Geology of the Grand Canyon: The Trias.


Kanab village is situated under the eaves of the Vermillion Cliffs, in the jaws of the cañon of Kanab Creek. It has for several years been the base of operations of the surveying parties working in the Grand Cañon district, and is well located for the purpose. After due preparation, we may leave the village, proceeding about twenty miles southwestward to the southernmost promontory of the Triassic escarpment. Here is Pipe Spring, famous in this far-off region as a watering place. The reader would do well to find the locality on the map, for it is a notable point. The Vermilion Cliffs here change their trend to the northwestward, and we shall presently follow them to admire their beauty and magnitude; but before doing so it is well to take a brief view of their geological relations.

The Trias is in most places separated from the Jura by a purely provisional horizon which marks a change in the lithological aspect of the strata, and in the grouping and habit of the series. Sometimes the passage from one to the other is obscured, but more frequently it is abrupt. The Jurassic sandstone is without a likeness in any other formation, and the sandstones of the Trias can ordinarily, be distinguished from it miles away. One of the most conspicuous distinctions is the color, and it is a never-failing distinction. The Jurassic is white; the Trias is flaming red. Equally conspicuous is the difference in bedding and in the architecture. The Jura is a solid indivisible mass of 800 to 1,000 feet in thickness; the Trias is composed of a very great number of beds, most of which are only a few feet in thickness. One bed, however, attains vast proportions. The majority of the layers are common sandstones, and they predominate most in the upper portion of the series. In the middle part the sandstones still predominate but are individually thinner, and are more often separated by shaly layers and by bands of gypsum. In the lower portions, sandy and argillaceous shales of [U. S. GEOLOGICAL SURVEY. VERMILION CLIFFS AT KANAB. TRIASSIC. ANNUAL REPORT 1881. PL. XVIII.] wonderful colors predominate. Lime is found in these rocks, and in notable quantity, but it is almost always in the form of gypsum or selenite. No fossils in these parts have yet been found which are of paleontological value; but fish-scales, and fragments of bony scutes are sometimes obtained, which are useless for the purposes of the geologist. In the lower shales we find a great abundance of fossil trees completely silicified and several bulky layers are composed very largely of their fragments.

The Trias makes its appearance upon the outermost western flank of the Markágunt, a little north of the Mormon town Cedar, rising by a fault out of the valley alluvium. With a constantly expanding exposure, it extends southward along the west flank of the Markágunt and along the upthrow of the Hurricane fault, until the whole of its mass comes to the surface; the broadening out into a wide terrace, it sweeps around the southwestern limit of the Colob and over into the valley of the Virgen, where it breaks into cliffs, temples, towers, and buttes of ineffable splendor and beauty. Thence, with a still wider terrace, bounded by a giant wall, it stretches to the southeast as far as Pipe Spring. Here is its southernmost promontory, from which its front trends away northeast and east in proportions diminished somewhat, but still imposing, as far as the Paria River. Thus far the distance is more than 120 miles, in which the sinuosities of the front are not reckoned. Throughout this entire sweep it presents to the southward a majestic wall richly sculptured and blazing with gorgeous colors. The cliff line is very tortuous, advancing in promontories, with intervening bays and broad cañon valleys setting far back into the terrace, and resembling a long stretch of coast-line gashed with fiords. Perhaps also the contour of a maple-leaf may be a suggestive analogy. The altitudes of the cliffs are greatest in their western portions,for there we find greater thickness of strata. They often exceed 2,000 feet, while in the portion extending from Pipe Spring to the Paria the altitude ranges from 1,000 to 1,400 feet.