The youngest [rocks] in Palo Duro Canyon State Park were formed during the [Pleistocene] [Epoch] of the Quaternary [Period] of the [Cenozoic] [Era] (see [geologic time scale], [p. 11]). Pleistocene rocks are rather widespread in much of the Panhandle-Plains area and they are mostly composed of [sediments] which were deposited in stream valleys, in lakes or ponds, or by the wind. Most of the Pleistocene [strata] in the park area consist of loose deposits of [silt] and sand which were deposited by wind action. Known locally as “blow sand,” this reddish-brown, silty sand overlies the Ogallala [caliche] at most points along the canyon’s rim.
HOW THE CANYON WAS CARVED
The visitor seeing Palo Duro Canyon for the first time may find it difficult to believe that this yawning chasm began as a simple gully. But to the geomorphologist—the geologist who studies the origin and development of landscapes—Palo Duro Canyon is but a gully magnified many times over. This is evident because the shape of the canyon, the nature of its tributaries, and the character of its walls indicate that it has been deepened and lengthened by the downcutting of a stream and widened by other geologic processes.
THE GEOLOGIC WORK OF RUNNING WATER
Palo Duro Canyon is a classic example of a land-form that has been created by the geologic work of running water. Undoubtedly the most important single agent of erosion, running water probably does more to wear away the land than all the other geologic agents combined. This is not surprising considering the fact that the earth’s annual precipitation (such as rain and snow) equals about four billion tons of water. Although the amount of precipitation varies greatly from place to place, the average annual precipitation on land is about 40 inches of water. Of this, roughly 25 percent runs off from the land to form streams.
When one drives through the park and fords the normally gently flowing waters of the Prairie Dog Town Fork of the Red River he may well wonder if this unimposing stream actually is the geologic agent that is responsible for this deep gorge. But the visitor who happens to be present during a severe rainstorm will soon be convinced, for during heavy rains this gentle stream becomes a raging torrent. As the river increases in size it also becomes a more effective land-shaping tool, for the larger and swifter the stream, the more [rock] material it can carry. Thus, when flowing at peak capacity, this branch of the Red River becomes a moving ribbon of sandpaper whose load of sand, [silt], and gravel has cut and scoured the canyon walls and floor for hundreds of thousands of years. How long has it taken the river to carve this remarkable chasm? Although there is no way of knowing for sure, geologic evidence indicates that the canyon has formed during the last one million years—a relatively short time, geologically speaking.
The work of the river is made still more effective by water and [sediment] which it receives from its tributaries; this added water substantially increases the volume and velocity of the river. Although many of the tributary streams are dry throughout much of the year, they carry large quantities of water during heavy rains. Moreover, because most of these streams flow over [rock] surfaces which are not protected by thick soil or vegetation, their waters are quickly transported to the master stream. Thus, the volume and velocity of the Prairie Dog Town Fork of the Red River make it possible—especially during flood [periods]—for the river to carry a large load of rock particles which effectively erodes the stream channel. Where does this rock debris come from? Most of it is eroded from the sides and bottom of the river’s channel.
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]).