Productive soil and ground water, closely allied resources, are North Dakota's greatest assets. The Wisconsin glacier and Lake Agassiz are largely responsible for the fertile soils that cover three-fifths of the State's surface. Through the Red River Valley the lake left a fine claylike silt 20 to 30 feet deep. The successive shore lines of the lake, showing its gradual recession, can be plainly seen in the ridges of sand and gravel that rise 10 to 25 feet along the western edge of the valley. Also on the west border of the valley are three extensive sand plains, the deltas of the Pembina, Sheyenne, and Elk Rivers, formed by glacial debris mingled with river silt. The Souris glacial lake bed, in the loop of the present Souris River, resembles the Red River Valley in geological history, but covers a much smaller area. Immediately under the silt of the old lake beds and on the surface of the Drift Prairie is glacial drift or till. In much of the southwestern part of the State, particularly along the western tributaries of the Missouri, there are no glacial deposits; the topsoil is composed largely of shale and sandstone, and, though not so fertile as the old lake beds and glacial plains to the east, provides fine range country. NATURAL RESOURCES Especially valuable to those who depend on the land for their livelihood are the numerous artesian wells and natural springs which furnish necessary water supplies. The artesian basin on the southern border of the State and extending into South Dakota has been designated by a Federal authority as the most important in America and probably in the world. People of the State have been awakened in recent years to a consciousness of the need for water conservation. Long abuse of seemingly unlimited artesian supplies resulted in lessening pressure in the wells. Simultaneously, drought, high winds, and the broken unwooded plains conspired to deplete the surface waters left by rains and winter snows. Within 20 years one-third of the lakes in North Dakota became extinct. To counteract these disastrous effects, a program of Federal, State, and private water and soil conservation has begun. Trees are being planted to hold the soil and conserve the moisture of rain and snow. A program of dam construction is under way in every county in the State. Dry-land farming and supplemental irrigation have been adopted to conserve the soil and return to it the elements which it has lost through constant cultivation. North Dakota is indebted to the ancient seas and glaciers not only for the fertility of its soil but also for many of its most important mineral resources. Almost inexhaustible is the vast supply of lignite, estimated at 600 billion tons, which underlies the western half of the State. The veins, once the luxuriant plant life of a far distant age, vary from a fraction of an inch to 40 feet in thickness.