=Topography.=—Many of the minor irregularities of the surface of a glacier are the result of crevassing. After the ice is crevassed, the sun’s rays and the air which has been warmed by them penetrate the openings and melt the ice. The melting is most rapid at the top, and decreases downward. The result is that the sections of ice between adjacent crevasses are narrowed into wedges. If there be cross-crevassing, as is common, points instead of wedges result. As the sort of surface shown in Fig. 239 develops, any débris which was on the ice slides into the crevices, and the upper surface becomes clean. Where ice is crevassed transversely, and where melting is not rapid, the crevasses may close as the ice moves forward, and the regelation of adjoining faces heals the rents in the surface. Even in this case, however, the surface is likely to be more or less undulating because of the waste on the sides of the crevices before they are closed. After regelation, surface ablation tends to obliterate the protuberances. The topography of the surface of the ice is affected by other conditions. All parts of the surface of the ice are not equally compact, and the least compact portions melt most rapidly, giving rise to depressions, while the more solid parts occasion protuberances. Both depressions and protuberances may be regular or irregular in form (Figs. 240 and 241). Undulations of the bed often show themselves in the surface of the ice as suggested by Fig. 242. In such cases, ponds or lakelets sometimes accumulate on the surface of the ice. The topography of the ice in such cases seems to show that the ice is forced up slope. =Surface moraines.=—The surface of a glacier is often affected by débris of one sort or another, and this also influences its topography. The débris is sometimes disposed in the form of belts or _moraines_ (Figs. 237, 243). The surface moraines may be _lateral_, _medial_, or _terminal_. A _lateral moraine_ is any considerable accumulation of débris in a belt on the side of a glacier. A _medial moraine_ is a similar accumulation at some distance from the margins, but not necessarily in or near the middle. There may be several medial moraines on one glacier, in which case some of them may be far from the center. In alpine glaciers, the _surface terminal_ moraine is less well-defined; in polar glaciers it often connects two lateral moraines, making a loop roughly concentric with the terminus of the glacier. [Illustration: +Fig.+ 240.—End of Mount Dana glacier, Cal. Shows irregularities of surface due to crevassing farther up the glacier, and to unequal melting.] [Illustration: +Fig.+ 241.—Shows irregularities due to unequal melting of veined ice. End of small glacier south of Forno hut, Engadine, Switzerland. (Reid.)]