Geology of the Grand Canyon: The Eocene.
The foreground of the picture is full of strength and animation. At our feet is the brink of a precipice where the profiles descend 800 feet upon rugged sloped which shelve sway downwards and mingle with the inequalities of a broad platform deeply indented with picturesque valleys. The cliff on which we stand is of marvelous sculpture and color. The rains have carved out of it rows of square obelisks and pilasters of uniform pattern and dimensions, which decorate the front for many miles, giving the effect of a giantic colonnade from which the entablature has been removed or has fallen in ruins. The Plateau Country abounds in these close resemblances of natural carving to human architecture, and nowhere are these more conspicuous or more perfect than in the [U. S. GEOLOGICAL SURVEY. PINK CLIFFS-EOCENE-PAUNSAGUNT. ANNUAL REPORT 1881. PL. XV.] scarps which terminate the summits of the Markí gunt and Paunsí gunt Plateaus. Their color varies with the light and atmosphere. It is a pale red under ordinary lights, but as the sun sinks towards the horizon it deepens into a rich rose color, which is seen in no other rocks and is beautiful beyond description. These cliffs are of lower Eocene age, consisting of lake marls very uniformly bedded. At the base of this series the beds are coarser, and contain well-marked, brackish-water fossils; but as we ascend to the higher beds we find the great mass of the Eocene to consist of fresh-water deposits.
These beds are identical in age with the lower divisions of the Eocene which are seen in great volume both north and south of the Uinta Mountains, in the basins of the Green River, of Bitter Creek, and of the White and Uinta Rivers. Their geological relations and associations, too, are quite the same, for the same lake bottom received the deposits of the southern Uinta slopes and those of the Markí gunt. Those of the Green River basin north of the Uintas appear to have accumulated in a separate lake basin communicating with the one which submerged the southern Plateau province. The interval separating the Markí gunt from the Uinta region is 250 miles and more, but the lower Eocene is continuous between them. It occupies a marginal belt sometimes narrow, but more frequently wide, which was once the locus of the northwestern portion and shore line of the great lake. The summits of the High Plateaus, wherever the volcanic masses are absent, disclose this formation, and its presence is decisively inferred beneath the lavas and their débris. A common bond between the two regions is also indicated by the physical conditions attending the deposition of these strata. The lower Eocene rests upon the underlying formations, conformably in some places, unconformably in others. Where conformity prevails, both the upper and lower series were at the time of deposition sensibly horizontal. But in many places the Cretaceous, prior to the deposition of the Eocene, was greatly disturbed and greatly eroded. And in general the base of the Eocene marks an epoch in he geological history of the country, in which an old order of vents was closing and a new order was making its advent. This revolution was the transition of the region from the oceanic condition to that of an estuary and lake, and subsequently to that of dry land. The Lower Eocene beds are brackish-water deposits in the basal members, while higher up they become fresh-water. The basal members are coarse and even conglomeratic in their texture, while the middle and higher ones are fine and marly. Thus is indicated the complete severence of the lake from the access of oceanic waters. Both in the Uinta district and throughout the High Plateaus these events are recorded in the same order and their meaning is the same in both.
The beds now found in the southern extremities of the High Plateaus represent less than half of the duration of Eocene time. No middle and no upper Eocene strata are found there. But as we go northward towards the Uintas we find later and later formations successively appearing until upon the flanks of the Uintas we find the whole Eocene series in enormous volume, exceeding perhaps 5,000 feet. Could these middle and Upper Eocene masses once have existed upon the southern portion of the High Plateaus and been swept away be erosion? There is strong evidence to the contrary.* The facts then, indicate that when the desiccation of the lake took place, the portion which first emerged was the southern and southwestern-or the Grand Cañon district; that its shore line gradually receded northward during middle and upper Eocene time, leaving dry land behind it; and the last remnant of the lake disappeared near the base of the Uintas. Wherever the physical geology and evolution of the Grand Cañon district touches the question of time, the earlier date of its emergence than that of other portions of the Plateau Province appears-sometimes dimly, sometimes forcibly.
The principal mass of the Eocene terminates at the “Pink Cliffs,“ as they are called, in the southern margins of the Markágunt and Paunságunt Plateaus. There are a few outliers beyond. Around the base of the Pine Valley Mountains to the southwest, and beyond them in the same direction, some remnants have escaped destruction. But this part of the country has not been sufficiently explored to indicate more than the bare fact of their existence. Far to the eastward a single outlier stands upon the summit of the great Kaiparowits Plateau, forming the apex of Kaiparowits Peak. But generally speaking, the Eocene is wholly absent, so far as known, from the country south of the terraces.
|1||* This evidence will be fully discussed in my monograph on the Geology of the Grand Cañon district.-C. E. D.|