Fig. 29.—Top of Notch Peak, Bighorn Mountains, Wyo. Shows the thoroughly broken character of the rock on the summit, the absence of soil, vegetation, etc. (Kümmel.)
Fig. 30.—A detail from [Fig. 29] showing the size of the rock blocks. (Kümmel.)
Fig. 31.—Peak north of Kearsarge Pass, the Sierras. Shows the way in which serrate peaks break up into angular blocks.
The freezing of water in the pores of rock is effective in disrupting them only when the pores are essentially full at the time of freezing. Otherwise there is room for the expansion attending the freezing. If the pores of the rock are large, the expansion on freezing may force out sufficient water to balance the increase of volume, even though the rock was completely saturated. If the pores be very small the water passes out less readily, and if the rock is saturated, freezing is more likely to be attended with disruption.[21]
In view of these considerations the breaking of rock by changes of temperature should be greatest on the bare slopes of isolated elevations of crystalline rock, where the temperature conditions of temperate latitudes prevail, and where the atmosphere is relatively free from moisture. All these conditions are not often found in one place, but the disrupting effects of changing temperatures are best seen where several of them are associated (Figs. [29], [30], and [31]).
The importance of this method of rock-breaking has rarely been appreciated except by those who have worked in high and dry regions. Climbers of high mountains know that almost every high peak is covered with broken rock to such an extent as to make its ascent dangerous to the uninitiated. High serrate peaks, especially of crystalline rock, are, as a rule, literally crumbling to pieces ([Fig. 31]). The piles of talus which lie at the bases of steep mountain slopes are often hundreds of feet in height, and their materials are often in large part the result of the process here under discussion. In mountain regions where atmospheric conditions favor sudden changes of temperature, the sharp reports of the disruption of rock masses are often heard. Masses of rock, scores and even hundreds of pounds in weight, are frequently thus detached and started on their downward course.[22] Small pieces of rock are of course much more commonly broken off than large ones. The disruption of rock by changes of temperature is not usually the result of a single change of temperature, but rather of many successive expansions and contractions.
The sharp needle-like peaks which mark the summits of most high mountain ranges ([Fig. 32]) are largely developed by the process here outlined. The altitude at which the serrate topography appears varies with the latitude, being, as a rule, higher in low latitudes and lower in high. But even in the same latitude it varies notably with the isolation of the mountains and with the aridity of the climate. Thus within the United States the sharply serrate summits appear in some places, as in Washington and Oregon, at altitudes of 6000 to 10,000 feet, while in the isolated Wichita range of Oklahoma, much farther south, but in a much drier climate, the same sort of topography is developed at altitudes of 2500 to 3000 feet.
Even in low latitudes and moist climates the effects of temperature changes are often seen. Thin beds of limestone at the bottom of quarries are sometimes so expanded by the heat of the sun as to arch up and break.[23] In desert and arid regions,[24] whatever the altitude, the effects of temperature changes are often striking.