. The mean temperature for the months of June, July, and August is 65.7° F., and for December, January, and February, 9.7° F. Relatively low humidity, averaging 68 percent, makes these extremes less uncomfortable, however, than if the atmosphere contained more moisture.
The sections of the State vary more in the matter of precipitation than in any other climatic phase. The average is about 18 inches annually, ranging from about 22 inches in the southeastern corner to about 14 inches in the southwestern corner. Most of the precipitation occurs in the late spring and summer. During the period of 1925-34, three years had a rainfall in excess of normal, and seven were deficient in precipitation.
Long and severe winters are typical of this region. Nevertheless the summers, though comparatively short, are favorable for agriculture, owing to the long hours of sunshine. At the maximum, about June 21, there are as many as 16 hours of sunlight a day; and this, together with cloudless skies, contributes to the rapid growth and early maturity of crops.
HOW THE LAND WAS FORMED
The surface of North Dakota, comparatively unvaried, is no more simple than the geological pattern that lies beneath it. Deposited by the seas of three geologic ages, horizontal layers of rock top each other in methodical and unintricate succession.
In far-off Paleozoic times, when all creatures of the earth were invertebrates, strange shellfish, unlike any existing today, lived among the rich foliage at the bottom of the shallow sea that covered this region. Hundreds of varieties of fossil plants and shells are embedded in the sandstone, limestone, and shale which the sea deposited on the uneven surface of the then-existent crystalline rocks. These Paleozoic rocks have been encountered in deep wells in eastern North Dakota, although nowhere in the State are they found at the surface.
Toward the close of the Paleozoic era, changing climatic conditions caused the death of many forms of life upon the earth, and the development of new and hardier types. With these, in the Mesozoic era, came the new lords of the earth, the reptiles. Much of the globe from the Arctic Ocean to New Mexico was covered by a great sea on whose swampy shores huge dinosaurs, alligators, and crocodiles made their homes. Largest of these grotesque creatures was the brontosaurus, with his long snakelike neck and face and huge body. Struggling with him for supremacy of the swamps was the armored stegosaurus, whose row of vertical plates along his backbone from head to tip of tail made him a formidable enemy. Among the plant and animal life that throve on the sea bottom were shellfish three feet or more in diameter.
The earliest seas of the Mesozoic era deposited the Dakota sandstone that underlies all of the State except the Red River Valley. It is a soft white or gray stone, containing many marine fossils. Although it does not appear at the surface anywhere in the State, it has been studied from specimens obtained from deep wells here or from outcroppings in other States.
Many rivers flowing into the prehistoric seas brought mud and clay to mix with the soils of the sea bottom, forming the shales that today underlie most of the Great Plains, including North Dakota. The lowest of these, Benton shale, is dark gray, almost jet black in places, and contains bits of pyrite (fool's gold) and gypsum. Over it lies the bluish-gray Niobrara shale, in which natural cement is found. In the Pembina Mountains and the Sheyenne Valley, where these rocks appear at the surface, they have yielded shells of lamellibranchs (ancestors of today's clams and oysters) and bones of great sharklike fish. Over these two strata lies the Pierre shale deposit, dark bluish-gray in color. It is frequently seen in the valleys of streams east of the Missouri Plateau, but outcrops in only two places west of the Missouri Escarpment—where the Missouri leaves the State, and in the valley of the Little Beaver Creek in southwestern North Dakota. In it have been found fossils of the chambered nautilus, the oyster, and other marine animals, the crocodile, and the plesiosaur—that ungainly reptile which had "the body of a turtle strung on a snake."