Volume 3 - No. 3
THE ORIGIN OF THE ROYAL GORGE
By Dr. Don B. Gould,
The Royal Gorge of the Arkansas River, in Colorado, is a bottleneck for the upper Arkansas Valley; it is a deep and unusually narrow canyon which cuts across the grain of the Rocky Mountains. The Rocky Mountains have a grain, something like a piece of wood. This is shown by the northwest-southeast trend of the main ranges. Between the ranges are lower areas, in which several major rivers have their source. The Arkansas River heads in one of these linear depressions in central Colorado, being fed by the melting snows of the Sawatch Range on the west, and the Mosquito Range on the east. The crests of these parallel ranges are about 15 miles apart, and their peaks rise more than a mile above the valley to elevations greater than 14,000 feet.
The upper Arkansas Valley is relatively open and conforms perfectly to the grain of the Rockies. From Tennessee Pass, at the north end of the Sawatch Range, the river flows southeastward for about 75 miles to a point below Salida, following the "straight and narrow path." But here, in spite of topographic and geologic arguments that the river should continue to follow the grain of the Rockies, it makes a sharp right angle turn to the northeast and flows "cross-grained" for about 30 miles through a granite-walled canyon which gets narrower and deeper downstream. The narrowest and deepest part of this canyon is known as the Royal Gorge. The Gorge opens abruptly onto the flat lowlands surrounding Canon City, where the Arkansas River begins its long journey eastward across the plains.
To visitors who look down into the Royal Gorge from its rim, the Arkansas River far below looks so small and insignificant that it seems incapable of cutting such a deep gash into the earth's surface. It is not surprising that these visitors occasionally suggest that the gash must have been opened by a mighty earthquake. To passengers on the railroad, which follows the bottom of the Gorge, the river is much more impressive as it rushes turbulently through the narrow passage. Those who realize the cutting power of sand-laden water are willing to concede the ability of the Arkansas River to saw its way down through the hard rocks of the canyon walls.
Several phases of the history of the Royal Gorge and the upper Arkansas Valley are not fully known, but geologists agree that the Gorge has been formed by the incessant grinding of pebbles and sand grains against the river bed as they are carried along by the stream. But why is the Gorge so deep, and why so narrow in relation to its depth, and in relation to its much greater width upstream and downstream? These questions can be answered more clearly after a detailed examination of the Gorge and the land surface in its vicinity.
At its narrowest point, the Gorge is approximately 1,250 feet wide and 1,050 feet deep; its walls consist of granite and gneiss of pre-Cambrian age. On each side of the Gorge a gently rolling plateau extends away from the rim for several miles. Rising above this plateau on the north are hills which represent the southernmost extension of the Front Range, and on the south rises the northern end of the Wet Mountains. Both of these ranges have been pushed up and later eroded sufficiently to expose the granite core which is typical of Rocky Mountain ranges. The Royal Gorge Plateau is a connecting link which ties these two ranges together, since the granite and gneiss of the Gorge walls are continuous with the great masses of similar rocks which make up the cores of the two ranges.
The Front Range extends from the Royal Gorge northward to the Wyoming line. Throughout its length, most of its summit area is a relatively flat upland, above which the higher peaks of the range, such as Long's Peak and Pike's Peak, rise to elevations greater than 14,000 feet. At the southern end of the range the flat upland has an elevation of approximately 10,000 feet, and this portion has been dissected by stream erosion to form the hills which rise above the Royal Gorge Plateau. The elevation of the plateau is approximately 6,600 feet; the Gorge itself has been cut more than 1,000 feet below the plateau.
To illustrate these facts and figures more clearly, suppose that a passenger got off the railroad at the bottom of the Royal Gorge and started to walk to the summit of Pike's Peak. Leaving the Arkansas River, at an elevation of about 5,550 feet, he would have to clamber more than 1,000 feet up the steep canyon wall, and when he reached the rim he would be only 600 feet away from his starting point, measured horizontally. From the rim he would take off across the flat Royal Gorge Plateau to the northeast, at an elevation of about 6,600 feet, for nearly 3 miles, where he would enter the hilly tract resulting from the dissection of the southern end of the Front Range upland. After crossing the hilly area for about 15 miles, he would arrive at Cripple Creek, built on the upland at an elevation of nearly 10,000 feet. Leaving Cripple Creek, he would cross the upland surface of the Front Range for about 5 miles before beginning the steep ascent to the summit of Pike's Peak. In the next 4 miles of his journey, he would climb more than 4,000 feet to the summit of Pike's Peak at 14,110 feet above the sea.
The history of the Front Range has been investigated more thoroughly than that of any other range in the Rockies. Details have been added to this history by many workers since the masterly study by Davis 30 years ago, but the sequence of events is still recognized to be as he described them. Davis used the Front Range as an example of a two-cycle mountain range; this means that it has passed through one complete cycle of uplift and erosion, and is now in its second cycle, having been reelevated so as to revive the streams and put them back to work cutting canyons into the uplifed mass. The second cycle is sometimes called the canyon cycle.
The first uplift occurred during the folding of the Rockies, at the end of the Cretaceous period about 60,000,000 years ago. Streams carved the uplifted area into rugged ridges and valleys, and aided by weathering, eventually removed most of the land above the level of the streams, leaving a gently rolling surface. A few hills stood above this flat surface as much as 4,000 feet. Then a second uplift raised this face to its present elevation of 10,000 feet, probably during the Pliocene period about 10,000,000 years ago. Since that time, streams have been working away at their appointed task of removing everything above stream level, although most of them have not progressed beyond the initial stage, which is the cutting of canyons. The foregoing summary of the history of the Front Range provides a background for the history of the Arkansas River and the Royal Gorge.
Prior to the second uplift, the Arkansas River was a major stream flowing in approximately its present location across the old erosion surface worn flat during the first cycle. It carried a large amount of water received from numerous tributaries. After the second uplift, which revived the drainage of the Rocky Mountain region, this large volume of water enabled the ancient Arkansas River to transport a heavy load of sand and gravel. Then, due to this combination of large volume and heavy load, it cut into its bed more effectively than did its tributaries, and rapidly passed through the canyon-cutting stage, after which it began to widen its valley. That is, it soon sawed its channel downward to the lowest point and the gentlest gradient at which a stream of that size could carry such a load, and then, by undercutting its banks as it swung from side to side in wide bends, it slowly ate away the walls of the canyon until a wide valley floor was established. This process continued until the valley floor, consisting of granite and gneiss and veneered with alluvium, was 5 or 6 miles wide. Upstream from the area of hard rocks, the valley was wider, and downstream in the soft rocks of the Great Plains, it was wider still.
This broad-valley, gentle-gradient condition would characterize the valley of the Arkansas River today, were it not for the occurrence of one more event of great importance - another uplift. This uplift began just before the Glacial period, possibly 1,000,000 years ago. Its effect was apparently local, since the entire Rocky Mountain region was not involved, as in the first and second uplifts. This third movement raised the area now occupied by the Royal Gorge Plateau along an axis which lay directly athwart the course of the Arkansas River. The adjoining ranges to the north and south may have participated, but this is not certain. The rise of the old valley floor was probably intermittent, and fairly rapid, but slow enough for the downcutting of the river to keep pace with the uplift. By grinding away its bed, with the sand and gravel it carried, the river sawed a deep trench into the rising mass of granite. Slow as this process must have been according to human standards, it was much faster than the action of weathering by which canyons are normally widened. This trench is the Royal Gorge.
In short, the Royal Gorge was caused by the deep intrenchment of the Arkansas River into its rising valley floor so recently that weathering has not had time to widen the trench to normal canyon proportions. The complete history of the Arkansas River may be summarized into three stages:
1. Uplift of the Rockies at the end of the Cretaceous period, followed by stream erosion to a flat surface during the early Tertiary. The course of the Arkansas River was established at this time.
2. Reelevation of the Rocky Mountain region during the Pliocene period, followed by canyon-cutting. The Arkansas River passed through the canyon-cutting stage and developed a valley floor several miles wide.
3. Local uplift of the old valley floor in the Royal Gorge area began just before the Glacial period. Intrenchment of the Arkansas River into the rising mass to form the Royal Gorge.
Most of the many people who see the Royal Gorge each year probably do not realize that it has had a complex history, but all are impressed by its depth and scenic grandeur. The construction in recent years of a suspension bridge across the Gorge from rim to rim, and an inclined railway from the rim to the railroad platform at the bottom has increased the feasibility of seeing and appreciating the Gorge from all possible angles.
Many of the visitors have been passengers on the Denver and Rio Grande Western Railroad. This railroad, after overcoming many knotty engineering problems and legal difficulties, finally completed the water level railway route through the Rockies in 1880, by following the Arkansas River through the Royal Gorge. Officials of the railroad estimate that 7,800,000 people have passed through the Gorge during the 60-year period since trains started operating over this route. A large number of people visit the rim of the Gorge by automobile, as a side trip from U. S. Highway 50, about 10 miles from Canon City.
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