Other theories speedily sprang up, endeavouring to account for the phenomena by supposing great bodies of water rushing with terrific force across whole continents, sweeping away the tops of hills, tearing up and dispersing entire geological formations, and strewing the ocean-bottom with scattered debris. But this explanation had the disadvantage of being woefully unphilosophical and not very clearly orthodox. Such debacles did not appear to have ever taken place in any previous geologic era, and experience was against them. Besides, they did not account for some of the most evident characteristics of the phenomena, such as the northern character of the formation, the long parallel striations of the rock surfaces, and the perching of huge boulders on lofty hills, often hundreds of miles distant from the parent rock. Geologists were completely at fault, and the boulder-clay remained a mystery for years.
When we consider the physical aspects of the countries where the question was studied, we cannot much wonder that the truth was so hard to find. In the midst of corn-fields and meadows, one cannot readily realize the fact that the spot where they stand has been the site of a wide-spread sea; and that where now villages and green lanes meet the eye, there once swam the porpoise and the whale, or monsters of a still earlier creation, unwieldy in bulk and uncouth in form. Such changes, however, must have been, for their traces meet us on every hand. We have the sea dashing against our shores, and there seems nothing at all improbable in the assertion that once it dashed against our hill-tops. No one, therefore, has any difficulty in giving such statements his implicit belief. But who could have dreamed that these fields, so warm and sunny, were once sealed in ice, and sunk beneath a sea that was cumbered with many a wandering iceberg? Who could have imagined, that down these glens, now carpeted with heath and harebell, the glacier worked its slow way amid the stillness of perpetual snow? And yet strange as it may seem, such is the true solution of the problem. The boulder-clay was formed during the slow submergence of our country beneath an icy sea, and the rock-surfaces owe their polished and striated appearance to the grating across them of sand and stones frozen into the bottom of vast icebergs, that drifted drearily from the north. That we may the better see how these results have been effected, let us glance for a little at the phenomena observable in northern latitudes at the present day.
Icebergs are formed in three principal ways:—1st, By glaciers descending to the shore, and being borne seawards by land-winds; 2d, By river-ice packed during spring, when the upper reaches of the rivers begin to thaw; 3d, By coast-ice.
I. There is an upper stratum of the atmosphere characterized by intense cold, and called the region of perpetual snow. It covers the earth like a great arch, the two ends resting, one on the arctic, the other on the antarctic zone, while the centre, being about 16,000 feet above the sea,[2] rises directly over the tropics. Wherever a mountain is sufficiently lofty to pierce this upper stratum, its summit is covered with snow, and, as the snow never melts, it is plain that, from the accumulations of fresh snow-drifts, the mountain-tops, by gradually increasing in height and width, would become the supporting columns of vast hills of ice, which, breaking up at last from their weight and width, would roll down the mountain-sides and cover vast areas of country with a ruin and desolation more terrible than that of any avalanche. Olympus would really be superposed upon Ossa. By a beautiful arrangement this undue growth is prevented, so that the hill-tops never vary much in height above the sea. The cone of ice and snow which covers the higher part of the mountain, sends down into each of the diverging valleys a long sluggish stream of ice, with a motion so slow as to be almost imperceptible. These streams are called glaciers. As they creep down the ravines and gorges, blocks of rock detached by the frosts from the cliffs above, fall on the surface of the ice, and are slowly carried along with it. The bottom also of the glaciers is charged with sand, gravel, and mud, produced by the slow-crushing movement; large rocky masses become eventually worn down into fragments, and the whole surface of the hard rock below is traversed by long parallel grooves and striæ in the direction of the glacier's course. Among the Alps, the lowest point to which the glacier descends is about 8500 feet. There the temperature gets too high to allow of its further progress, and so it slowly melts away, choking up the valleys with piles of rocky fragments called moraines, and 'giving rise to numerous muddy streams that traverse the valleys, uniting at length into great rivers such as the Rhone, which enters the Lake of Geneva turbid and discoloured with glacial mud.
[2] The average height of the snow-line within the tropics is 15,207 feet, but it varies according to the amount of land and sea adjacent, and other causes. Thus, among the Bolivian Andes, owing to the extensive radiation, and the ascending currents of air from the neighbouring plains and valleys, the line stands at a level of 18,000 feet, while, on mountains near Quito, that is, immediately on the equatorial line, the lowest level is 15,795.—See Mrs. Somerville's Physical Geography, 4th edit. p. 314.
In higher latitudes, where the lower limit of the snow-line descends to the level of the sea, the glaciers are often seen protruding from the shore, still laden with blocks that have been carried down from valleys far in the interior. The action of storms and tides is sufficient to detach large masses of the ice, which then floats off, and is often wafted for hundreds of miles into temperate regions, where it gradually melts away. Such floating islands are known as icebergs.
II. In climates such as that of Canada, where the winters are very severe, the rivers become solidly frozen over, and, if the frost be intense enough, a cake of ice forms at the bottom. In this way sand, mud, and rocky fragments strewing the banks or the channel of the stream, are firmly enclosed. When spring sets in, and the upper parts of the rivers begin to thaw, the swollen waters burst their wintry integuments, and the ice is then said to pack. Layer is pushed over layer, and mass heaped upon mass, until great floes are formed. These have often the most fantastic shapes, and are borne down by the current, dropping, as they go, the mud and boulders, with which they are charged, until they are stranded along some coast line, or melt away in mid-ocean.
III. But icebergs are also produced by the freezing of the water of the ocean. In high latitudes, this takes place when the temperature falls to 28·5° of Fahrenheit. The surface of the sea then parts with its saline ingredients, and takes the form of a sheet of ice, which, by the addition of successive layers, augmented sometimes by snow-drifts, often reaches a height of from thirty to forty feet. On the approach of summer these ice-fields break up, crashing into fragments with a noise like the thundering of cannon. The disparted portions are then carried towards the equator by currents, and may be encountered by hundreds floating in open sea. Their first form is flat, but, as they travel on, they assume every variety of shape and size.
On the shores of brackish seas, such as the Baltic, or along a coast where the salt water is freshened by streams or snow-drifts from the land, sheets of ice also frequently form during severe frosts. Sand and boulders are thus frozen in, especially where a layer of ice has formed upon the sea-bottom.[3] The action of gales or of tides is sufficient to break up these masses, which are then either driven ashore and frozen in a fresh cake of ice, or blown away to sea. The bergs formed in this way have originally a low flat outline, and many extend as ice-fields over an area of many miles, while, at a later time, they may be seen towering precipitously as great hills, some 200 or 300 feet high.
[3] I was informed by the late Mr. Hugh Miller, that a seam of shale abounding in liassic fossils, had been found intercalated among the boulder-clay beds in the vicinity of Eathie. He explained its occurrence there by supposing that it had formed a reef along a shore where ground-ice was forming; and so having been firmly frozen in, it was torn up on the breaking of the ice, and deposited at a distance among the mud at the sea-bottom.