Hidden beneath the earth's surface are other minerals deposited during the geologic formation of the various strata. These include fuller's earth, sandstone, granite, gneiss, and gold; but because of their limited quantity and inaccessibility, they are commercially unimportant. The glacial deposits are important because they include the sand and gravel used extensively for road surfacing. Some of the eastern lake beds contain marl, a clay from which Portland cement is made. The extent and purity of the deposits are not definitely known. (For discussion of industrial development of mineral resources see INDUSTRY AND LABOR.) PLANT AND ANIMAL LIFE North Dakota falls into three distinct zones of plant and animal life: the Turtle Mountain region and a few scattered areas in the Canadian, or cold, zone; the Missouri and Little Missouri Valleys in the upper austral, or warmer, zone; and the remainder of the State in the transition zone. Because of its semiarid climate, the State has only 600 square miles of wooded area. Native forests are found chiefly along streams and lakes, and in the Turtle and Killdeer Mountains. Despite the limited forested area, a surprising variety of trees is found. Throughout the Red River Valley, Turtle Mountains, and Devils Lake region plant life is similar to the Minnesota type, while such trees as the elm, green ash, box elder, poplar, and cottonwood are also common. Although the cottonwood's ability to withstand drought makes it one of the most desirable species of trees in North Dakota, efforts are being made in many towns to eradicate the tree because of the ubiquitous soft white "cotton" which floats from its branches like summer flurries of snow. During the fall and early winter, the thickets of the northern Red River Valley are aflame with the highbush cranberry, which lent its Indian name to Pembina, first permanent white settlement in the State. Other berries grow profusely along all the eastern streams, and many families assure themselves of a winter supply of jams and jellies by picking the June berries, chokecherries, wild plums, and wild grapes. In the woods along the Missouri and Little Missouri grow trees of the Missouri type--the broadleaf cottonwoods, willows, ash, elm, buffalo berry, and flowering currant. A trace of the Rocky Mountain type of forest is found in the Badlands and on the buttes of the Little Missouri, where the yellow pine and red cedar grow. Not only trees but other forms of vegetation differ widely from the eastern to the western sections of the State. The long Indian-grass and blue grass typical of the east is replaced on the western ranges by short buffalo grass and grama grass, the two forming a dense mat over the ground. Due to differences in rainfall, the western grasses are much duller and more grayish in color than those of the eastern section.