This is but a small fraction of the thickness to which snow would be piled on the Alps were it not constantly being drained away.  Below the snow fields which mantle the heights the mountain valleys are occupied by glaciers which extend as much as a vertical mile below the snow line.  The presence in the midst of forests and meadows and cultivated fields of these tongues of ice, ever melting and yet from year to year losing none of their bulk, proves that their loss is made good in the only possible way.  They are fed by snow fields above, whose surplus of snow they drain away in the form of ice.  The presence of glaciers below the snow line is a clear proof that, rigid and motionless as they appear, glaciers really are in constant motion down valley.

The neve field.  The head of an Alpine valley occupied by a glacier is commonly a broad amphitheater deeply filled with snow.  Great peaks tower above it, and snowy slopes rise on either side on the flanks of mountain spurs.  From these heights fierce winds drift the snows into the amphitheater, and avalanches pour in their torrents of snow and waste.  The snow of the amphitheater is like that of drifts in late winter after many successive thaws and freezings.  It is made of hard grains and pellets and is called neve.  Beneath the surface of the neve field and at its outlet the granular neve has been compacted to a mass of porous crystalline ice.  Snow has been changed to neve, and neve to glacial ice, both by pressure, which drives the air from the interspaces of the snowflakes, and also by successive meltings and freezings, much as a snowball is packed in the warm hand and becomes frozen to a ball of ice.

The bergschrund.  The neve is in slow motion.  It breaks itself loose from the thinner snows about it, too shallow to share its motion, and from the rock rim which surrounds it, forming a deep fissure called the bergschrund, sometimes a score and more feet wide.

Size of glaciers.  The ice streams of the Alps vary in size according to the amount of precipitation and the area of the neve fields which they drain.  The largest of Alpine glaciers, the Aletsch, is nearly ten miles long and has an average width of about a mile.  The thickness of some of the glaciers of the Alps is as much as a thousand feet.  Giant glaciers more than twice the length of the longest in the Alps occur on the south slope of the Himalaya Mountains, which receive frequent precipitations of snow from moist winds from the Indian Ocean.  The best known of the many immense glaciers of Alaska, the Muir, has an area of about eight hundred square miles (Fig. 95).