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]).
[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]).
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]).
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.
Thus, within a relatively short time—geologically speaking—the familiar land-shaping processes described above have joined forces to provide Texas with one of its most remarkable natural attractions. But interestingly enough, the same geologic processes that created these unusual [formations] are busily at work destroying them. As time passes and erosion progresses, the caps of the pedestals are worn away and the underlying [shales] crumble and are washed into the valley below. Yet even as the old land-forms are being destroyed, wind, water, ice, and man are attacking the canyon walls to produce still more of these interesting erosional remnants.