_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). Differential erosion.— Even the most casual observer will soon note that not all of the canyon’s rocks have been equally affected by erosion. Indeed, it is the nature of this _differential erosion_ that gives Palo Duro Canyon the rugged sculptured appearance that accounts for much of its beauty. Visitors to Palo Duro Canyon commonly ask why the rock formations are so diversely shaped. The answer to this question lies in the rocks themselves. Because the various rock strata are of unequal hardness, they erode at different rates of speed. Hence, the harder, more resistant rocks, such as the sandstones and conglomerates of the Trujillo Formation, form the shelves, ledges, and “caps” of the rock sculptures. The Lighthouse (fig. 31) and other pedestal rocks (fig. 16) are good examples of land-forms produced by differential erosion. The “hoodoos” mentioned earlier are also the products of this type of erosion (figs. 16 and 20). [Illustration: Fig. 20. Talus slopes (arrow) are well developed on the east side of Capitol Peak and in places obscure the Quartermaster red beds. Note the “hoodoo” at the south (left) end of the structure.] Softer rocks like shales and clay are more readily eroded and they normally form slopes rather than cliffs or ledges (fig. 12). Grooves, recesses, and caves have also developed in some of the less resistant rocks such as the shales and gypsum beds of the Quartermaster Formation. Catarina Cave (fig. 27) which has formed in the red and white shales of the Spanish Skirts (fig. 26) is a good example of this type of feature. Caves of this type afforded protection to both man and wild animals since the dawn of history, for their remains have been found in a number of similar caves.