Geology: The Science of the Earth's Crust. William J. Miller

Canyon, Arizona. (From Darton’s “Story of the Grand Canyon.”)

      Strikingly narrow and deep valleys, called gorges and canyons, are rather exceptional features of stream action. Most wonderful of all features of this kind is the Grand Canyon of the Colorado River in Arizona. In fact, this canyon takes high rank among the most remarkable works of nature. The canyon is over 200 miles long, from 4,000 to 6,000 feet deep, and from 8 to 15 miles wide. Contrary to popular opinion, this mighty canyon is not a result of some violent process, such as volcanic action, or the sudden sinking of part of the earth’s crust. Nor is it the result of the scouring action of a great glacier. It is simply a result of the operation of the ordinary processes of erosion where the conditions have been exceptionally favorable. Some of the favorable conditions have been, and are, a large volume of very swift water (Colorado River) continually charged with an abundance of rock fragments for the work of corrasion, and a great thickness of rock which the river must cut through before reaching base-level. Aridity of climate also tends to preserve the canyon form. The whole work has been accomplished in very late geological time, and the tremendous volume of rock which has been weathered and eroded to produce the canyon has all been carried away by the Colorado River and accumulated in the great delta deposit near where the river empties into the Gulf of California. Even now the canyon is growing deeper and wider because the very active Colorado River is still from 2,000 to 3,000 feet above sea level. Standing on the southern rim near Grand Canyon station at an altitude » 41 «

       » 42 « of nearly 7,000 feet, and looking down into the canyon, one beholds a vast maze of side canyons, high, vertical rock walls which follow very sinuous courses, giving rise to a steplike topography, and countless rock pinnacles, towers, and mesas often of mountain-like proportions. The side canyons are the result of erosion by tributaries to the main river which have gradually developed and worked headward as the main river has cut down. The mountain-like sculptured forms which rise out of the canyon are erosion remnants, or, in other words, masses of rock which were more favorably situated against erosion by either the main river or any of its tributaries. All of the rocks of the broader, main portions of the canyon are strata of Paleozoic age, arranged as a vast pile of almost horizontal layers, including sandstone, limestone, and shale. Some of these layers, being distinctly more resistant than others, stand out in the canyon wall in the form of conspicuous cliffs, in some cases hundreds of feet high. The very striking color bands (mostly light gray, red, and greenish gray), which may be traced in and out along the canyon sides, represent the outcropping edges of variously colored rock layers. Far down in the canyon lies the steep-sided, V-shaped inner gorge, or canyon which is fully 1,000 feet deep. The rocks are there not ordinary strata, but rather metamorphic and igneous rocks, mostly dark gray, not in layers, and about uniformly resistant to erosion. There is reason to believe that this inner gorge has developed mainly since a distinct renewed uplift (rejuvenation) of the Colorado Plateau after the river began its canyon cutting. The narrow, steep-sided inner gorge may thus be readily accounted for and the general lack of steplike forms on its sides is due to essential uniformity of the rock material as regards resistance to erosion.

      Fig. 4.—Profile and structure section across the line A-A in Fig 3. Length of section 10 miles, vertical scale not exaggerated. The main relief features, and the relations of the rocks below the surface are shown. The granite and gneiss are of Archeozoic age, and the overlying nearly horizontal strata are of Paleozoic age. (After Darton, U. S. Geological Survey.)

      The wonderful King’s River Canyon of the southern Sierras in California is remarkable for its combined narrowness and depth. It is a steep V-shaped canyon whose maximum depth is 6,900 feet, carved out in mostly solid granite by the action of weathering and running water. Some idea of the vast antiquity of the earth may be gleaned from the fact that this tremendously deep canyon has been produced by erosion in one of the most resistant of all known rocks in very late geologic time! Conditions favorable for cutting this canyon have been volume and swiftness of water and a liberal supply of grinding tools.

      Among the many other great canyons of the western United States brief mention may be made of the Grand Canyon of the Yellowstone River in the National Park. The plateau into which the river has cut its steep-sided, narrow, V-shaped canyon, with a maximum depth of 1,200 feet, has been geologically recently built up by outpourings of vast sheets of lava. The large volume of very swift water, aided by decomposition and weakening of the ordinarily very hard rock by the action of the hot springs, has been able to carve out this deep canyon practically within the last period of earth history. The deepening process is still vigorously in progress. The wonderful coloring of the rock, mostly in tones of yellow and brown, is due to the hydrated iron oxides developed during the decay of the iron-bearing minerals of the lava, the chemical action having been greatly aided by the action of the hot waters. (See Plate 2.)

      In regard to its origin, the marvelous Yosemite Valley, or canyon, falls in a somewhat different category, and it is discussed beyond in connection with the work of ice. Suffice it to say here that running water has been a very important factor in its origin.

      In New York and New England there are many gorges which have developed by the action of running water since the Great Ice Age. Famous among these are Ausable Chasm and Watkins Glen of New York, and the Flume in the White Mountains of New Hampshire.

      Fig. 5.—Sketch map showing the retreat of the crest of Niagara Falls from 1842 to 1905, based upon actual surveys. The retreat of the inner part of the Horseshoe Fall was more than 300 feet. (Modified by the author after Gilbert, U. S. Geological Survey.)

      Before leaving our discussion of the work of running water, we should briefly consider waterfalls. True waterfalls originate in a number of ways. Most common of all is what may be termed the “Niagara type” of waterfall. Niagara Falls merit more than passing mention not only because of their scenic grandeur, but also because of the unusual number of geologic principles which their origin and history so clearly illustrate. Niagara Falls are divided into two main portions, the Canadian, or so-called “Horseshoe Fall,” and the “American Fall,” separated by a large island. The crest of the American Fall is about 1,000 feet long and nearly straight, while the crest of the Canadian Fall is notably curved inward upstream, and it is about 3,000 feet long. The height of the Falls is 167 feet. Downstream from the Falls there is a very steep-sided gorge about 200 feet deep and seven miles long. The exposed rocks of the region are nearly horizontal layers of limestone underlain with shales. Relatively more resistant limestone forms the crest of the falls, and directly underneath are the much weaker shales. Herein lies the principle of this type of waterfall because, due to weathering and the swirling action of the water, the weaker underlying rocks erode faster, thus causing the overlying rock to overhang so that from time to time blocks of it already more or less separated by cracks (joints), fall down and are mostly carried away in the swift current. Thus the waterfall maintains itself while it steadily retreats upstream. Careful estimates based upon observations made between 1827 and 1905 show that the Canadian Fall retreated at the rate of from three to five feet per year, while the American Fall retreated during the same time at the rate of only several inches per year. It has been well established that Niagara Falls came into existence soon after the ice of the great Ice Age had retreated from the district. The falls started by plunging over a limestone escarpment, situated at what is now the mouth of the gorge seven miles downstream from the present falls. If we consider the rate of recession of the falls to have been always five feet per year, the length of time required to cut the gorge would be something over 7,000 years. But the problem is not so simple, because we know that, at the time of, or shortly after, the beginning of the falls, the upper Great Lakes drained farther north and not over the falls; and that this continued for a considerable, though unknown, length of time. During this interval the volume of water in Niagara River was notably diminished, and hence the recession of the falls must have been slower. On the other hand, judging by the width of the gorge, the length of the crest of the falls has generally been considerably less than at present, which in turn means greater