The Antiquity of Man

by Sir Charles Lyell

If any one of the existing Swiss lakes were now lowered by deepening its outlet, or by raising the higher portion of it relatively to the lower, we should see similar deltas of comparatively modern date exposed to view, some of them with embedded trunks of pines of the same species drifted down during freshets. Such deposits would be most frequent at the upper ends of the lakes, but a few would occur on either bank not far from the shore where torrents once entered, agreeing in geographical position with the lignite formations of Utznach and Durnten.

There are other freshwater formations with lignite, besides those on the Lake of Zurich, as those of Wetzikon near the Pfaffikon Lake, of Kaltbrunnen, of Buchberg, and that of Morschweil between St. Gall and Rorschach, but none probably older than the Durnten beds. Like the buried forest of Cromer they are all pre-glacial, yet they by no means represent the older nor even the newer Pliocene period, but rather the beginning of the Pleistocene. It is therefore true, as Professor Ramsay remarks, that, as yet, no strata "of the age of the English Crag" have been detected in any Alpine valley. In other words, there are no freshwater formations yet known corresponding in date to the Pliocene beds of the upper Val d'Arno, above Florence—a fact from which we may infer (though with diffidence, as the inference is based on negative evidence), that, although the great Alpine valleys were eroded in Pliocene times, the lake-basins were, nevertheless, of Pleistocene date—some of them formed before, others during, the glacial epoch.

Sixthly. In what manner then did the great lake-basins originate if they were not hollowed out by ice? My answer is, they are all due to unequal movements of upheaval and subsidence. We have already seen that the buried forest of Cromer, which by its organic contents seems clearly to be of the same age as the lignite of Durnten, was pre-glacial and that it has undergone a great oscillation of level (about 500 feet in both directions) since its origin, having first sunk to that extent below the sea and then been raised up again to the sea-level. In the countless Post-Miocene ages which preceded the glacial period there was ample time for the slow erosion by water of all the principal hydrographical basins of the Alps, and the sites of all the great lakes coincide, as Professor Ramsay truly says, with these great lines of drainage. The lake-cavities do not lie in synclinal troughs, following the strike and foldings of the strata, but often, as the same geologist remarks, cross them at high angles; nor are they due to rents or gaping fissures, although these, with other accidents connected with the disturbing movements of the Alps, may sometimes have determined originally the direction of the valleys. The conformity of the lake-basins to the principal watercourses is explicable if we assume them to have resulted from inequalities in the upward and downward movements of the whole country in Pleistocene times, after the valleys were eroded.

We know that in Sweden the rate of the rise of the land is far from uniform, being only a few inches in a century near Stockholm, while north of it and beyond Gefle it amounts to as many feet in the same number of years. Let us suppose with Charpentier that the Alps gained in height several thousand feet at the time when the intense cold of the glacial period was coming on. This gradual rise would be an era of aqueous erosion and of the deepening, widening, and lengthening of the valleys. It is very improbable that the elevation would be everywhere identical in quantity, but if it was never in excess in the outskirts as compared to the central region or crest of the chain, it would not give rise to lakes. When, however, the period of upheaval was followed by one of gradual subsidence, the movement not being everywhere strictly uniform, lake-basins would be formed wherever the rate of depression was in excess in the upper country. Let the region, for example, near the head waters of the great rivers sink at the rate of from 4 to 6 feet per century, while only half as much subsidence occurs towards the circumference of the mountains—the rate diminishing about an inch per mile in a distance, say of 40 miles—this might convert many of the largest and deepest valleys at their lower ends into lakes.

We have no certainty that such movements may not now be in progress in the Alps; for if they are as slow as we have assumed, they would be as insensible to the inhabitants as is the upheaval of Scandinavia or the subsidence of Greenland to the Swedes and Danes who dwell there. They only know of the progress of such geographical revolutions because a slight change of level becomes manifest on the margin of the sea. The lines of elevation or depression above supposed might leave no clear geological traces of their action on the high ridges and table-lands separating the valleys of the principal rivers; it is only when they cross such valleys that the disturbance caused in the course of thousands of years in the drainage becomes apparent. If there were no ice, the sinking of the land might not give rise to lakes. To accomplish this in the absence of ice, it is necessary that the rate of depression should be sufficiently fast to make it impossible for the depositing power of the river to keep pace with it, or in other words to fill up the incipient cavity as fast as it begins to form. Such levelling operations once complete, the running water, aided by sand and pebbles, will gradually cut a gorge through the newly raised rock so as to prevent it from forming a barrier. But if a great glacier fill the lower part of the valley all the conditions of the problem are altered. Instead of the mud, sand, and stones drifted down from the higher regions being left behind in the incipient basin, they all travel onwards in the shape of moraines on the top of the ice, passing over and beyond the new depression, so that when at the end of fifty or a thousand centuries the glacier melts, a large and deep basin representing the difference in the movement of two adjoining mountain areas—namely, the central and the circumferential—is for the first time rendered visible.

By adopting this hypothesis, we concede that there is an intimate connection between the glacial period and a predominance of lakes, in producing which the action of ice is threefold; first, by its direct power in scooping out shallow basins where the rocks are of unequal hardness; an operation which can by no means be confined to the land, for it must extend to below the level of high water a thousand feet and more in such fjords as have been described as filled with ice in Greenland.

Secondly. The ice will act indirectly by preventing cavities caused by inequalities of subsidence or elevation from becoming the receptacles first of water and then of sediment, by which the cavities would be levelled up and the lakes obliterated.

Thirdly. The ice is also an indirect cause of lakes, by heaping up mounds of moraine matter and thus giving rise to ponds and even to sheets of water several miles in diameter.

The comparative scarcity, therefore, of lakes of Pleistocene date in tropical countries, and very generally south of the fortieth and fiftieth parallels of latitude, may be accounted for by the absence of glacial action in such regions.

POST-GLACIAL LAKE-DWELLING IN THE NORTH OF ITALY.