Although the explorations of the central field of Greenland are as yet incomplete, several of these excursions into or across the interior have been made, and the identity of the observations is such that we can safely assume the whole region to be of one type. We can furthermore run no risk in assuming that what we find in Greenland, at least so far as the unbroken nature of the central ice field is concerned, is what must exist in every land where the glacial envelope becomes very deep. In Greenland it seems likely that the depth of the ice is on the average more than half a mile, and in the central part of the realm the sheet may well have a much greater profundity; it may be nearly a mile deep. The most striking feature--that of a vast unbroken expanse, bordered by a region where the ice is ruptured--is traceable wherever very extensive and presumably deep deposits of ice have been examined. As we shall see hereafter, these features teach us much as to the conditions of glacial action--a matter which we shall have to examine after we have completed our general survey as to the changes which occur during glacial periods.
In the present state of that wonderful complex of actions which we term climate, glaciers are everywhere, so far as our observations enable us to judge, generally in process of decrease. In Switzerland, although the ancients even in Roman days were in contact with the ice, they were so unobservant that they did not even remark that the ice was in motion. Only during the last two centuries have we any observations of a historic sort which are of value to the geologist. Fortunately, however, the signs written on the rock tell the story, except for its measurement in terms of years, as clearly as any records could give it. From this testimony of the rocks we perceive that in the geological yesterday, though it may have been some tens of thousands of years ago, the Swiss glaciers, vastly thickened, and with their horizontal area immensely expanded, stretched over the Alpine country, so that only here and there did any of the sharper peaks rise above the surface. These vast glaciers, almost continually united on their margins, extended so far that every portion of what is now the Swiss Republic was covered by them. Their front lay on the southern lowlands of Germany, on the Jura district of France; on the south, it stretched across the valley of the Po as far as near Milan. We know this old ice front by the accumulations of rock _débris_ which were brought to it from the interior of the mountain realm. We can recognise the peculiar kinds of stone, and with perfect certainty trace them to the bed rock whence they were riven. Moreover, we can follow back through the same evidence the stages of retreat of the glaciers, until they lost their broad continental character and assumed something like their present valley form. Up the valley of any of the great rivers, as, for instance, that of the Rhône above the lake of Geneva, we note successive terminal moraines which clearly indicate stages in the retreat of the ice when for a time it ceased to go backward, or even made a slight temporary readvance. It is easily seen that on such occasions the stones carried to the ice front would be accumulated in a heap, while during the time when day by day the glacier was retreating the rock waste would be left broadcast over the valley.