The river carries its load in a number of ways. Material such as salt and other soluble matter is transported in a dissolved state or in _solution_. Still more, for example, silt and fine sand, is carried in _suspension_. These sediments are suspended between the surface of the water and the bottom of the stream channel. Those particles that will not dissolve in water and are too heavy to be carried in suspension, constitute the _bottom load_ of the stream. These larger sediments, such as gravel, cobbles, and boulders, roll, bounce, or slide along the stream bed. As flash floods course through Palo Duro Canyon, the river uses its load to erode further the rocks over which it passes. Each moving rock fragment literally becomes a cutting tool for _abrasion_ as the loose rock particles slowly wear away the banks and bed of the stream. Eventually the abraded rock fragments become smooth and rounded and the stream channel is gradually worn down to a lower level; it is also widened. The river also erodes by _hydraulic action_ as loose rock fragments are lifted and moved by the force of the stream’s current. This process is similar to the effect produced when soil is churned up and washed away when water from a garden hose is sprayed on loose earth. The effects of hydraulic action have played an important role in widening the canyon, for recession of the cliffs away from the middle of the canyons has been caused in part by undercutting. Thus, as the soft shale and gypsum beds were removed by the stream, the overlying sandstone formations gradually broke off and fell into the canyon. Once on the canyon floor, most of the slabs and blocks of sandstone were eventually broken up and carried away by the streams as sand and mud. Not all of the boulders have been destroyed in this manner; in places (for example, the Rock Garden) similar boulders are seen today (fig. 34). 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.—