. Elementary physical geography . Fig. Alpine snow field. GLACIERS. 307 where the warmth of the sun is sufficient to melt them, andtherefore to stop their further movement. This place is nota fixed line, but may vary from year to year, so that thefront of a glacier often retreats and advances. The conditions at present favoring the formation ofglaciers, are found either in high mountains, or else in lati-. FiG. glacier, Mt. Shasta. tudes within the Arctic or Antarctic circles. There was atime when these conditions existed further south, and thengeneral glaciation was brought
. Elementary physical geography . Fig. Alpine snow field. GLACIERS. 307 where the warmth of the sun is sufficient to melt them, andtherefore to stop their further movement. This place is nota fixed line, but may vary from year to year, so that thefront of a glacier often retreats and advances. The conditions at present favoring the formation ofglaciers, are found either in high mountains, or else in lati-. FiG. glacier, Mt. Shasta. tudes within the Arctic or Antarctic circles. There was atime when these conditions existed further south, and thengeneral glaciation was brought about in regions now withinthe temperate zone. There are two quite distinct classes ofglaciers: the valley or alpine, and the continental glacier. Alpine or Valley Glacier This form of glacier receives its name from the fact that it is generally developed in 308 PHYSICAL GEOGRAPHY. mountain valleys, and is particularly well developed amongthe Alps (Fig. 175). We also find valley glaciers amongmost of the mountains of Alaska (Plate 26), in BritishColumbia, in some of the high mountains of Washington,such as Mt. Shasta (Fig. 174), and in several places inthe Sierra Nevadas (Fig. 177). The glaciers of the westare small and insignificant, but those of Alaska are among the best developedin the world. Val-ley glaciers are byno means uncom-mon in other partsof the earth ; and,among otherplaces, we findthem in N
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