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THE GLACIAL OE ICE EPOCH.
THE GLACIAL EPOCH AS DISTINGUISHED FROM THE PRE - GLACIAL OR LATER TERTIARY—ITS BOULDER-CLAYS, GRAVEL-DRIFTS, AND ERRATIC BLOCKS—LIMIT OF THESE IN THE NORTHERN HEMISPHERE —CONDITIONS UNDER WHICH THEY APPEAR TO HAVE BEEN FORMED—ICE-MANTLE, GLACIERS, AND ICEBERGS—DURATION OF THE PERIOD—PAUCITY OF LIFE—DEPRESSION AND RE-ELEVATION OF LAND—GRADUAL RETURN OF MORE GENIAL CONDITIONS—ESTABLISHMENT OF EXISTING ARRANGEMENTS, PHYSICAL AND VITAL— RECURRENCE OF COLDER AND WARMER CYCLES IN TIME.
There are few chapters in geological history possessed of more interest than that which is usually known as the Glacial Epoch, and none that has received a larger share of attention from modern investigators. Indeed it is still to a great extent an unsolved problem, and hence the conflicting views that prevail as to the physical conditions of the period, and the causes by which these conditions were produced. Though beset with many difficulties, the general features of the period are well known, and it is to place these broadly before the general reader, rather than enter upon debatable hypotheses, that we attempt the present Sketch.
As mentioned in a preceding paper, the genial temperature that prevailed during the deposition of the earlier tertiaries began gradually to decline during the middle and later portions, till towards the close of the period an intense cold set in, and ice seems to have prevailed alike over the land and waters. Of course we refer more especially to the higher latitudes of Europe, Asia, and North America, the regions within which the phenomena of the glacial epoch are most strikingly displayed, and to which (from the 40th or 42d parallel northwards) they were in all likelihood restricted. Over these limits the ice-epoch long held its iron sway, annihilating, or all but annihilating, terrestrial life; grinding and rounding and moulding the land-surface as no other agent but ice can do; and loading the bottom of the ocean with miscellaneous masses of mud, shingle, and boulders. This paucity of life, these land-surfaces, and these miscellaneous accumulations, are the principal proofs of the conditions of the glacial epoch, and these we must first consider under the knowledge we have gained by a perusal of the preceding chapter.
At first it seems evident that towards the close of the tertiary period the climate of a large portion of the northern hemisphere was gradually growing colder and colder. From the occurrence of pre-glacial river-courses several hundred feet below the present sea-level, we know that the land was then higher than it is now; and as this cold increased, the loftier mountains would become perennially enveloped with snow and glacier, and the surrounding seas with an annual covering of ice. Under this increasing rigour all the more delicate tertiary plants and animals would succumb, and those endowed with greater elasticity of constitution would shift ground to lower and more southern situations. As the cold still increased, the ice-sheet seems to have spread itself even over the lower grounds, to have pushed its way out to sea, and during the thaw and currents of a brief summer to have been drifted off in floes and bergs, as the ice is now from the coasts of the arctic and antarctic regions. At this stage the terrestrial flora and fauna would be at their minimum, and paralleled, perhaps, by what we now find in Greenland and the islands of the Arctic Ocean. During this setting-in of the glacial epoch, the land, as we shall shortly see, seems to have been gradually subsiding, and this subsidence went on to the extent of 1800 or 2000 feet below the existing sea-level, converting a large portion of what is now Europe and America into series of frozen states and ice-clad islands. When the land seems to have been at its greatest depression the cold appears to have attained its greatest intensity, and at this stage we have the zenith and turning-point of the glacial period. After the lapse, perhaps, of ages, a reverse action sets in; the land begins to be re-elevated; a new cycle of temperature commences; and the cold, though still clinging in snow and glacier to the higher hills, is less felt along the lower grounds and neighbouring sea-shores. By-and-by, as the elevation continues, the glaciers melt away from the hill-sides; the icebergs and ice-packs disappear from the seas; the general climate improves; plants and land animals in newer species gradually take possession of the land; and the existing order of things is imperceptibly established. Such seem to have been the setting-in, the creeping-on, the culmination, and the departure of the glacial epoch. Let us now glance at the proofs by which this advent, this subsidence, and this re-elevation can be logically established.
That the ice-epoch, like other great events in nature, came on slowly and gradually, is abundantly evidenced by the temperate or even coldly-temperate aspects of the flora and fauna of the later, as compared with those of the middle and earlier tertiaries. The eocene palms, crocodiles, turtles, and monkeys do not appear in miocene strata; the miocene sycamores, chestnuts, and maples are replaced by pliocene pines, beeches, and birches; and thus over the tertiary areas of Europe at least the declension of climate had been going on for ages before the advent of the glacial period. How far this declension was simultaneous over Asia and America has not been determined, but that a similar declension took place in those areas is sufficiently obvious from a similar change in their flora and fauna.* That the pre-glacial land was somewhat higher than the present is shown by old river-courses and land-surfaces that lie below the existing sea-level, as well as by ice-marked rocks that dip away beneath the waters. Had the pre-glacial lands been lower, these rock-surfaces would not have been smoothed and furrowed by ice, nor would the old land-surfaces have made their appearance, t It was on this more elevated surface, therefore, that the glacier and ice-sheet first began their operations; and it is at this stage that we find the lowest tenacious clays (" lower till"), and angular blocks and boulders, little removed from the rocks from which they were severed. Here, then, we have the first stage of the glacial epoch—the operation of ice on a land-surface somewhat more elevated, in its average altitude at least, than the existing continents of Europe, Asia, and North America. This operation, as we have learned from the preceding Sketch, must have been to grind and gouge in the valleys, to smooth and round the higher eminences, and generally to polish the harder rocks—the detritus or abraded material being carried forward to lower levels, there to be laid
* According to Professor Dana, there are no tertiary strata in North America to the north of the New England States, the northern area having been dry land while the southern was under water and received the tertiary deposits. To this elevation of the northern lands, and the subsequent gradual uprise of the southern or tertiary portion to the height of 3000 or 5000 feet, he attributes the first setting-in of the glacial epoch.
+ The attention of geologists has not been sufficiently directed to these pre-glacial land-surfaces. It is true that the "lower till" rests for the most part on abraded rock-surfaces, but there are many localities (wehave noticed them in Kincardine, Ayr, Fife, and Durham—to say nothing of the well-known Cromer beds in Norfolk) where it reposes on sands, gravels, and even peaty beds, which were undoubtedly the soils and rivergravels of the period immediately preceding; and in these we may expect to find the remains of the true pre-glacial flora and fauna.
down as clay and mud, mounds of shingly gravel, and masses of blocks and boulders. Whatever the nature of the parent rock, these mounds and masses would partake of it—yellow clays and schists and granites in granite districts, red clays and red sandstone blocks in Old Eed Sandstone areas, and dark - coloured clays and blocks of limestone and sandstone in Carboniferous basins; and as a general rule these blocks and boulders not far transported from the cliffs and precipices from which they had been torn by the ice-giant. Indeed, in most instances, this proximity of glacial clays, ice-worn and ice-scratched blocks, is one of the best proofs of the first stage of the ice-epoch, and all over the northern and middle portions of Scotland we have never found it to fail in its indications.
But as the cold set in more intensely, the downward movement of the land seems to have commenced; and hence much of the ice-worn debris of this first stage was removed by denudation, step by step, as the terrestrial surface descended. What may have been the precise character of the climate at this epoch of descent—that is, how long the winter frosts and how short the summer's thaw—we have no means of determining, for the earlier clays and moraine blocks are destitute of organic remains; but if we may judge by the comparatively small amount of obliteration bj' denudation, it would appear that the seas were more icelocked than free-flowing, and that consequently the land went down encased, as it were, in ice of prodigious thickness.* This descent or subsidence of the land forms the
* The ice-sheet at this stage may have been two or three thousand feet in thickness. The great antarctic ice-barrier, met by Sir James Ross and his companions, was estimated at a thousand or fifteen hundred feet. Ice of this thickness would rest on the beds of all the shallower friths and seas, and act upon thfin precisely as upon the rocky surfaces of the dry land. This circumstance should be carefully borne in mind in reasoning on the phenomena of the glacial epoch.