Читать книгу The Romance of Modern Geology - Edwin Sharpe Grew - Страница 16

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The astronomers who look at the planet Mars tell us that at the Northern and Southern Poles there are great areas of snow, very much greater than the arctic regions of the earth, for the south polar area alone occupies 11,330,000 square miles. But the geological records of the earth show that our own arctic regions once extended very much farther than they do at present, a fact which need not in any way surprise us, for as we have already remarked, snow and ice are very largely a matter of the nearness of the sea to the land. We may put the same thing in another way by saying that winter cold and summer heat depend largely on the distribution of sea and land. Thus Venice, which is not very much farther from the North Pole than Vladivostok, has an altogether different climate, and inhabitants of the Shetland Isles have a very different kind of climatic experience from those who delve by the frozen Yukon. There is another consideration which is sometimes overlooked. We do not think of the earth as a very warm body. But at its very coldest part, where the thermometer goes down to seventy or ninety degrees below freezing, it is several hundreds of degrees warmer than the space outside the earth. Midway between the earth and the moon the temperature must be 430° F. below freezing; so that if we take the surface of the earth as a whole we may say that it is between four hundred and five hundred degrees warmer than the space by which it is surrounded. Every one knows what is happening when he warms his hands at a fire. The fire being hotter than its surroundings is giving out heat towards them, and the hands catch some of this radiated heat. Similarly the earth is radiating heat, and the atmosphere round the earth catches some of it. So also do the seas. While therefore it is certain that the heat of the sun warms the earth and the air and the sea, and so gives rise to currents of air and perhaps of water, so also is it likely that the heat of the earth causes warm air to rise, and so plays its part in forming the winds, the currents of air, and the currents of water. When the earth was warmer than it is now it had more and greater effects in this direction. It caused more evaporation of the water, more clouds, and therefore more rain, and in winter more snow.

Suppose, then, a period when there was very much more snow in winter than now. As the snow accumulated layer on layer the lower part would become squeezed into a mass half ice, half snow; and it is quite likely that the heat of the summer sun (especially if, as we have supposed, the atmosphere was much cloudier then than now) would be unable to dissolve it. Thus the snow age would gradually merge into an ice age, and we can imagine a period when a great deal of Europe was covered with snow or ice as Greenland is to-day. What records would it leave behind? Now, on slopes the sheet of snow would tend to slip just as it slips from sloping house-roofs. In doing so it would push before it any loose material which lay in front of it; and trees or bushes, stone or soil, would be gradually pushed downhill. If the slopes were steep enough the snow-sheets would occasionally break off and sweep down as avalanches. Sometimes these great masses are many thousands of square yards in area and fifty feet thick, and in the late winter and early spring often do immense damage, carrying away houses, trees, and great masses of rocks in their progress. They leave their imprint not only in ground swept bare, but in huge mounds of debris piled up in the valleys below.

But when the snow has taken the form of a glacier its record is left in more unmistakable characters. Imagine a great mass of snow and ice descending between the clefts of mountains to lower levels. As it slips slowly down its valley, like a very slow river—slower than a river of thickest mud or pitch or lava would be—earth, sand, mud, gravel, boulders, and masses of rock sometimes are washed down on its surface from the slopes on either side. Avalanches will occasionally bring other contributions. Nearly all this rubbish accumulates on the edges of the glacier nearest the slopes, and it is slowly borne to the journey's end on the glacier's shoulders. Some of it falls into rents or crevasses in the ice, and may be imprisoned there and carried down as an inside passenger, or it may reach the rocky floor over which the ice is sliding. Its progress then resembles that of the Irish gentleman who was travelling in a Sedan chair out of which the seat and the bottom had fallen, and who said that if 'twere not for the fashion of the thing he'd as lief walk. The rubbish borne onward on the surface of the glacier is known as moraine-stuff, and the mounds of it at the edge of the glacier are called lateral moraines. Where two glaciers unite like two rivers, their moraines, right-hand and left-hand, will join, and in the new glacier a new moraine will appear running down the middle, and so called a medial moraine. Where a glacier has many tributaries bearing a good deal of moraine-stuff, its surface may be like a bare plain so covered with stones that the ice beneath can hardly be discerned except here and there. At the end of the glacier where the ice melts, the heaps of stones, ever adding to their numbers, are deposited in heaps, to which are given the name of the terminal moraine.

With such tokens of their existence as this, glaciers, as will readily be understood, leave visiting-cards in history that cannot easily be mistaken. Even existing glaciers tell strange stories. Nowadays glaciers are carefully measured and examined both in Switzerland and in Canada. During the last decade of the nineteenth century and the first decade of the twentieth the Swiss glaciers were found to show signs of receding farther up their valleys. The same thing has been observed about some of the Canadian glaciers. There are several plausible reasons for this. Professor Schaeberle says that the earth is growing cooler, and that in the temperate regions the winter rainfall (which would turn to falls of snow in the mountains) is less than it has been. It is certain that a shortage of winter rain over a number of years in succession would account for the shrinkage of the glaciers, but it is not by any means certain that a number of dry winters will not be followed by wet ones, in which case the glaciers would increase again. Some of the glaciers show that during their existence they have shrunk and lengthened alternately like gutta-percha in a variable climate. How do we know that they have shrunk and lengthened? The moraines of which we have spoken give us the testimony. As a glacier shrinks either in length or in breadth and depth it leaves the blocks of rock at its edges stranded on the sides of the valley. Such perched blocks or erratics are the best of glacier marks, and their great size, some of them as large as a cheap villa residence, is such that no current of water could have brought them there. They are often poised on the tops of crags, on the very edges of precipices, or on steep slopes, where they could never have been left by any flood, even had the flood been able to move them. The only thing that could have carried them must have been a vehicle that moved very, very slowly and deposited them very, very gently—in fact, glacier ice. We can see blocks like this on the glaciers now, and others stranded at the sides. In the Swiss valleys the scattered ice-borne boulders may be seen by hundreds far above the glaciers and far beyond the places where the glaciers end. We know they must have been left by glaciers, and by inference we surmise that when we find a valley filled with them, then, though the valley may have no glacier now, it must have once been occupied by one.

These erratic blocks, now found all over Europe, tell us a good deal about the ice on which they were transported. The blocks that fall on the edges of the glacier remain on the side where they descend. Hence, if there is any notable difference in the composition of the rocks on either side of the valley, the existence of this difference will be preserved in the moraines. If, therefore, in a country where the glaciers have disappeared we can trace the scattered blocks up to their sources among the mountains, we can say what was the track followed by the prehistoric glaciers. In Europe there are several examples of the uses of this detective evidence. Thus the peculiar blocks of the Valais mountains can be traced right on to Lyons; and this shows us that the glacier from which the River Rhone sprang extended once right across the east of France to Lyons, and probably farther. It was therefore once at least 170 miles longer. Similarly, blocks which are exactly like the characteristic rocks peculiar to Southern Scandinavia are found in Northern Germany, Belgium, and East Anglia; and we therefore believe that a great sheet of ice once filled up the Baltic and the German Ocean, carrying with it immense numbers of northern "erratics." In our own country, in fact, glacier boulders are found in nearly every county, and show that once the greater part of the country was buried under ice.

But, as we have said, it is not only on its shoulders, but in its interior and beneath its base that a glacier rolls and pushes its rubbish along. It is not all stones. Clay and earth mingle with it, often enclosing the stones; and the debris left by extinct glaciers of ages ago is sometimes called the boulder-clay. This is the deposit, earthy and stony, that the glacier leaves on the floor of the valley as it shrinks—unless the river which usually springs from the end of glaciers sweeps it away. Most of the stones thus left are smoothed or polished and covered with scratches or ruts, such as would be made by rubbing against other hard pointed fragments of stone. This is to be explained by the fact that these stones as they were carried on by the glacier were rubbed on the floor of rock over which the glacier was slipping. If their journey was long enough, they stood a chance of being rubbed away altogether and of finishing their existence as sand or mud. What the valley did to the glacier's stones, the stones did to the valley. They scratched it and scored it. Every promontory of rock which stood in the path of the ice had its angles and corners ground away. The polish and the directions of the scratches are especially remarkable, because, whether the marks are mere lines or deep-worn ruts, they are all on smooth surfaces, and they all run one way. That way is the direction in which the glacier moved. How high a degree of polish or how deep the markings may be depends a good deal on the kind of rock over which the glacier moved. Tough, close-grained rocks, such as hard limestone, are sometimes polished to look like marble. But there is a great deal of difference between the smoothing effected by a river or a torrent and that which is produced by a glacier, because the river tosses the rocks and stones in all directions, polishes them on every side, and leaves no distinctive parallel scratches or grooves on them. That can only be done by glaciers which hold the rocks, the rubbers and the rubbed, pressed firmly together and grind them continually in the same way.

These scratchings or striations of rocks, the smoothed and grooved surfaces, and the deposited boulder-clay and boulders enable us to trace the march of great ice-sheets over regions of the earth which are now of totally different aspect. From this kind of evidence we have been able to find that the whole of Northern Europe was once buried under a great expanse of snow and ice. The sheet was, as we should expect, thickest in the north and west, and thinned away southward and eastward. Over Scandinavia it was between 6000 and 7000 feet in thickness—as we can tell from the scratches on the sides of the high mountains. Similar marks 3000 feet above the sea-level in the Scottish Highlands lead us to believe that over Scotland the glaciers were 5000 feet thick, and even as far south as the Hartz Mountains in Germany it could not have been far short of 1500 feet in thickness. Imagine this great mass of ice ever slowly moving and ever creeping solemnly down to the sea. By the markings it left we can trace where the greater glaciers slid grandly along. In Scandinavia it swept westwards to the Atlantic and eastwards to the Gulf of Bothnia, then frozen as solid as the Pole. Southward the ice ground its way across Denmark to the Low Countries and North Germany. The Baltic was choked with ice, and so was the North Sea as far south as London. Ice in that day flowed in glaciers from the British Isles, eastwards from Scotland into the hollow of the North Sea, and westwards down all the clefts of the mountains, burying the western isles and breaking off in icebergs that drifted far into the Atlantic. Sir Archibald Geikie says that the western margin of the ice-fields from the south-west of Ireland to the North Cape of Norway must have presented a vast wall of ice 1200 to 1500 miles long and hundreds of feet high—like that great barrier which the Antarctic explorers tell us frown on the waters that lap the boundaries of the south polar land. Northern Europe must have been like North Greenland of to-day. The rock scratches tell us (since even the southern coast of Ireland is intensely ice-worn) that the edge of ice must have extended some distance beyond Cape Clear, rising out of the sea with a precipitous front that faced to the south. Thence the ice-cliff swung eastwards, passing probably along the line of the British Channel and keeping to the north of the valley of the Thames. Its southern margin ran across what is now Holland and skirted the high grounds of Westphalia, Hanover, and the Hartz Mountains—which probably barred its further progress southward. "There is evidence that the ice swept round into the lowlands of Saxony up to the chain of the Erz, Riesen, and Sudenten Mountains, whence its southern limit turned eastward across Silesia, Poland, and Galicia, and then swung round to the north, passing across Russia by way of Kieff and Nijni Novgorod to the Arctic Ocean."

In North America there are similar traces of the great ice-sheet, one of the branches of which streamed southward into the basin of the Mississippi, the second moving westward from Hudson Bay to the Rockies, and southward to Iowa, and the third setting out from the great mountain ranges of British Columbia. Right across North America to-day for thousands of miles stretch accumulations and mounds of rock which were pushed forward by the ice, and were dropped by the glaciers when they reached "farthest south." These accumulations are called, from their origin, the great "terminal moraines" of the North American Ice Age.

It must not be thought that these great ice-sheets of both hemispheres remained constant in extent and thickness. There were periods of retreat and advance, of progress and shrinking, and the shrinkings of these took place on a large scale, and perhaps lasted for hundreds or thousands of years; so that mixed with the strata of boulder-clay, which are the characteristic strata of the glacial periods, are other strata of sand, ordinary clay, and even peat. Remains of plants and animals are found in these strata, showing that sometimes the glaciers retreated so far and for so long that vegetation sprang up and animals lived on the ground that they had covered—in the intervals when the cold of centuries was replaced by other centuries of mild and equable climate.

At last, after many of these swallow-like retreats and advances, the warmer climate at length came to stay, and the ice retreated farther and farther to the north. It still remained among the mountains, so that we might describe the glaciers of the Alps and of the Canadian Rockies as the last relics existing to-day of the great Ice Age of the past. The retreat to the Arctic Circle left many other relics behind it, the great lakes, for example, like Winnipeg and Manitoba, and the Great Salt Lake of Utah. All were once mightier sheets, because during the Ice Age their waters were held back. Other smaller lakes formed by the dumping-heaps or terminal moraines of the glaciers still exist, and are especially noticeable in Finland. During the later stages of the Ice Age the level of the land was lower than it is now in Western Europe. When the ground began to rise—in slow upheavals with long pauses for rest—it left its impress in raised beaches, which can be seen on both sides of Scotland and on the Norwegian coast. The climate grew gradually milder, the animals and the plants followed in the train of the retreating ice, and even the traces of man's existence began to appear. The change was not sudden; it was so gradual that the Ice Age slipped as imperceptibly as its own glaciers into the age in which Man's activities in Europe began.