The term frost, in this connection, is restricted to the freezing of water. Water is one of the comparatively few substances which expand considerably on solidifying. This expansion amounts to about one-eleventh of the original bulk of the water, and, exerting a pressure of somewhat more than 2000 pounds per square inch, takes place with irresistible power, bursting thick iron vessels like egg-shells.

Excepting loose, incoherent masses, like sand and gravel, no rocks are formed of continuous sheets of material, but are rather to be considered as masses of blocks, divided by joints (see pp. 5 and 136). In addition to these visible clefts, the blocks are traversed by minute crevices, rifts, and pores, all of which openings take up and retain quantities of water, as may readily be seen by examining freshly quarried stone. When exposed to a low temperature, the water freezes and forces out the large blocks and shatters them into pieces of smaller and smaller size. The fragments thus formed are called talus, and great accumulations of such blocks are found at the foot of cliffs in all regions where the winters are at all severe. Talus accumulations are also formed by other agencies, as will be seen in the sequel. Alternate freezings and thawings not only break up rocks, but cause the broken fragments and soil to work their way down slopes. Each freezing causes the fragments to rise slightly at right angles to the inclined surface, and each thawing produces a reverse movement; hence the slow creep down the slope.

The action of frost is, of course, practically absent in the lowlands of the tropics, but in high mountains and in all countries which have cold winters, frost is an agent of great importance in the mechanical shattering of rocks and slow destruction of cliffs. The hardest rocks are shivered into fragments and, dislodged from their places, the fragments roll down the mountain side till they come to rest, perhaps thousands of feet below. Immense accumulations of frost-made talus are to be found in such places as the foot of the Palisades of the Hudson, the abrupt southern slope of the Delaware Water Gap, and'wherever cliffs or peaks of naked rock are exposed to severe cold. Many mountain passes are so bombarded by falling stones as to be extremely dangerous; in the Sierra Nevada of California, talus slopes as much as 4000 feet high are reported, all the work of frost. At Sherman, where the Union Pacific Railroad crosses the "continental divide," the ground is covered for miles with small, angular fragments of granite broken up by the frost.

Shales creeping under the action of frost. (U. S. G. S).

Fig. 39. - Shales "creeping " under the action of frost. (U. S. G. S).

In the polar regions frost is probably the most important of the disintegrating agents. In Spitzbergen Beechy found that in summer the mountain slopes absorb quantities of water, which freezes in winter with very destructive effect. "Masses of rock were, in consequence, repeatedly detached from the hills, accompanied by a loud report, and falling from a great height, were shattered to fragments at the base of the mountain, there to undergo more rapid disintegration." Similar phenomena are reported from the Aleutian Islands of Alaska.

Cliff and talus slope, Delaware Water Gap, N.J.

Fig. 40. - Cliff and talus slope, Delaware Water Gap, N.J.

The action of frost is, in itself, purely mechanical; no chemical change is occasioned by it, and the smallest fragments into which a block may be riven are sharp and angular, and the minerals have unaltered and shining faces. But, on the other hand, frost pre pares the way for the more rapid action of rain and percolating waters. The effects of these agents are produced upon the surface of the rocks and the walls of the crevices which run through them. By breaking up the blocks, the frost greatly increases the surface and thus facilitates the work of the rain. A breaking up of one cubic foot into cubic inches multiplies the exposed surface by 12. Frost is an extremely superficial agent and acts effectively only a few feet below the surface. In the polar regions the ground is permanently frozen to a depth of several hundred feet, but the shattering of rocks requires alternate freezings and thawings.

Rain and frost are agents whose effects are most important in regions of moist climate and abundant rainfall, for both are forms of the activity of water. Few regions of the earth's surface are altogether rainless, but nearly all of the continents have great desert areas in which atmospheric precipitation is very light. It might seem that in such deserts the work of rock disintegration must be practically at a standstill, and that the circulation of material must be so slow as to be hardly distinguishable from complete stagnation. Even in these regions, however, the rain accomplishes something, and it is aided by other agencies which in moist climates play a much more modest role; these are the changes of temperature and the wind.