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We have now discovered not only that the glacial period was of vast duration, but that it passed through various phases and oscillations of temperature; so that, although the chief polishing and furrowing of the rocks and transportation of erratics in Europe and North America may have taken place contemporaneously, according to the ordinary language of geology, or when the same testacea and the same Pleistocene assemblage of mammalia flourished, yet the extreme development of cold on the opposite sides of the ocean may not have been strictly simultaneous, but on the contrary the one may have preceded or followed the other by a thousand or more than a thousand centuries.
The stony fragments dispersed irregularly through the till usually belong, especially in mountainous countries, to rocks found in some part of the same hydrographical basin; but there are regions where the whole of the boulder clay has come from a distance, and huge blocks, or "erratics," as they have been called, many feet in diameter, have not unfrequently travelled hundreds of miles from their point of departure, or from the parent rocks from which they have evidently been detached.
Types of moraines, distance of blocks. Weathering of glaciers. Types of surface. Amount of water silt bands, stratified, or irregular folded or broken. Cross section, of valleys 35° slopes? Weather slopes debris covered, height to which. Nunataks, height of rounded, height of any angle in profile, erratics. Evidence of order in glacier delta.
I may here remark that a sheet of ice of moderate thickness, if it extend over a wide area, may suffice to buoy up the largest erratics which fall upon it. The size of these will depend, not on the intensity of the cold but on the manner in which the rock is jointed, and the consequent dimensions of the blocks into which it splits when falling from an undermined cliff.
Throughout the Alps, on lofty crags, great bowlders were often found, which had no relation to the geology of the region and which were called erratics, because they had evidently come there from a distance. But how? Scientists explained it in many ways, but it remained for the mountaineer to suggest that the bowlders had been left in their present positions by glaciers.
And even the catastrophists, though they met Lyell amicably on almost no other theoretical ground, were inclined to admit the plausibility of his theory of erratics. Indeed, of all Lyell's nonconformist doctrines, this seemed the one most likely to meet with general acceptance.
Fundamental Rocks, polished, grooved, and scratched. Abrading and striating Action of Glaciers. Moraines, Erratic Blocks, and "Roches Moutonnees." Alpine Blocks on the Jura. Continental Ice of Greenland. Ancient Centres of the Dispersion of Erratics. Transportation of Drift by floating Icebergs. Bed of the Sea furrowed and polished by the running aground of floating Ice- islands.
First we had Mount Hope at the lower end of the Glacier. Mount Hope is a nunatak of granite, about 2800 feet in height, of which the summit is strewn with erratics, giving evidence of former glaciation of far greater extent.
We learn, therefore, from a study both of the arctic and antarctic regions, that a great extent of land may be entirely covered throughout the whole year by snow and ice, from the summits of the loftiest mountains to the sea-coast, and may yet send down angular erratics to the ocean.
Forests clothed the shore on every side, and the shelving beach met the wood in a bank of wild Bromelia, most brilliant in color. Not only were excellent collections made on this beach, but the shore was strewn with large accumulations of erratics.
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