The culmination of the progressive climatic change may ere this have been reached and milder conditions have ensued. If, however, the severity of the climate should be still further increased, the expanded fronts of neighboring glaciers will coalesce to form a common ice fan or apron along the foot of the upland (Plate 18 B). This could hardly take place without a still further deepening of the ice within the valleys above, and, probably, a progressive submergence of the lower crests in the valley walls. This may even continue until all parts of the upland area have been buried. The snow and ice now take the form of a covering cap or carapace, and the upper portions being no longer restrained at the sides, now spread into a broad dome, as would a viscous liquid like thick molasses when poured out upon the floor (Fig. 293). The lower zones of the mass and the thinner marginal portions still have their motion to a greater or less extent controlled by the irregularity of the rock floor against which they rest. The reverse series of changes in the glacier is inaugurated by an amelioration of the climate, and here, therefore, the advancing hemicycle becomes merged in the receding hemicycle of glaciation. =Continental and mountain glaciers contrasted.=—The time when the rock surface becomes submerged beneath the glacier is, as regards both the surface forms and the erosive work, a critical point of much significance; for the ice cap and larger continental glacier obviously protect the rock surface from the action of those chemical and mechanical processes in which the atmosphere enters as chief agent, and which are collectively known as weathering processes. Until submergence is accomplished, larger or smaller portions of the rock surface project either through or between the ice masses and are, therefore, exposed to direct attack by the weather (see below, p. 370). [Illustration: FIG. 294.—Section through a mountain glacier (in solid black), showing how its surface is determined by the irregularities in the rock basement (after Hess).] Snow which falls in the mountains is not allowed to remain long where it falls. By the first high wind it is swept off the more elevated and exposed surfaces and collected under eddies in any existing hollows, but especially those upon the lee slopes of the range. We are to learn that glaciers carve the mountains by enlarging the hollows which they find and producing great basins for the collection of their snows; but with the initiation of glaciation the inherited hollows are in most cases the unimportant depressions at the heads of streams. Whatever they may be and however formed, the snow first fills those hollows which are sheltered from the wind, and as it accumulates and becomes distributed as ice, assumes a surface of its own that is dependent upon the form and the position of the basin which it occupies (see Fig. 294).