[WEATHERING] AND GRAVITY ADD THE FINAL TOUCH
Most of the energy of the river has been expended in downcutting, for the canyon has apparently been deepened more rapidly than it has been widened. But as the stream gouged its channel deeper into the bedrock, an ever-increasing expanse of canyon wall was exposed to other agents of erosion. Slowly—almost imperceptibly—the walls of the canyon have been eroded by the processes of [weathering] and mass-wasting.
[Weathering].—
Wherever [rocks] are exposed on the earth’s surface, they are attacked by the agents of [weathering]. They are dissolved by rainwater, pried apart by frost and ice, and blasted by windblown sand. Some of the changes produced by weathering are purely mechanical, that is, the rock is simply reduced to smaller fragments without being broken down chemically or undergoing any change in its [mineral] composition. This mechanical weathering, or disintegration, takes place in a number of ways. Changes are especially noticeable in rocks that are subjected to large daily temperature variations. If a crack in these rocks becomes filled with water and the temperature drops below freezing, ice forms. When water freezes it expands by about 10 percent of its volume—this is the reason why water pipes often split open during the winter. Just as in a water pipe, the pressure of the expanding ice is commonly great enough to widen and deepen the crack in the rock. This process, called frost wedging, may ultimately cause the rock to split and fall apart. The cumulative effects of frost wedging have probably played a significant role in prying off large blocks of rocks from the walls and rim of the canyon.
Animals and plants may also hasten [rock] disintegration. Plant roots commonly grow in rock crevices and as the roots become larger they wedge the rock apart. Burrowing animals such as rabbits, gophers, and ground squirrels also promote rock disintegration. Although they do not attack the rocks directly, their digging exposes new rock surfaces to [weathering] processes. The holes these creatures make also permit water and air to enter the earth more easily, thereby hastening rock destruction.
Man, of course, promotes more [rock] disintegration than all other animals combined. Thus, as one explores the canyon’s trails and climbs its walls, he will not only see evidence of the various types of mechanical [weathering], he will also be contributing to the further wearing away of the rocks.
Decomposition, or chemical [weathering], works hand in hand with mechanical weathering. But unlike disintegration, decomposition produces [rock] materials that are basically different from the original unweathered rock. These changes are brought about as the result of chemical reactions between [minerals] in the rocks and water, carbon dioxide, and oxygen. Although the arid climate and severe winters of the Panhandle generally facilitate mechanical weathering, some of the red [shales] and [gypsum] deposits show the effect of oxidation, hydration, and other forms of chemical weathering ([fig. 10]).
Mass-wasting.—
Mass-wasting, the erosional process by which [rock] and soil move downslope in response to the force of gravity, has also been instrumental in shaping Palo Duro Canyon. This type of erosion has been especially active on the walls of the canyon, for here the slopes are steep enough to promote downward movement of earth materials. In a few places there have been landslides which have moved large quantities of rock in a short span of time. But most mass movements have been imperceptibly slow as masses of [talus] (accumulations of rock debris) on steeper slopes have inched slowly downhill because of their own weight. Talus deposits produced in this way can be seen at the foot of most of the cliffs and erosional remnants throughout the canyon ([fig. 20]).