Part 5

The lower portion of the Ogallala Formation is composed of a reddish-brown, fine- to medium-grained sandstone that contrasts sharply with the underlying red and green shales that are exposed in the top of the Trujillo Formation. Much of this sandy rock is characterized by pebbles consisting of a variety of igneous, sedimentary, and metamorphic rocks. Because it consists of rock and mineral fragments of varied composition and size, this kind of sedimentary rock is called a conglomerate. The type of rock fragments found in basal Ogallala conglomerates suggests that they were transported to the Panhandle-Plains area by streams flowing southeastward from the Rocky Mountains. As these streams deposited their loads, they left behind a wide spread blanket of sand, gravel, and mud which formed an extensive alluvial plain that extended from western Nebraska to northwest Texas. Although it is less than 100 feet thick in Palo Duro Canyon, in places this great mantle of fluvial (stream-deposited) sediments is as much as 900 feet thick.

Most of the Ogallala Formation consists of a mixture of diverse rock types such as conglomerate, sandstone, siltstone, clay and marl. But the upper part of the formation is characterized by thick caliche deposits. A dull, earthy calcite deposit, caliche typically forms in areas of scant rainfall. It is believed to originate when ground moisture, containing dissolved calcium bicarbonate, moves to the surface where the moisture steadily evaporates leaving a calcium carbonate crust on or near the surface (fig. 17).

Caliche, which derives its name from the Latin calix, meaning “lime,” may be firm and compact or loose and powdery. It is also commonly found mixed with other materials such as clay, sand, or gravel. Caliche commonly occurs in the Trans-Pecos, southwestern Gulf Coastal Plain, and the High Plains area of Texas (see fig. 5, p. 8). In the latter area it typically makes up the “caprock.” Caliche is commonly quarried in these parts of Texas where it is used as road material and as an aggregate.

Good exposures of Ogallala caliche can be seen on the surface around the overlook at Coronado Lodge on the northwest rim of the canyon (fig. 17). Ogallala strata also crop out along the upper reaches of Park Road 5 as it starts to descend into the canyon. But probably the most spectacular exposures of the Ogallala are exposed in the precipitous face of the Fortress Cliff (fig. 33) which forms part of the eastern rim of the canyon.

Also located within the Ogallala Formation is a very important aquifer—a porous, water-bearing rock formation. This fine-to coarse-grained sandstone is very porous and permeable and is the most important single water-producing formation in the Panhandle-Plains area.

Opal and chert are locally abundant in the Ogallala conglomerates. The opal, which is found in small cavities in the conglomerate is not of the gem variety but it does fluoresce. Minerals that exhibit fluorescence emit visible colors when exposed to ultraviolet light. For this reason, the Ogallala opal is sought after by rock and mineral collectors. The chert, a flint-like variety of quartz, occurs as nodules in the conglomerate and in a well-developed layer near the base of the formation. Both of these siliceous (silica-bearing) rocks were apparently prized by the Indians, who used them to fashion knives, scrapers, projectile points, and other artifacts. The Indians also learned that flat slabs of caliche were ideal for lining fireplaces and to construct primitive rock shelters.

A number of Pliocene vertebrates have been found in the Palo Duro area. Known as the “Age of Mammals,” the Tertiary Period was characterized by mammals as diverse as were the reptiles of the Mesozoic Era. Among these unusual creatures were such now-extinct species as the saber-tooth cat and the elephant-like shovel-jawed mastodon (fig. 18). The remains of these as well as bones of giraffe-like camels, pony-sized horses, and sloths have been found in the vicinity of the canyon. The grassy plains of Pliocene time were also inhabited by large tortoises which reached lengths of up to 3 feet (fig. 19). Dioramas showing how these animals might have looked, as well as their actual remains, are on display in the Hall of Pre-History in the lower floor of the Panhandle-Plains Historical Museum in Canyon, 13 miles west of the park (p. 35).

Rocks of the Pleistocene.—

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).

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.—