Читать книгу Walks and Talks in the Geological Field - Alexander Winchell - Страница 11

Shall we proceed? This subject has its alphabet, like most others; and every child can testify that there is little inspiration in the alphabet. A few more letters of our alphabet will be found in the arrangement of the loose materials which cover the surface of the northern states. These materials are called Drift. The bowlders are a part of the Drift. Now we wish to know more about the internal constitution of this deposit. This will be easy, for the Drift is all about us, and numberless deep excavations have been made for sundry purposes. Let us visit a gravel pit, or some deep railroad cut through a pile of these incoherent materials.

Do you find these loose sands and gravels arranged in regular courses? Yes, you say; and then you hesitate; and well you may; for the semblance of courses is exceedingly interrupted. Here is indeed, a layer or bed, or stratum of sand, but it thins out in one direction, and in the other, loses its upper and lower boundaries, and merges in a general mass of sand. Here is a bed of gravel, but it lies at a different inclination from the last, and in one direction it changes to sand, while in the other, it becomes split up into a number of subordinate layers which bend down and lose themselves. This bed also is composed of many oblique laminæ, coarser and finer in alternation, which are cut off completely by the upper and lower surfaces of the bed or stratum. What is singular, the very next bed below this, which is also obliquely laminated, has its laminæ tilted in the opposite direction. And then next to this is a long straight course of cobble-stones and pebbles. Is not this a correct description of what you have all seen somewhere?

In some places are large beds of fine sand, which are taken out and used for mortar-making. In others we find extensive deposits of gravel and pebbles, which are used for paths and streets. Mixed in the sands are some cobble-stones and large bowlders. Here and there, too, are some beds containing much clay; and these are impervious to water. Now, all this is not a regular nor a perfect bedding or stratification. We may say the Drift here is semi-stratified. You can all recall some locality where this arrangement of materials occurs.

This cut or exposure, however, extends only fifteen or twenty feet down. How is the arrangement below? There are places where the bed-rock is not reached in less than a hundred or two hundred feet. There are wells fifty to eighty feet deep, without reaching bed-rock. Those who have seen such wells have observed the deeper structure of the Drift; and they report it much like what we see in the gravel-pit. I will tell you how we shall ascertain the arrangement to the depth of perhaps two hundred feet. Go to the lake-shore, or the sea-shore. Of course it must be a place where the shore is not formed of bed-rocks. Here the whole thickness of the Drift may be cut through, exposing at the bottom the solid foundation on which the Drift reposes. Well, here we find two kinds of Drift. The semi-stratified Drift passes down into a sheet of Drift quite unstratified. It consists of blue clay and a large quantity of imbedded bowlders. These are rounded like those at the surface. They are in every respect the same thing—made, apparently, by the same agency; transported in the same company. This is the Bowlder Clay or Till.

We must state, however, that in some situations the semi-stratified Drift rests directly on the bed-rock. Perhaps in these places the Bowlder Clay was washed off before the semi-stratified Drift was laid down. Again, there are many places where the semi-stratified Drift does not rest on the Bowlder Clay—perhaps because it was never laid down; but more probably because it has been removed. In such places the stiff, blue clay is exposed over the surface, and the soil is full of bowlders. Can you not call to mind such a place?

The sheets of sand and gravel, often obliquely laminated, which we saw in the gravel-pit, were there cut through in a vertical section presented edgewise. You must think of these sheets as extending into the earth a certain distance, but very irregular in extent as well as in form and position. Some of them are flat; some are concave upwards, and some are convex. Now and then one is nearly horizontal, but most are considerably inclined.

Did you ever see a huge mound of rock-rubbish at the foot of a torrent rushing down a steep ravine to the open, level land—a torrent sometimes suddenly swollen to a terrific and maddened volume, which tears stones and trees from their fastenings? And have you ever seen such mound cut through for a highway or other purpose? If you have, you have witnessed a semi-stratified order of deposition somewhat like that in the Drift. Those who have thought on this resemblance have reached the conclusion that the semi-stratified Drift must have been moved and laid down by some kind of torrential action.

But however this was, the origin of the bed of Bowlder Clay must have been very different. Here is no sort of bedding. The whole is in a state of uniform confusion. Evidently, then, Nature employed two kinds of action successively in transporting and dispersing the Drift. In the semi-stratified Drift, water in tumultuous movement may have been the chief agent. In the Bowlder Drift water was not the chief agent, since here is none of the assortment and stratification due to water, and here also are rock-masses moved scores or hundreds of miles, and these results are not ascribable to water.

Let us take another glance over the general distribution of the Drift. We have seen the bowlders increasing in bulk and abundance northward. We have seen the whole Drift formation terminating southward on about the parallel of Cincinnati. We find incoherent surface deposits in Kentucky and southward; but they contain no bowlders; and they have mostly resulted from the disintegration and decay of the bed-rocks in place. The Drift, then, is a northern phenomenon.

If we notice more carefully the detailed distribution of bowlders, we find that, while they have generally moved southward, there has also been a radial distribution from high mountains. In New Hampshire the bowlders move east and west from the White Mountains, as well as south. In Switzerland, the Pierre à bot and thousands of other bowlders moved north-westward from the Mont Blanc range—though on the opposite sides of Mont Blanc the movement was in the opposite direction. In the Rocky Mountains and the Sierra Nevada, the movement of the bowlders was east and west from the mountain axis. So, too, the southward distribution of bowlders was greatest along mountain elevations.

Thus the distribution of Drift materials sustains a relation to altitude similar to that which it sustains to latitude. What factor, or force, or agency exists in altitude which exists identically in latitude? Temperature, certainly. To ascend a high mountain range is the same as to ascend to a high latitude. All high mountains support animals and plants related to species farther north. On the summit of Mount Washington are the butterflies and plants of Labrador. Ascending the Andes, you have tropical products at the foot, temperate products at ten thousand feet, and arctic conditions at the summit. The distribution of the Drift, then, has relation to heat and cold. Greater cold has been accompanied by larger results. Bowlders are more numerous and more massive in northern and in elevated regions, because the cold is there more intense.

Now, how does cold act to effect transportation of rock-fragments? Our thoughts run over the world to scrutinize the modes of action of cold. Much cold implies much snow and ice, if moisture and water are abundant. Most far northern regions and high mountain summits are covered much of the year, or the whole of it, by a sheet of snow. Winter snow, under the action of thawing and freezing temperatures in alternation, becomes granular, as we often observe in old snow, especially in early spring. With a more advanced stage of granulation, the icy grains coalesce into larger grains, and finally merge completely into a solid mass of ice. This, also, we have often noticed in the last lingering patches of last winter’s snow.

We have many observations of this kind on a large scale. On high mountains broad fields of granular snow come into existence, and at a certain elevation the average annual temperature is not sufficient to dissolve it before autumnal snows begin to increase the amount. The old snow becomes a permanent granular sheet on the high slopes. In the Alps the Germans designate it Firn, and the French, Névé. When the firn-masses are accumulated in valleys, the amount of snow is so great that it may reach to a much lower altitude before finding a temperature which will suffice to melt it all away before the next winter. So tongues of granular snow stretch down the mountain valleys, and being, like our late spring snow, exposed to increased action of warmth, these valley prolongations of the upper firn become completely changed into solid ice. This is now a glacier.

We may reason a step further from facts of observation. All substances expand with increase of temperature, and contract with reduction of temperature. The glacier is certainly at a lower temperature in winter than in summer—though it can never be warmed above thirty-two degrees Fahrenheit, which is the thawing temperature. The surface of the glacier is also at a lower temperature during the night than during the day. The glacier, therefore, must sometimes expand and sometimes contract. Now, when it expands, the whole expansion will be developed at the free lower border, since the upper border is frozen to the earth, and pressed also, by the snows beyond. Also, if both were free, most of the expansion would be developed below, because gravity aids motion downwards. Next, when the glacier contracts, the lower border does not retreat, because the ice is not strong enough to bear the pull of the mass up the slope. The ice breaks in innumerable little cracks. These are soon filled with water, which freezes, and thus restores the complete solidity of the glacier. Thus, when the next expansion takes place, the glacier takes another slide down the valley. So the glacier travels. So, if a whole state should become glacier-covered, the ice-sheet would have a motion from higher to lower, and from colder to warmer. Every thing on its surface would be transported; every loose object beneath it or in front of it would be pushed forward.

Now, here are some hints toward an explanation of the method of transportation of our millions of bowlders. If we go to the Alps we find exactly such glaciers, on a small scale, performing precisely such work. Thus our theory receives confirmation. We can not pretend that glacier action explains all the phenomena of the Drift. Nor do we pretend that any thing more than a hint has been given toward an explanation of transportation. The action which arranged the semi-stratified Drift must have been exerted by water rather than ice. But we leave the subject now to your thoughts. You may speculate as much as you please for the purpose of forming a complete theory. You will find such occupation interesting and profitable. By and by we shall come upon this subject again from another direction. (Talk XLVII.)