Geology: The Science of the Earth's Crust. William J. Miller

is about 5,000 feet above sea level, and many of the glaciers come down to sea (Plate 4), while in the Alps, the lower limit of perpetual snow is at about 9,000 feet, and the glaciers descend as much as 5,000 feet below it.

      In regions of perpetual snow there is a tendency for more or less snow to accumulate faster than it can be removed by evaporation or melting. As such snow accumulates it gradually undergoes a change, especially in its lower parts, first into granulated snow (so-called “névé”) and then into solid ice. Snow drifts in the northern United States often undergo similar transformation, after a few months first to névé, and then to ice. This transformation seems to be brought about mainly by weight of overlying snow which compacts the snow crystals; by rain or melting snow percolating into the snow to freeze and fill spaces between the snow crystals; and by the actual growth of the crystals themselves. When ice of sufficient thickness has accumulated (probably at best several hundred feet), the spreading action or flowage begins and a glacier has developed. Renewed snowfalls over the gathering ground keep up the supply of ice.

      There are several types of glaciers: valley or alpine glaciers; cliff or hanging glaciers; piedmont glaciers; ice caps; and continental ice sheets. A valley or alpine glacier consists essentially of a stream of ice slowly flowing down a valley and fed from a catchment basin of snow within a region of perpetual snow. In the Alps, where glaciers of this sort are very typically shown, they vary in length up to eight or nine miles. Perhaps the grandest display of great valley glaciers is in southern Alaska where they attain lengths up to forty or fifty miles and widths of one or two miles (Plate 4).

      Hanging or cliff glaciers are in many ways like valley glaciers, but they are generally smaller; they develop in snow-filled basins above the snow line usually on steep mountain sides; and they do not reach down into well-defined valleys. Most of the glaciers of the Glacier National Park in Montana and many of those in the Cascade Mountains are of this type. Mount Rainier in Washington is one of the most remarkable single large mountain peaks in the world, in regard to development of glaciers over it. Great tongues of ice, starting mostly at 8,000 to 10,000 feet above sea level, flow down the sides of the mountain for distances of to four and even six miles. The total area of ice in this remarkable system of radiating glaciers on this one mountain is over forty square miles. These Mount Rainier glaciers are in general best classified as intermediate in type between valley and hanging glaciers.

      Fig. 6.—Map of Mount Rainier, Washington, showing its wonderful system of glaciers which covers more than 40 square miles. Dotted portions represent moraines. (U. S. Geological Survey.)

      In some high latitude areas, as in Iceland and Spitzbergen, snow and ice may accumulate on relatively level plains or plateaus and slowly spread or flow radially from their centers. These are called ice caps. Ordinary ice caps usually do not cover more than some hundreds of square miles.

      Continental glaciers or ice sheets are, in principle, much like ice caps, but they are larger. Greenland is buried under an ice sheet of moderate size (about 500,000 square miles), the motion being outward in all directions toward the sea. Tongues of ice, like valley glaciers, are commonly sent off from the main body of ice across the land border of Greenland into the sea. The size of the great ice sheet of Antarctica is not definitely known, but it covers probably at least several million square miles. Two continental ice sheets of special interest to the geologist are those which existed during the great Ice Age of the Quaternary period. One of these then covered nearly 4,000,000 square miles of North America, while the other covered about 600,000 square miles of northern Europe. The main facts regarding the Ice Age are given in a succeeding chapter. The facts brought out in the present discussion of existing glaciers will greatly aid in understanding the Ice Age.

      How fast do glaciers flow? Based upon many observations, we may say that an average rate of flow for the glaciers of the world is not more than a few feet per day. A very exceptional case is a large glacier, branching off as a tongue from the ice sheet of Greenland, which is said to move sixty to seventy-five feet per day. Some of the great Alaskan glaciers have been found to flow from four to forty feet per day. Most glaciers of the Alps move only one to two feet per day. A glacier advances only when the rate of motion is greater than the rate of melting of its lower end and vice versa in the case of retreat. Thus it is true, though seemingly paradoxical, to assert that a glacier has a constant forward motion even when it is retreating by melting.

      By watching the changing position of marked objects placed in the ice, it has been proved that, in a valley glacier, the top moves faster than the bottom; the middle moves faster than the sides; the rate of motion increases with thickness of ice, slope of floor over which it moves, and temperature.

      Ice, like molasses candy, tends to crack when subjected to a relatively sudden force, and where the ice rides over a salient on the bed of the glacier, transverse cracks or fissures often develop. Due to more rapid motion of the central part of a valley glacier, stresses and strains are set up and crevasses are formed, usually pointing obliquely upstream. Where the ice tends to spread laterally in a broad portion of a valley, longitudinal cracks may develop. Crevasses vary in size up to several feet in width and hundreds of feet in depth. Owing to the forward motion of the ice, old fissures tend to close up and new ones form, and, aided by uneven melting, the surface of a glacier is generally very rough.

      Like running water, ice may have considerable erosive power when it is properly supplied with tools. The total erosive effect which has been, and is now being, accomplished by ice compared with that of running water is, however, slight. One of the main processes by which ice erosion is accomplished is “corrasion” due to the rubbing or grinding action of hard rock fragments frozen into the bottom and sides of the glacier. Thick ice, shod with hard rock fragments and flowing through a deep, narrow valley of soft rock, is especially powerful as an erosive agent because the abrasive tools are supplied; the work to be done is easy; and the deep ice causes great pressure on the bottom and lower sides of the valley. Rock surfaces which have been thus subjected to ice erosion are characteristically smoothed and more or less scratched, striated, or ground due to the corrosive effects of small and large rock fragments. This affords one of the best means of proving the former presence of a glacier over a region or in a valley. A typical V-shaped stream cut (eroded) valley is changed into one with a U-shaped profile or cross section by glacier erosion (Plate 5).

      Another important process of ice erosion is “plucking,” which consists in pushing among already more or less loosened joint blocks by the pressure of the moving ice. The pressure thus exerted, especially by a deep valley glacier, may be enormous. This process was an important factor in the development of the famous Yosemite Valley, a very brief account of whose history it will now be instructive to give.

      Plate 3.—The Gorge of Niagara River Below the Great Falls. The strata (containing fossils) were accumulated on the bottom of the Silurian sea which overspread the region at least 18,000,000 years ago. Since the Ice Age or within 20,000 to 40,000 years, the river has carved out the gorge. (Courtesy of the Haines Photo Company, Conneaut, Ohio.)

      Plate 4.—(a) A Winding Stream in the St. Lawrence Valley of New York. Due to its low velocity the stream cuts its channel down very little, but it swings or “meanders” slowly from one side of its valley to the other, developing a wide flood plain. The stream once flowed against the valley wall shown at the middle left. (Photo by the author.)

      Plate 4.—(b) Davidson Glacier, Alaska. This glacier is at work slowly grinding down the valley floor and cutting back its walls, thus changing the original stream-cut, V-shaped profile, like that of Plate 5. (Photo by Wright, U. S. Geological Survey.)

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