=The advancing hemicycle.=—There is little reason to doubt that whatever be the cause of the climatic changes which bring on glacial conditions, these changes come on by insensible gradations. The first visible evidence of the increased severity of the climate is the longer persistence of the winter snows, at first within the more elevated districts. In such positions drifts must eventually continue throughout the warm season and so contribute to the snow accumulations of the succeeding winter. This point once reached, small glaciers are inevitable, even should the average temperature fall no further, for the snow left over in each season must steadily increase the depth of the deposits until the weight brings about an internal motion of the mass from higher to lower levels. [Illustration: FIG. 292.—An Alaskan glacier spreading out at the foot of the range which nourishes it.] The inherited depressions of the upland—the gentle hollows at the heads of rivers—will first be filled, and so the valleys below become the natural channels for the outflow of the early glaciers. With a continued lowering of the annual temperature and consequent increased snowfall, the early glaciers become more and more amply nourished. Snow and ice will, therefore, cover larger areas of the upland, and the glaciers will push their fronts farther down the valleys before they are wasted in the warm air of the lower levels. As the valleys become thus more completely invested by the glacier they are likewise filled to greater and greater depths, and they may thus submerge portions of the walls that separate adjacent valleys. Reaching at last the front of the upland area, the glaciers may now be so well nourished at their heads that they push out upon the flatter foreland and without restraint from retaining walls spread broadly upon it (Fig. 292). [Illustration: FIG. 293.—Surface of a glacier whose upper layers spread with slight restraint from retaining walls. Surface of the Folgefond, an ice cap of southern Norway.]