The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Leverett, F. “Comparison of North American and European glacial deposits.” Zs. f. Gletscherkünde 4, 1910, pp. 280, 323.

Wright, W. B. “The Quaternary Ice Age.” London, 1914, Chs. 8-9.

Attwood, W. W. “The glaciation of the Uinta Mountains.” J. Geol., 15, 1907, p. 790.

Henderson, J. “Extinct glaciers of Colorado.” Colo. Univ. Studies, 3, 1905, p. 39.

Gilbert, G. K. “Lake Bonneville.” Washington, U.S. Geol. Survey Monograph I, 1890.

Russell, I. C. “The geological history of Lake Lahontan.” Washington, U.S. Geol. Survey Monograph XI, 1885.

Coleman, A. P. “An estimate of post-Glacial and interglacial time in North America.” Rep. 12 Internat. Congr. Geol., 1913, p. 435.

CHAPTER IX
CENTRAL AND SOUTH AMERICA

The scarcity of data which was bewailed in dealing with Asia is still more marked in the case of South America, and it will be necessary to present the glacial history of that continent in the barest outline only. This is the more unfortunate as the chain of the Andes, extending from north of the equator to high southern latitudes, is of enormous importance in glacial theory, and especially in the question of simultaneity of glaciation in the two hemispheres.

The beginnings of glaciation in South America are obscure. The distribution of animals shows that towards the close of the Tertiary the Falkland Islands were greatly elevated and were united to Tierra del Fuego and Patagonia, and this enlarged land area was connected in some way with Australia and Tasmania, but the mode of this latter connexion is not definitely known. This question will be discussed more fully in [Chapter XI]; it is sufficient to say here that the amount of elevation may have reached 12,000 feet in Tierra del Fuego. Equatorwards the elevation diminished, and near the equator the land probably lay somewhat lower than now.

In South Georgia the present glaciers greatly expanded, until practically the whole island was buried in ice, and the same is true of the Falkland Islands and Tierra del Fuego, only the highest peaks remaining above the ice. In the latter district there is some evidence of two glaciations separated by an interglacial, the earlier glaciation being due to a regional ice-sheet and the later to smaller valley glaciers. The intricate coast-line of the Falkland Islands and Tierra del Fuego points to fiord erosion by ice which extended well beyond the present limits of the land, and can only have occurred during considerable elevation. As to the character of the interglacial, little is known. In the Falklands there is a bed of black vegetable soil full of tree-trunks, indicating the existence of luxuriant forests and a temperate climate. This deposit is overlain by boulder-clay, and may be either interglacial or pre-glacial, but since it was formed when the land stood at a comparatively low level, while we have reason to believe (see [Chapter XII]) that during the close of the Tertiary period these islands were greatly elevated, it is probably an interglacial formation, and indicates a great amelioration of climate. In Gable Island, Tierra del Fuego, Halle found beneath boulder-clay a Quaternary fauna of barnacles and marine mollusca indicating a climate slightly warmer than the present, and this probably belongs to the same period. To the concluding stages of the Glacial period in the Falklands belong the curious “stone rivers,” great streams of moss-grown boulders which fill the valleys, and under the influence of temperature changes are probably still slowly advancing.

Passing further north to the Andes, between 39° and 44° south latitude, the glaciation was not so severe, and its records are therefore clearer. The first result of elevation was the cutting of deep canyons by the rivers. This was followed, possibly without much further elevation, by a fall of temperature, which in this connexion may be attributed to the extension of the Antarctic and Tierra del Fuego ice-sheets. Glaciers now developed and spread down the canyons, leaving moraines of great volume and height, associated with all the other criteria of glaciation. The snow-fields from which these glaciers originated lay between 5000 and 6000 feet above the sea, and the snow-line lay at about 3000 feet instead of above 6000 as at present.

This glaciation was followed by a long interglacial, during which the glaciers retreated to the highest summits of the Andes. The length of this period is indicated by the fact that the earlier moraines have been eroded to such an extent that they no longer present distinctly the typical features of glacial topography, while the materials of which they are composed are decayed to somewhat the same extent as the older moraines of North America, the granite boulders especially being rotten and friable. This interglacial was followed by a re-development of the glaciers, but to nothing like the same extent as formerly; their moraines are smaller and fresh-looking, indicating that this glaciation was comparatively recent.

Still further north, in latitude 20°-25° S., we come to a region of very slight snowfall, where the snow-line lies higher than anywhere else on the face of the earth. The glaciation here was comparatively unimportant, the snow-line descending only 1600 to 2500 feet. Here Keidel found moraines of three glacial advances, and from his description it appears probable that the earliest and greatest was separated by a considerable interval from the two younger, the interglacial between which was short and not characterized by a return to present-day climatic conditions, since during this interval there was very little weathering. Probably we have here to do with two glaciations, of which the second was double. In fact, some writers have described no less than five glacial advances in the Argentine Andes, but most of these are probably merely retreat stadia.

In Peru, W. Sievers reports the existence of two glaciations separated by a considerable interval. The present limit of the glaciers is about 15,200 feet; during the first glaciation they descended to about 11,000 feet, and during the second to 12,800 feet. The evidence is very complete. In Ecuador, H. Meyer records a similar bipartition. The oldest glaciation is represented by trough-like valleys, enormous gravel terraces, and old moraines much weathered; the limits are far below the present limits of glaciers, but have been much obscured by subsequent erosion. This glaciation was followed by a long period of steppe climate resembling the present, during which the loess-like Cangagua formation was deposited. This in turn was followed by a readvance of the glaciers to a level about 2700 feet below the present limit. This glaciation is associated with crescent-shaped moraines, corrie lakes, hanging valleys and gravel terraces, covered with vegetation, but otherwise fresh-looking. The snow-line lay about 1600 feet below the present. Probably during the first glaciation the Andes were invaded by numerous mountain plants and animals related to North American forms—a valuable piece of evidence which indicates that the glaciation was contemporaneous with that in North America. In Columbia and Venezuela there are traces of Glacial periods, but these have not yet been studied in detail. The most northerly evidence of a Glacial period comes from the Sierra Nevada de Santa Maria, near the north coast of Venezuela in 11° N.

Except in Tierra del Fuego and Patagonia the ice did not extend far from the mountains. But in the eastern Argentine there is a great series of Quaternary deposits known as the Pampean. This formation covers 200,000 square miles, and consists of at least ninety feet of fine loam without a single pebble (except for a few thin calcareous layers), but containing large numbers of complete skeletons of mammals. It raises several interesting problems. Apparently it represents the whole course of the Glacial period. By some geologists it is considered to be a delta deposit of the combined Parana and Paraguay rivers, but the absence of mollusca, except in a marine intercalation near its summit, is against this view, and Steinmann attributes it to æolian agencies and compares it to the loess of Europe and North America. If this view is correct the Pampean represents steppe conditions prevailing on the equatorial side of the Patagonia-Falkland Islands ice-sheet. Apparently before the incoming of the greatest cold the Pampas were in part at least forest-clad, for in the older beds are found peculiar forms of ground-sloths which were adapted for forest life and have been found also in cave-deposits of Brazil. At the maximum of glacial conditions the Pampas probably had a steppe climate, but the disappearance of the forests is to be attributed rather to drought than to cold. Elevated glacier-bearing Andes to the west and ice-sheets to the south would render the Argentine extremely arid, and this accounts for the gradual extinction of so many giant forms whose remains are found in the Pampean deposits. Conditions ultimately became too severe even for the horse, which died out in South America. The marine transgression which left its mark near the top of the Pampean is probably post-glacial.

In Brazil, on the other hand, there is no evidence that the climate has ever been drier than the present, and in the semi-arid regions of the north-east it is even probable that during the Glacial period the climate was moister, presumably owing to the greater strength of the rain-bearing east and north-east winds. Further west in the Andes the existence of this wet period is borne out by the former greater size of Lake Titicaca, and there seems to be additional evidence to the same effect in the Chilian deserts.