Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

We have now to consider the causes which could bring about such great extensions of the ice sheet as occurred in the last Glacial period. Here again we are upon the confines of geological knowledge, and in a field where there are no well-cleared ways for the understanding. In facing this problem, we should first note that those who are of the opinion that a Glacial period means a very cold climate in the regions where the ice attained its extension are probably in error. Natural as it may seem to look for exceeding cold as the cause of glaciation, the facts show us that we can not hold this view. In Siberia and in the parts of North America bordering on the Arctic Sea the average cold is so intense that the ground is permanently frozen—as it is, for instance, in the Klondike district—to the depth of hundreds of feet, only the surface thawing out during the warm summers. All this region is cold enough for glaciers, but there is not sufficient snowfall to maintain them. On the other hand, in Greenland, and in a less though conspicuous degree in Scandinavia, where the waters of the North Atlantic somewhat diminish the rigour of the cold, and at the same time bring about a more abundant snowfall, the two actions being intimately related, we have very extensive glaciers. Such facts, which could be very much extended, make it clear that the climate of glacial periods must have been characterized by a great snowfall, and not by the most intense cold.

It is evident that what would be necessary again to envelop the boreal parts of North America with a glacial sheet would not be a considerable decrease of heat, but an increase in the winter's contribution of frozen water. Even if the heat released by this snowfall elevated the average temperature of the winter, as it doubtless would in a considerable measure, it would not melt off the snow. That snowfall tends to warm the air by setting free the heat which was engaged in keeping the water in a state of vapour is familiarly shown by the warming which attends an ordinary snowstorm. Even if the fall begin with a temperature of about 0° Fahr., the air is pretty sure to rise to near the freezing point.

It is evident that no great change of temperature is required in order to bring about a very considerable increase in the amount of snowfall. In the ordinary succession of seasons we often note the occurrence of winters during which the precipitation of snow is much above the average, though it can not be explained by a considerable climatal change. We have to account for these departures from the normal weather by supposing that the atmospheric currents bring in more than the usual amount of moisture from the sea during the period when great falls of snow occur. In fact, in explaining variations in the humidity of the land, whether those of a constant nature or those that are to be termed accidental, we have always to look to those features which determine the importation of vapour from the great field of the ocean where it enters the air. We should furthermore note that these peculiarities of climate are dependent upon rather slight geographic accidents. Thus the snowfall of northern Europe, which serves to maintain the glaciation of that region, and, curiously enough, in some measure its general warmth, depends upon the movement of the Gulf Stream from the tropics to high latitudes. If by any geographical change, such as would occur if Central America were lowered so as to make a free passage for its waters to the westward, the glaciers of Greenland and of Scandinavia would disappear, and at the same time the temperature of those would be greatly lowered. Thus the most evident cause of glaciation must be sought in those alterations of the land which affect the movement of the oceanic currents.

Applying this principle to the northern hemisphere, we can in a way imagine a change which would probably bring about a return of such an ice period as that from which the boreal realm is now escaping. Let us suppose that the region of not very high land about Bering Strait should sink down so as to afford the Kuro Siwo, or North Pacific equivalent of our Gulf Stream, an opportunity to enter the Arctic Sea with something like the freedom with which the North Atlantic current is allowed to penetrate to high latitudes. It seems likely that this Pacific current, which in volume and warmth is comparable to that of the Atlantic, would so far elevate the temperature of the arctic waters that their wide field would be the seat of a great evaporation. Noting once again the fact that the Greenland glaciers, as well as those of Norway, are supplied from seas warmed by the Gulf Stream, we should expect the result of this change would be to develop similar ice fields on all the lands near that ocean.

Applying the data gathered by Dr. Croll for the Gulf Stream, it seems likely that the average annual temperature induced in the Arctic Sea by the free entrance of the Japan current would be between 20° and 30° Fahr. This would convert this wide realm of waters into a field of great evaporation, vastly increasing the annual precipitation. It seems also certain that the greater part of this precipitation would be in the form of snow. It appears to the writer that this cause alone may be sufficient to account for the last Glacial period in the northern hemisphere. As to the probability that the region about Bering Strait may have been lowered in the manner required by this view, it may be said that recent studies on the region about Mount St. Elias show that during or just after the ice epoch the shores in that portion of Alaska were at least four thousand feet lower than at present. As this is but a little way from the land which we should have to suppose to be lowered in order to admit the Japan current, we could fairly conclude that the required change occurred. As for the cause of the land movement, geologists are still in doubt. They know, however, that the attitudes of the land are exceedingly unstable, and that the shores rarely for any considerable time maintain their position. It is probable that these swayings of the earth's surface are due to ever-changing combinations of the weight in different parts of the crust and the strains arising from the contraction of its inner parts.

In the larger operations of Nature the effects which we behold, however simple, are rarely the products of a single cause. In fact, there are few actions so limited that they can fairly be referred to one influence. It is therefore proper to state that there are many other actions besides those above noted which probably enter into those complicated equations which determine the climatal conditions of the earth. To have these would carry us into difficult and speculative inquiries.

As before remarked, all the regions which have been subjected to glaciation are still each year brought temporarily into the glacial state. This fact serves to show us that the changes necessary to produce great ice sheets are not necessarily of a startling nature, however great the consequences may be. Assuming, then, that relatively slight alterations of climate may cause the ice sheet to come and go, we may say that all the influences which have been suggested by the students of glaciation, and various other slighter causes which can not be here noted, may have co-operated to produce the peculiar result. In this equation geographic change has affected the course of the ocean currents, and has probably been the most influential, or at least the commonest, cause to which we must attribute the extension of ice sheets. Next, alterations of the solar heat may be looked to as a change-bringing action; unfortunately, however, we have no direct evidence that this is an efficient cause. Thirdly, the variations in the eccentricity of the earth's orbit, combined with the precession of the equinoxes and the rotation of the apsides, may be regarded as operative. The last of all, changes in the constitution of the atmosphere, have to be taken into account. To these must be added, as before remarked, many less important actions which influence this marvellously delicate machine, the work of which is expressed in the phenomena assembled under the name of climate.

Evidence is slowly accumulating which serves to show that glacial periods of greater or less importance have been of frequent occurrence at all stages in the history of the earth of which we have a distinct record. As these accidents write their history upon the ground alone, and in a way impermanently, it is difficult to trace the ice times of ancient geological periods. The scratches on the bed rocks, and the accumulations of detritus formed as the ice disappeared, have alike been worn away by the agents of decay. Nevertheless, we can trace here and there in the older strata accumulations of pebbly matter often containing large boulders, which clearly were shaped and brought together by glacial action. These are found in some instances far south of the region occupied by the glaciers during the last ice epoch. They occur in rocks of the Cambrian or Silurian age in eastern Tennessee and western North Carolina; they are also found in India beyond the limits to which glaciers have attained in modern times.

In closing this inadequate account of glacial action, a story which for its complete telling would require many volumes, it is well for the reader to consider once again how slight are the changes of climate which may alternately withdraw large parts of the land from the uses of life, and again quickly restore the fields to the service of plants and animals. He may well imagine that these changes, by driving living creatures to and fro, profoundly affect the history of their development. This matter will be dealt with in the volume concerning the history of organic beings.

When the ice went off from the northern part of this continent, the surface of the country, which had been borne down by the weight of the glacier, still remained depressed to a considerable depth below the level of the sea, the depression varying from somewhere about one hundred feet in southern New England to a thousand feet or more in high latitudes. Over this region, which lay beneath the level of the sea, the glacier, when it became thin enough to float, was doubtless broken up into icebergs, in the manner which we now behold along the coast of Greenland. Where the shore was swept by a strong current, these bergs doubtless drifted away; but along the most of the coast line they appear to have lain thickly grouped next the shores, gradually delivering their loads of stones and finer débris to the bottom. These masses of floating ice in many cases seem to have prevented the sea waves from attaining the shore, and thus hindered the formation of those beaches which in their present elevated condition enable us to interpret the old position of the sea along coast lines which have been recently elevated. Here and there, however, from New Jersey to Greenland, we find bits of these ancient shores which clearly tell the story of that down-sinking of the land beneath the burden of the ice which is such an instructive feature in the history of that period.