_Effects of freezing water._—The irresistible force exerted by the expansion of water in freezing, amounting to about 9 per cent of its bulk, is a powerful factor in widening and deepening fissures and cracks of rocks; not uncommonly, whole masses of rock are rent into fragments by this agency, which is one of the most common causes of “rock falls” on the brink of precipices. By the freezing process cracks and crevices are enlarged, and the surfaces exposed to weathering are still farther increased; and the rock fragments or soil particles are loosened and rendered more liable to be removed from the original site, whether by gravity, wind or water. _Glaciers._—Ice in the form of the glaciers that descend from mountain chains (see figure 1), and of the moving ice sheets that have covered large portions of North America and Europe in past ages and now cover Greenland and the South Polar continent, exerts a most potent action in abrading and grinding even the hardest rocks; not so much by the direct friction of the moving ice itself, as by the cutting, scoring, grinding and crushing action which the stones imbedded in the ice, or carried and shoved by it, exert upon the rocky channels in which the ice stream moves, as well as upon each other. The product of this grinding process is largely very fine (hence “glacier flour”), so that it remains suspended in the water of the glacier-streams until their velocity is permanently checked when reaching a plain or lake. This suspended stone-flour imparts to the glacier streams their distinctive character of “white rivers,” as contradistinguished from the clear, dark “green rivers” that have their origin outside of glaciated areas. This difference can be readily observed in traveling along any of the glacier-bearing mountain chains of the world, and is frequently expressed in the names of the streams. [Illustration: FIG. 1.—Zermatt Glacier (Agassiz).] The physical analysis of mud from the foot of Muir glacier,[5] Alaska, at its sea front, made by Professor Loughridge, shows the prevalent fineness of the materials brought down by the glacier waters. [5] Collected by Dr. W. E. Ritter of the University of California. PHYSICAL COMPOSITION OF GLACIER MUD. ============+====================+=============== MATERIAL. | DIAMETER. | PER CENT. ------------+--------------------+--------------- Clay | ? | 16.57 } 70.31 Fine silt | .0023 -- .016 mm. | 53.74 } Fine silt | .016 to .025 mm. | 4.38 Medium silt | .025 to .036 mm. | 7.06 Coarse silt | .036 to .047 mm. | 5.91 Coarse silt | .047 to .072 mm. | 3.76 Fine sand | .072 to .12 mm. | 1.14 Medium sand | .12 to .16 mm. | 1.56 | | ----- Total | | 94.12 ------------+--------------------+---------------