CHAPTER V

GLACIERS AND THEIR WORK

A glacier may be defined as a mass of flowing ice. The motion may not be that of flowage in the usually accepted sense of the term. A discussion of the various theories of glacier motion will not here be attempted. Glaciers form only in regions of perpetual snow, but they commonly move down far below the line of perpetual snow of any given region. In the polar regions they may form near sea level, while in the tropics they form at altitudes of two to three miles, and there only rarely. In southern Alaska, the lower limit of perpetual snow 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.

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.

The Yosemite Valley, about 7 miles long, less than one mile wide, and from 2,000 to 4,000 feet deep, lies on the western slope of the Sierra Nevada Mountains of California. Great cliffs of granite, mostly from 1,000 to over 3,000 feet high, bound the valley on either side. The floor of the valley is wide and remarkably flat (Plate 6). Just prior to the Ice Age, by the processes of erosion already set forth, the Merced River had carved out a great steep-sided V-shaped canyon commonly from 1,000 to 3,000 feet deep. During the Ice Age, two glaciers joined to form an extra deep powerful glacier, which flowed through a deep part of the Merced Canyon and modified it into the Yosemite Valley, essentially as we see it to-day. Because the ice was shod with many fragments of hard rock (granite), and the pressure at the bottom and lower sides of the glacier (several thousand feet thick) was so great, the V-shaped stream-cut canyon was changed to a U-shaped canyon with very steep to even vertical walls. A factor of great importance which notably aided the erosive power of the glacier in this case was the existence of an unusual number of large vertical joint cracks in the granite in this local region. The plucking action of the ice was thus very greatly facilitated and great slabs of rock, separated by the vertical joints, especially toward the lower sides and bottom of the valley, were pushed away one after another by the ice. When the ice disappeared, great precipitous joint faces from 1,000 to 3,000 feet high were left along the valley sides. At its lower end the glacier left a dam of glacial débris (moraine) across the valley, thus causing a lake to form over the valley floor. The wide flat bottom of the valley was caused by filling up of the lake with sediment. The uniqueness of the Yosemite Valley is, then, due to a remarkable combination of several main factors; one, the presence of a large swift river well supplied with tools which carved out a deep V-shaped canyon; two, a mighty glacier which plowed its way through this canyon and converted it by erosion into a U-shaped canyon; three, the weakening of the rock by many joint cracks, thus greatly facilitating the ice erosion; and four, a postglacial lake covering the valley floor which became filled with sediment. As a result of the ice work, several streams, tributary to the main stream (Merced River) which flows through the bottom of the valley, were forced to plunge over great vertical rock walls (joint faces), thus producing high and beautiful true waterfalls, including the very high Upper Yosemite Fall where Yosemite Creek makes a straight drop of 1,430 feet. A tributary valley like that of Yosemite Creek, which ends abruptly well above the main valley, is known as a "hanging" valley. The valley of Bridal Veil Creek is another good example. (See Plate 6.) Valleys which were once occupied by active glaciers are generally characterized by their U-shaped cross sections and their hanging (tributary) valleys, but the great height and steepness of the valley walls in Yosemite are exceptional.

A type of glacial erosion which is of special interest is the sculpturing of so-called "cirques" or "amphitheaters" in mountains within the region of perpetual snow. Where the main mass of snow and ice in the catchment basin or gathering ground of a valley glacier pulls away from the snow and névé on the upper slopes, the rock wall is more or less exposed in the deep crevasse. During warm days water fills the joint cracks in the rocks down in this crevasse (so-called "Bergschrund"), and during cold nights the water freezes and forces the blocks of rock apart. This is greatest toward the bottom of the crevasse and so, by this excavating or quarrying process, vertical or very steep walls are developed around a great bowlike basin or cirque. Such cirques, now free from glacial ice, with precipitous walls 500 to 2,000 feet high and one-fourth of a mile to one-half of a mile across, are common in the Sierra Nevada and Cascade Ranges and in the Rocky Mountains.

What becomes of the materials eroded by the ice? An answer to this question involves at least a brief discussion of the deposition of glacial débris, this constituting an important feature of the work of ice. The débris transported by a glacier is carried either on its surface or within it, or pushed along under it. It is generally heterogeneous material ranging from the finest clay through sand and gravel, to bowlders of many tons' weight. Various types of glacial deposits are abundantly illustrated by débris left strewn over much of the northeastern United States and some reference to these will be made.

Most valley glaciers carry considerable débris on their surfaces, this representing material which falls or is carried down from the valley walls upon the margins of the ice, thus forming marginal moraines. When two glaciers flow together, one marginal moraine from each will coalesce to form a medial moraine. The material carried along at the bottom of a glacier is called the ground moraine. Where it contains much very fine grained material with pebbles or bowlders scattered through its mass, it is called "till" or "bowlder clay." The pebbles or bowlders of the ground moraine are commonly facetted and striated as a result of having been rubbed against the bedrock on which the glacier moved. Ground moraine material is the most extensively developed of all glacial deposits. It is so widely scattered over the glaciated northeastern portion of the United States that most of the soils consist of it, having been left strewn over the country during the melting of the vast ice sheet.

When a glacier remains practically stationary for some time, more or less material which it carries is piled up at its lower end to form a terminal moraine. Repeated pauses during general glacier retreat permit the accumulations of so-called recessional moraines. A wonderful display of recessional moraines occurs from the Great Lakes south, where they are festooned one within another and remain almost exactly as they were formed during pauses in retreat of great lobes of ice during the closing stages of the Ice Age. A great terminal moraine marks the southernmost limit of the ice sheet during the Ice Age, a very fine illustration being the ridge of low irregular hills extending the whole length of Long Island. Some of the material in that morainic ridge was transported by the ice from northern New England.

Considerable rock débris is transported within the ice, and such "englacial" material in part results from rock débris which falls on the surface in the catchment basin and becomes buried under new snowfalls which change to ice, and in part from material which falls into the crevasses in the glacier farther down the valley. Marked objects thrown into the catchment basin have, after many years, emerged at or near the end of the glacier; thus the rate of motion can be very accurately told. A very remarkable case of transportation through the body of a glacier is the following: In 1820, three men were buried under an avalanche in the catchment basin of the Bossons Glacier in the Alps. Forty-one years later several parts of the bodies, including the three heads together with some pieces of clothing, emerged at the foot of the glacier after traveling most of its length at the rate of eight inches per day. The heads were so perfectly preserved after their remarkable journey in cold storage that they were clearly recognized by former friends!

Where a valley floor slopes downward away from the end of a glacier, waters emerging from the ice, heavily loaded with rock débris, cause more or less deposition of the débris on the valley floor often for miles beyond the ice front. Such a deposit is called a "valley train." When the ice front pauses for a considerable time upon a rather flat surface, the débris-laden waters emerging from the ice develop an "outwash plain" by deposition of sediment rather uniformly over the flat surface. A very fine example is the plain which constitutes most of the southern half of Long Island just beyond the southern limit of the great terminal moraine ridge.

A type of glacial deposit of particular interest is the "drumlin" which is, in reality, only a special form of ground moraine material (commonly till), and, therefore, essentially unstratified. Typical drumlins are low, rounded mounds of till with roughly elliptical bases and steeper fronts facing the direction from which the ice flowed. Their long axes are always parallel to the direction of ice movement. In height they commonly range from 50 to 200 feet. Their mode of origin is not yet definitely known, but they form near the margins of broad lobes of ice either by erosion of earlier glacial deposits, or by accumulation beneath the ice under peculiarly favorable conditions, as perhaps in the longitudinal crevasses. One of the finest and most extensive exhibitions of drumlins in the world is in western New York between Syracuse and Rochester. Thousands of drumlins there rise above the general level of the Ontario plain, the New York Central Railroad passing through the very midst of them. Drumlins are also abundant in eastern Wisconsin.

Another type of glacial deposit in the form of low hills is the "kame" which, unlike the drumlin, always consists of more or less stratified material. Kames are seldom over 200 feet high, and they are of various shapes. In many cases they form irregular groups of hills, and in other cases fairly well defined kame ridges. Kames form as deposits from débris-laden streams emerging from the margins of glaciers, the water sometimes rising as great fountains because of the pressure. Such deposits are now actually in process of formation along the edge of the great Malaspina Glacier of Alaska. Kames are commonly associated with terminal and recessional moraines. "Eskers" are similar except that they are long winding low ridges of stratified material deposited by débris-laden streams, probably in longitudinal fissures in the ice near its margin. (See Plate 20.)

Glacial bowlders, or "erratics" are blocks of rock or bowlders left strewn over the country during the melting of the ice. They vary in size from small pebbles to those of many tons of weight, and most of them were derived from ledges of relatively hard, resistant rocks. (See Plate 20.) Erratics have very commonly been carried a few miles from their parent ledges, while more rarely they have traveled even hundreds of miles. They are extremely abundant in New York and New England, many occurring even high up on the mountains. In some cases erratics of ten or more tons' weight have been left in such remarkably balanced positions on bedrock that a child can cause one of them to swing back and forth slightly. Such a bowlder is literally a "rocking stone." In the Adirondack Mountains the writer recently observed a rounded erratic of very hard rock fourteen feet in diameter resting in a very remarkably balanced position on top of another large round glacial bowlder.