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

by Nathaniel Southgate Shaler

Waterfall near Gadsden, Alabama. The upper shelf of rock is a hard sandstone, the lower beds are soft shale. The conditions are those of most waterfalls, such as Niagara.

The history of plains, as well as that of mountains, will have further light thrown upon it when in the next chapter we come to consider the effect of rain water on the land. We may here note the fact that the level surfaces which are above the seashores are divisible into two main groups—those which have been recently lifted above the sea level, composed of materials laid down in the shallows next the shore, and which have not yet shared in mountain-building disturbances, and those which have been slightly tilted in the manner before indicated in the case of the plains which border the Rocky Mountains on the east. The great southern plain of eastern and southern United States, extending from near New York to Mexico, is a good specimen of the level lands common on all the continents which have recently emerged from the sea. The table-lands on either side of the Mississippi Valley, sloping from the Alleghanies and the Cordilleras, represent the more ancient type of plain which has already shared in the elevation which mountain-building brings about. In rarer cases plains of small area are formed where mountains formerly existed by the complete moving down of the original ridges.

There is a common opinion that the continents are liable in the course of the geologic ages to very great changes of position; that what is now sea may give place to new great lands, and that those already existing may utterly disappear. This opinion was indeed generally held by geologists not more than thirty years ago. Further study of the problem has shown us that while parts of each continent may at any time be depressed beneath the sea, the whole of its surface rarely if ever goes below the water level. Thus, in the case of North America, we can readily note very great changes in its form since the land began to rise above the water. But always, from that ancient day to our own, some portion of the area has been above the level of the sea, thus providing an ark of refuge for the land life when it was disturbed by inundations. The strongest evidence in favour of the opinion that the existing continents have endured for many million years is found in the fact that each of the great lands preserves many distinct groups of animals and plants which have descended from ancient forms dwelling upon the same territory. If at any time the relatively small continent of Australia had gone beneath the sea, all of the curious pouched animals akin to the opossum and kangaroo which abound in that country—creatures belonging in the ancient life of the world—would have been overwhelmed.

We have already noted the fact that the uplifting of mountains and of the table-lands about them, which appears to have been the basis of continental growth, has been due to strains in the rocks sufficiently strong to disturb the beds. At each stage of the mountain-building movement these compressive strains have had to contend with the very great weight of the rocks which they had to move. These lands are not to be regarded as firm set or rigid arches, but as highly elastic structures, the shapes of which may be determined by any actions which put on or take off burden. We see a proof of this fact from numerous observations which geologists are now engaged in making. Thus during the last ice epoch, when almost all the northern part of this continent, as well as the northern part of Europe, was covered by an ice sheet several thousand feet thick, the lands sank down under their load, and to an extent roughly proportional to the depth of the icy covering. While the northern regions were thus tilted down by the weight which was upon them, the southern section of this land, the region about the Gulf of Mexico, was elevated much above its present level; it seems likely, indeed, that the peninsula of Florida rose to the height of several hundred feet above its present shore line. After the ice passed away the movements were reversed, the northern region rising and the southern sinking down. These movements are attested by the position of the old shore lines formed during the later stages of the Glacial epoch. Thus around Lake Ontario, as well as the other Great Lakes, the beaches which mark the higher positions of those inland seas during the closing stages of the ice time, and which, of course, were when formed horizontal, now rise to the northward at the rate of from two to five feet for each mile of distance. Recent studies by Mr. G.K. Gilbert show that this movement is still in progress.

Other evidence going to show the extent to which the movements of the earth's crust are affected by the weight of materials are found in the fact that wherever along the shores thick deposits of sediments are accumulated the tendency of the region where they lie is gradually to sink downward, so that strata having an aggregate thickness of ten thousand feet or more may be accumulated in a sea which was always shallow. The ocean floor, in general, is the part of the earth's surface where strata are constantly being laid down. In the great reservoir of the waters the débris washed from the land, the dust from volcanoes, and that from the stellar spaces, along with the vast accumulation of organic remains, almost everywhere lead to the steadfast accumulation of sedimentary deposits. On the other hand, the realms of the surface above the ocean level are constantly being worn away by the action of the rivers and glaciers, of the waves which beat against the shores, and of the winds which blow over desert regions. The result is that the lands are wearing down at the geologically rapid average rate of somewhere about one foot in five thousand years. All this heavy matter goes to the sea bottoms. Probably to this cause we owe in part the fact that in the wrinklings of the crust due to the contraction of the interior the lands exhibit a prevailing tendency to uprise, while the ocean floors sink down. In this way the continents are maintained above the level of the sea despite the powerful forces which are constantly wearing their substance away, while the seas remain deep, although they are continually being burdened with imported materials.

Fig. 8.—Diagram showing the effect of the position of the fulcrum point in the movement of the land masses. In diagrams I and II, the lines a b represent the land before the movement, and a' b' its position after the movement; s, s, the position of the shore line; p, p, the pivotal points; l, s, the sea line. In diagram III, the curved line designates a shore; the line a b, connecting the pivotal points p, p, is partly under the land and partly under the sea.

It is easy to see that if the sea floors tend to sink downward, while the continental lands uprise, the movements which take place may be compared with those which occur in a lever about a fulcrum point. In this case the sea end of the bar is descending and the land end ascending. Now, it is evident that the fulcrum point may fall to the seaward or to the landward of the shore; only by chance and here and there would it lie exactly at the coast line. By reference to the diagram (Fig. 8), it will be seen that, while the point of rotation is just at the shore, a considerable movement may take place without altering the position of the coast line. Where the point of no movement is inland of the coast, the sea will gain on the continent; where, however, the point is to seaward, beneath the water, the land will gain on the ocean. In this way we can, in part at least, account for the endless changes in the attitude of the land along the coastal belt without having to suppose that the continents cease to rise or the sea floors to sink downward. It is evident that the bar or section of the rocks from the interior of the land to the bottoms of the seas is not rigid; it is also probable that the matter in the depths of the earth, which moves with the motions of this bar, would change the position of the fulcrum point from time to time. Thus it may well come about that our coast lines are swaying up and down in ceaseless variation.

In very recent geological times, probably since the beginning of the last Glacial period, the region about the Dismal Swamp in Virginia has swayed up and down through four alternating movements to the extent of from fifty to one hundred feet. The coast of New Jersey is now sinking at the rate of about two feet in a hundred years. The coast of New England, though recently elevated to the extent of a hundred feet or more, at a yet later time sank down, so that at some score of points between New York and Eastport, Me., we find the remains of forests with the roots of their trees still standing below high-tide mark in positions where the trees could not have grown. Along all the marine coasts of the world which have been carefully studied from this point of view there are similar evidences of slight or great modern changes in the level of the lands. At some points, particularly on the coast of Alaska and along the coast of Peru, these uplifts of the land have amounted to a thousand feet or more. In the peninsular district of Scandinavia the swayings, sometimes up and sometimes down, which are now going on have considerably changed the position of the shore lines since the beginning of the historical period.

There are other causes which serve to modify the shapes and sizes of the continents which may best be considered in the sequel; for the present we may pass from this subject with the statement that our great lands are relatively permanent features; their forms change from age to age, but they have remained for millions of years habitable to the hosts of animals and plants which have adapted their life to the conditions which these fields afford them.