MAN AND THE GLACIAL PERIOD. CHAPTER I. INTRODUCTORY. THAT glaciers now exist in the Alps, in the Scandinavian range, in Iceland, in the Himalayas, in New Zealand, in Patagonia, and in the mountains of Washington, British Columbia, and southeastern Alaska, and that a vast ice-sheet envelops Greenland and the Antarctic Continent, are statements which can be verified by any one who will take the trouble to visit those regions. That, at a comparatively recent date, these glaciers extended far beyond their present limits, and that others existed upon the highlands of Scotland and British America, and at one time covered a large part of the British Isles, the whole of British America, and a considerable area in the northern part of the United States, are inferences drawn from phenomena which are open to every one's observations. That man was in existence and occupied both Europe and America during this great expansion of the northern glaciers is proved by evidence which is now beyond dispute. It is the object of the present volume to make a concise presentation of the facts which have been rapidly accumulating during the past few years relating to the Glacial period and to its connection with human history. Before speaking of the number and present extent of existing glaciers, it will be profitable, however, to devote a little attention to the definition of terms. B A glacier is a mass of ice so situated and of such size as to have mction in itself. The conditions determining the character and rate of this motion will come up for statement and discussion later. It is sufficient here to say that ice has a capacity of movement similar to that possessed by such plastic substances as cold molasses, wax, tar, or cooling lava. The limit of a glacier's motion is determined by the forces which fix the point at which its final melting takes place. This will therefore depend upon both the warmth of the weather and upon the amount of ice. If the ice is abundant, it will move farther into the region of warm temperature than it will if it is limited in supply. Upon ascending a glacier far enough, one reaches a motionless part corresponding to the lake out of which a river often flows. Technically this motionless part is called the névé. Glacial ice is formed from snow where the annual fall is in excess of the melting power of the sun at that point. Through the influence of pressure, such as a boy applies to a snow-ball (but which in the névé field arises from the weight of the accumulating mass), the lower strata of the névé are gradually transformed into ice. This process is also assisted by the moisture which percolates through the snowy mass, and which is furnished both by the melting of the surface snow and by occasional rains. The division between the névé and the glacier proper is not always easily determined. The beginnings of the glacial movement-that is, of the movement of the ice-stream flowing out of the névé field-are somewhat like the beginnings of the movement of the water from a great lake into its outlet. The névé is the reservoir from which the glacier gets both its supply of ice and the impulse which gives it its first movement. There can not be a glacier without a névé field, as there can not be a river without a drainage basin. But there may be a névé field without a glacierthat is, a basin may be partially filled with snow which never melts completely away, while the equilibrium of forces is such that the ice barely reaches to the outlet from which the tongue-like projection (to which the name glacier would be applied) fails to emerge only because of the lack of material. FIG. 2.-Illustrates the formation of veined structure by pressure at the junction of two branches. A glacier is characterised by both veins and fissures. The veins give it a banded or stratified appearance, blue alternating with lighter-coloured portions of ice. As these bands are not arranged with any apparent uniformity in the glacier, their explanation has given rise to much discussion. Sometimes the veins are horizontal, sometimes vertical, and at other times at an angle with the line of motion. On close investigation, however, it is found that the veins are always at right angles to the line of greatest pressure. This leads to the conclusion that pressure is the cause of the banded structure. The blue strata in the ice are those from which the particles of air have been FIG. 3. FIG. 4. FIGS. 3, 4.-Illustrate the formation of mar. ginal fissures and veins. expelled by pressure; the lighter portions are those in which the particles are less thoroughly compacted. Snow is but pulverized ice, and differs in colour from the compact mass for the same reason that almost all rocks and min erals change their colour when ground into a powder. The fissures, which, when of large size, are called crevasses, are formed in those portions of a glacier where, from some cause, the ice is subjected to slight tension. This occurs especially where, through irregularities in the bot tom, the slope of the descent is increased. The ice, then, instead of moving in a continuous stream at the top, cracks open e along the line of tension, and S FIG. 5.-c, c, show fissures and seracs where the glacier tending from moves down the steeper portion of its incline; 8, 8, the top down to a greater or less distance, according to the degree of tension. Usually, however, the ice remains continuous in the lower strata, and when the slope is diminished the pressure reunites the faces of the fissure, and the surface becomes again comparatively smooth. Where there are extensive areas of tension, the surface of the ice sometimes becomes exceedingly broken, presenting a tangled mass of towers, domes, and pinnacles of ice called seracs. Like running water, moving ice is a powerful agent in transporting rocks and earthy débris of all grades of fineness; but, owing to the different consistencies of ice. and water, there are great differences in the mode and result of transportation by them. While water can hold. in suspension only the very finest material, ice can bear its surface upon bles which would Lateral Moraines. be unaffected ex- FIG. 6.-Section across Glacial Valley, showing old cept by torrential currents of water. We find, therefore, a great amount of earthy material of all sizes upon the top of a glacier, which has reached it very much as débris reaches the bed of a river, namely, by falling down upon it from overhanging cliffs, or by land-slides of greater or less extent. Such material coming into a river would either disappear beneath its surface, or would form a line of débris along the banks; in both cases awaiting the gradual erosion and transportation which running water is able to effect. But, in case of a glacier, the material rests upon the surface of the ice, and at once begins to partake of its motion, while successive accessions of material keep up the supply at any one point, so as to form a train of boulders and other débris, extending below the point as far as the glacial motion continues. |