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.

A _glacier_ is a mass of ice so situated and of such size as to have motion 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 comparatively motionless part corresponding to the lake out of which a river often flows. Technically this 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 glacier--that 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.

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 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 minerals 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 bottom, the slope of the descent is increased. The ice, then, instead of moving in a continuous stream at the top, cracks open along the line of tension, and wedge-shaped fissures are formed extending from 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 upon its surface rocks of the greatest magnitude, and can roll or shove along under it boulders and pebbles which would be Unaffected except 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.

Such a line of _débris_ is called a _moraine_. When it forms along the edge of the ice, it is called a _lateral_ moraine. It is easy to see that, where glaciers come out from two valleys which are tributary to a larger valley, their inner sides must coalesce below the separating promontory, and the two lateral moraines will become united and will move onward in the middle of the surface of the glacier. Such lines of _débris_ are called _medial_ moraines. These are characteristic of all extensive glaciers formed by the union of tributaries. There is no limit to the number of medial moraines, except in the number of tributaries.

A medial moraine, when of sufficient thickness, protects the ice underneath it from melting; so that the moraine will often appear to be much larger than it really is: what seems to be a ridge of earthy material being in reality a long ridge of ice, thinly covered with earthy _débris_, sliding down the slanting sides as the ice slowly wastes away Large blocks of stone in the same manner protect the ice from melting underneath, and are found standing on pedestals of ice, often several feet in height. An interesting feature of these blocks is that, when the pedestal fails, the block uniformly falls towards the sun, since that is the side on which the melting has proceeded most rapidly.

If the meteorological forces are so balanced that the foot of a glacier remains at the same place for any great length of time, there must be a great accumulation of earthy _débris_ at the stationary point, since the motion of the ice is constantly bearing its lines of lateral and medial moraine downwards to be deposited, year by year, at the melting line along the front.

Such accumulations are called _terminal_ moraines, and the process of their formation may be seen at the foot of almost any large glacier. The pile of material thus confusedly heaped up in front of some of the larger glaciers of the world is enormous.

The melting away of the lower part of a glacier gives rise also to several other characteristic phenomena. Where the foot of a glacier chances to be on comparatively level land, the terminal moraine often covers a great extent of ice, and protects it from melting for an indefinite period of time. When the ice finally melts away and removes the support from the overlying morainic _débris_, this settles down in a very irregular manner, leaving enclosed depressions to which there is no natural outlet. These depressions, from their resemblance to a familiar domestic utensil, are technically known as _kettle-holes_. The terminal moraines of ancient glaciers may often be traced by the relative abundance of these kettle-holes.

The streams of water arising both from the rainfall and from the melting of the ice also produce a peculiar effect about the foot of an extensive glacier. Sometimes these streams cut long, open channels near the end of the glacier, and sweep into it vast quantities of morainic material, which is pushed along by the torrential current, and, after being abraded, rolled, and sorted, is deposited in a delta about its mouth, or left stranded in long lines between the ice-walls which have determined its course. At other times the stream has disappeared far back in the glacier, and plunged into a crevasse (technically called a _moulin_), whence it flows onwards as a subglacial stream. But in this case the deposits might closely resemble those of the previous description. In both cases, when the ice has finally melted away, peculiar ridge-like deposits of sorted material remain, to mark the temporary line of drainage. These exist abundantly in most regions which have been covered with glacial ice, and are referred to in Scotland as _kames_, in Ireland as _eskers_, and in Sweden as _osars_. In this volume we shall call them _kames_, and the deltas spread out in front of them will be referred to as _kame-plains_.

With this preliminary description of glacial phenomena, we will proceed to give, first, a brief enumeration and description of the ice-fields which are still existing in the world; second, the evidences of the former existence of far more extensive ice-fields; and, third, the relation of the Glacial period to some of the vicissitudes which have attended the life of man in the world.

The geological period of which we shall treat is variously designated by different writers. By some it is simply called the "post-Tertiary," or "Quaternary"; by others the term "post-Pliocene" is used, to indicate more sharply its distinction from the latter portion of the Tertiary period; by others this nicety of distinction is expressed by the term "Pleistocene." But, since the whole epoch was peculiarly characterised by the presence of glaciers, which have not even yet wholly disappeared, we may properly refer to it altogether under the descriptive name of "Glacial" period.