THE PHYSICAL GEOLOGY OF THE GRAND CAñON DISTRICT. By Clarence E. Dutton.
The idea of a terrace is not so typically represented in the Permian as it is in the superior formations. In many parts of the great stairway it clearly forms the lowest step; in others it forms one cliff with the Trias; in still others it is beveled off and covered with alluvium. On the whole it is more frequently presented as a distinct terrace. There is another qualification which requires some mention, because when we refer to the geological map to study the surface distribution of the strata, we should find some anomalies unless the point referred to were duly explained.
Wherever we encounter a cliff which discloses the upper Permian beds we find at the summit of the escarpment a band of pale-brown sandstone of very coarse texture, often becoming a conglomerate. Its thickness is usually from 40 to 75 feet. In a few places it is wanting from its proper horizon, and in some others its thickness becomes more than 100 feet. But on the whole it is a remarkably persistent bed, and its persistence is all the more striking when we consider the coarseness of its texture; for no beds are so variable as the coarse ones. This member has been named by Powell the Shin-á-rump Conglomerate. The name Shinarump he also applies provisionally to a large group of beds in which the conglomerate is included.* For several years it was thought very probable that these beds were a part of the Triassic system, though no positive proof could be cited to sustain that presumption. In the summer of 1879 Mr. C. D. Walcott, of this survey, at length found some limestone bands near the base of Powell's Shinarump, which seem to establish pretty conclusively their Permian age. But the fossils so far discovered have only a small vertical range, and lie near the base of the group. Above them are many hundred feet of beds which yield no fossils at all. While some of them are unquestionably Permian, it still remains to find the horizon where the Permian ends and the Trias begins. The Trias is as destitute of fossils as the Permian, excepting, however, some which are useless for determining age. In cases like these the geologist finds himself in trouble. He is quite sure that he has beds of two distinct ages; and he must, for purposes of discussion, separate them somehow; if not by a natural and unmistakable dividing horizon, then by an arbitrary and provisional one, subject to amendment by future research. But he must look very carefully for a natural horizon of separation. His course of procedure would be somewhat a follows. Starting, for instance, with those strata which he was sure were Triassic, he would examine the beds downwards and finding to fossils would pay attention to their lithological characters. Finding no marked difference in the beds, and finding a strict parallelism or “conformity“ in the several members, he would infer that they were deposited under conditions which were substantially identical throughout the period of deposition. But if he at length reached a stratum of very different character, say, for instance, after passing down through a great series of sandstones and shales, he came to a heavy mass of limestone or a bulky conglomerate, he would have found at last a “break“ in the continuity or homogeneity of the group. Here, at last, is something which he can use. It may or it may not be synchronous with the dividing horizon used in Russia, England, or Kansas, but it is at all events not far from it; and it is something palpable, distinct, and recognizable by those who come after him. In this way Mr. Walcott [U. S. GEOLOGICAL SURVEY. PERMIAN BUTTE NEAR KANAB. ANNUAL REPORT 1881. PL. XXI.] seized upon the Shinarump conglomerate as a divisional stratum between the Trias and Permian. But another perplexing question arose. To which of the two series should the conglomerate itself be assigned? And the question is not at first an easy one to answer. Immediately above it is a series of sandy shales such as beggar description on account of their gorgeous colors. Immediately below it is another series of sandy shales so similar to the one above that we never know which of the two we are looking at unless the conglomerate is in sight as a “bench-mark.“ Mr. Walcott settled the question (provisionally, of course) in the following way. The summit of the lower series shows in many places that immediately after it was deposited it was slightly eroded, and the contact of the conglomerate shows an “unconformity by erosion.“ The contact of the conglomerate with the upper series shows no such unconformity. Now, an unconformity means to the geologist a break in the continuity of deposition, and in the absence of reasons to the contrary, and with no better divisional criterion at hand, it may be used to separate two series of beds. He therefore assigned the conglomerate to the Trias, and the beds below he placed in the Permian.
Mr. Walcott's conclusion is no doubt the best which can be reached with our present knowledge, but it is very inconvenient and awkward to the geologist who is required to map the distribution of the strata and their topographical features. In all of the other formations each group forms its own terrace of series of terraces. As we descend them we find ourselves, when we reach the foot of the Eocene cliff, upon the summit of the Cretaceous. Reaching the foot of the Cretaceous cliffs, or slopes, we are upon the broad expanse of the Jurassic platform. Descending the Jurassic, we find the Trias coming out from the base of the Jurassic Cliffs; but when we descend the Vermilion Cliffs, we have not reached the Permian. The Trias is still beneath us, pushing out its basal member, the Shinarump conglomerate, clear to the crest-line of the Permian wall. In the Jurassic terrace and in its terminal cliff we find none but Jurassic strata. Similarly, also, on the cliffs and terrace platforms of the Cretaceous and Eocene; but the Permian terrace is everywhere sheeted over with a solitary stratum of the Trias. Somehow we cannot help thinking that the conglomerate has no business there, and that it ought to have been cut off at the base of the Vermilion Cliffs, or else it ought to be relegated to the Permian. In delineating the distribution of the formations by means of colors on the map, the ordinary practice would require us to extend the Trias to the brink of the Permian Cliffs, for in such delineations we only profess to show the surface exposures of the several groups; but this would confound the Permian terrace with the Trias, and obliterate the individuality of the former, whereas in the topography both are as distinct as land and water. To preserve this distinction the Shinarump is denoted by a special modification of the color, which is to be interpreted as meaning an arbitrary subdivision of the Trias.
The Permian beds consist of sandy clay-shales in very many thin beds and a few thin beds of impure limestone. They are very striking on account of their dense, rich colors, which are sometimes also wonderfully delicate. They are belted in a surprising way. Horizontal streaks of chocolate, purple and red-brown are interstratified with violet, lavender, and white. Perhaps the richest tone is the red-brown, which is almost exactly like the color of the fumes of nitrous acid. Lower in the series are layers of a very peculiar shade of Indian red, alternating with grayish white. In the lower Trias and Permian the colors reach their climax. Surely no other region in the world, of which I have any knowledge, can exhibit anything comparable to it. Wonderfully even is the bedding. Thin layers may be traced for miles without showing any variation of thickness, color, or texture. In the escarpments the weathering has etched out the harder layers, leaving a line of shadow in the places of the softer layers, and this greatly emphasizes the stratification and gives it finer detail.
The Permian series is of considerable magnitude. In the western portion of the district its thickness is greater than elsewhere, reaching, probably, 1,400 feet, and possibly 1,600 feet, while in the vicinity of Kanab it is less than 1,000 feet. It gives rise to terminal cliffs, which in the northern part of the Uinkaret are from 800 to 1,000 feet high, while around Kanab their height seldom exceeds 500 or 600, and is often less than 300. But what they lack in magnitude they make up in refinement and beauty of detail and in sumptuous color. It is in the Permian that we find the most remarkable buttes. They are never large, but their resemblances to human architecture or works of design are often amazing. Very few Permian buttes are found in the Grand Cañon district, but further eastward, especially in the neighborhood of the junction of the Grand and Green Rivers, they are innumerable and of such definiteness that the geologist feels as if he were taxing the credulity of his hearers when he asks them to believe that they are the works of nature alone, and not of some race of Titans.
At the foot of the Permian cliffs begins the Carboniferous platform of the interior region of the Grand Cañon district. It stretches southward without visible bound, an almost featureless plain. It terminates beyond the San Francisco Mountains in the Aubrey Cliffs.
[U. S. GEOLOGICAL SURVEY. LAND OF THE STANDING ROCKS. ANNUAL REPORT 1881. PL. XXII.]