The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Scott Elliott, G. F. “The geology of Mount Ruwenzori and some adjoining regions of tropical Africa.” Q.J.G.S., 51, 1895, p. 669.

Hobley, C. W. “The alleged desiccation of East Africa.” Geogr. Journ., 44, 1914, p. 467.

Freydenberg, H. “Le Tchad et le Bassin du Chari.” Diss. Paris, 1908.

Passarge, H. “Die Kalahari.”

CHAPTER XI
AUSTRALIA AND NEW ZEALAND

The continent of Australia has a relatively low relief, only rising above the snow-line in Mount Kosciusko, and glacial traces have a relatively unimportant development. The history of the region appears to be as follows:

In late Tertiary times the shore-line lay some distance to the east towards New Zealand, this being a relic of a much earlier connexion between the two lands. Towards the close of the Tertiary earth-movements set in, which elevated the mountain belt of eastern Australia and formed a land connexion with Tasmania and the Antarctic continent. At the same time the land to the east and the closed basins of central Australia were also probably developed about this time. The climate was then somewhat warmer than the present, at least on the east coast, for the Australian barrier reef extended further south. Probably at this time the Antarctic ice-sheet did not reach the sea, and there was none of the floating ice which is such an important factor in cooling the Southern Ocean.

The next stage was the lowering snow-line on Kosciusko to about 3000 feet below the present and the development of extensive glaciers, which descended to 5500 feet above the sea, and attained an area of 80 to 100 square miles and a thickness of at least 1000 feet. Tasmania was also extensively ice-covered, probably by glaciers which coalesced at low levels, forming what is known as a “piedmont” ice-sheet, which possibly reached the sea. The lowering of the snow-line in Tasmania is estimated as 6000 feet, corresponding to a fall in temperature of 18° F. Probably a large part of this fall is accounted for by the increased elevation, which may have been several thousand feet in Tasmania and more than a thousand feet even in New South Wales. This glaciation, which was probably dependent on the growth of the Antarctic ice-sheet, was followed by a very long interglacial, the duration of which has been estimated by Professor David as 100,000 to 200,000 years. The old moraines are much weathered and denuded, resembling in this respect the older moraines of Europe. No information is available as to the climate of this interglacial period. Possibly some of the Quaternary raised beaches with warmth-loving mollusca found in unglaciated parts of Australia belong to this period, and if so the climate was warmer than the present for at least part of the time.

The interglacial was followed by uplift and a second much less severe Glacial period, characterized by valley glaciers on Kosciusko and in Tasmania, reaching the sea in places on the latter island. It was at the close of this Glacial period that man reached Tasmania; its conclusion is dated by Prof. David at about 10,000 years ago. It was terminated by a period of depression below the present level with a warm climate.

In the dry interior of Australia there is evidence that at one time, probably during the maximum glaciation, the rainfall was heavier than the present, and numerous lakes were developed which have now been dry for a very long time. It is possible that the artesian water supply of Australia, which Gregory considers to be “fossil water” accumulated under different conditions from the present, is a vestige of the rainfall of this period. Further north, in Java, the beds in which the famous Pithecanthropus skeleton was found, believed to be lower glacial, contain also plant remains similar to those now found in the Khassian mountains of Assam, one of the rainiest climates in the world. The climate of Java during the maximum glaciation was thus decidedly rainier, and probably somewhat cooler than the present.

An extraordinary find which may be referred to here is that of Professor Neuhauss, who discovered giant erratics, scratched and polished, and moraines at sea-level at the western end of Huon Gulf, New Guinea. The region is very unstable, and is known to have stood at a very much higher level, perhaps 10,000 feet or more, in Quaternary times, and if the moraines indicate glaciers terminating at 10,000 feet above the sea they are explicable by a slight fall of temperature and increase of snowfall.

Turning now to New Zealand, we find extensive glacial remains on South Island, though not on North Island. As in so many other countries, the Quaternary opened with great elevation, which reached at least 1500 feet over the whole group. North and South Islands were united with each other, with Stewart Island and probably also the outlying islands, even including the Chatham Islands, forming a great land-mass several times the present area of New Zealand. On the southern part of this land-mass extensive glaciers were formed; on the east these did not reach the present sea-level, but on the snowy south-west they extended far below it, so that the terminal moraines are now completely submerged; possibly they were never formed, but the debris was floated away seaward on icebergs. Further north moraines are found near the present shore-line at many places between Milford Sound and Hokitika, and morainic mounds cover a large part of the low ground. Still further north they retreat inland, and in the Nelson Province are not found below a level of 2000 feet at the foot of Lake Rotoiti.

In the south-east a great moraine has been described at the south end of Lake Wakatipu and others at the north-east ends of Lakes Manapouri and Te Anau, but none are found nearer the sea-coast. The glaciated area of New Zealand was at least ten times the present ice-covered area, and the Tasman, the longest glacier in New Zealand, was expanded from its present length of 16 miles to at least 30 miles. Much of the apparent fall of temperature shown by this glaciation was probably due to the great elevation, but apart from this the ice had a marked influence on climate. Outside the limits of glaciation on the east is a thick deposit of typical loess, which extends up to a level of 1000 feet on the flanks of the hills. The occurrence of this loess points to a steppe climate with dry, cold, southerly winds on the lee side of the glaciated mountains, and is probably also connected with the increase of land area. Further north, north of Auckland in North Island, the present treeless plains were covered by forests; for Kauri gum, apparently very old, has been found. The sub-antarctic islands—Campbell, Antipodes, etc.—were not covered by either New Zealand or Antarctic ice, but were the centres of local severe glaciations of their own.

The next stage was a great subsidence, during which the glaciers retreated. The land sank below its present level, raised beaches probably of interglacial age being found at various heights ranging from 10 feet above the sea at Manukau in the centre of North Island to 150 feet at Taranaki, 200 feet at Cape Palliser, 400 feet on the west coast of South Island, 500 feet at Amuri Bluff, and even 800 feet in the entrances to the south-western sounds. This great submergence was associated with the deposition of extensive gravel deposits by the rivers.

The interglacial was followed by a second period of elevation. It is not certain how far this went. Submerged peat-bogs have been found at a depth of nearly 600 feet below sea-level near Canterbury, but these may belong to the early stages of the interglacial and not to the post-Glacial period. On the other hand, the submerged forests which are found at many points on the coast of New Zealand are evidently post-glacial and indicate a slight rise above present level. At the same time there was a renewal of the glacial conditions, but the ice was confined to the valleys and had a much less extent than in the first glaciation. This period seems to have been followed by a slight submergence and a temporary warm period.