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regions. And they belong, moreover, to very different geological periods, some being of comparatively recent formation, while others date back to ages incalculably remote.
One of the most interesting and instructive regions of the kind is the remarkable plateau of the Grand Cafion district of Arizona and Utah. This plateau occupies an area of between 13,000 and 16,000 square miles, and is traversed by the Colorado River of the West, which follows a tortuous course towards west-south-west through a succession of profound ravines or cafions. The strata visible at the surface are approximately horizontal, and attain a thickness of many thousand feet. It may be said, therefore, that the prevalent plain-like character of the surface is an expression of the underground structure—that, in short, the Grand Cafion district is a plateau of accumulation. This, in a broad sense, is doubtless true; but when we come to examine the configuration and structure of the district more closely, we find reason to conclude that the original surface has been greatly modified by denudation. We learn, moreover, that the strata are not quite horizontal. The inclination is certainly gentle, but a slope of only one degree, if continued for a few miles, will result in a fall of several hundred feet. If a surface be inclined at an angle of one degree, then for every eleven miles of distance it will lose 1000 feet of elevation. Now, in the Grand Cafion district the general inclination of the strata is towards north and
north-east, while the slope of the surface is in the opposite direction. Thus it comes to pass that strata which lie open to the day upon the south-west margin of the plateau gradually descend towards north and north-east, until, in a distance of 120 miles or thereabouts, they lie buried at a depth of several thousand feet. It is not quite true, therefore, that in the Grand Cafion district the form of the ground is an exact expression of the underground structure. On the contrary, the average slope of the surface is against and not with the average dip of the strata. Nevertheless, it cannot be doubted that the general configuration of the region—its plateau-character—has, in the first place, been determined by the approximately horizontal disposition of the strata, and that it may be rightly termed a plateau of accumulation. A glance at the geological history of the district will show how far the plateau-character is original, and to what extent and by what means it has been subsequently modified.
Reference has been made to the fact that the rocks composing the plateau are chiefly of aqueous origin, and approximately horizontal.
Here and there in the bottoms of deep cafions we get peeps at another set of rocks that form the pavement upon which the horizontal strata repose. With the history of these older underlying rocks we need not concern ourselves further than to note that they are of Pre-Cambrian and early Palaeozoic age. It is with the superincumbent masses that we have to deal. Those attain a vast thickness, and range in age from Carboniferous down to Eocene times. At the beginning of the Carboniferous Period the district formed a portion of the sea-floor, and similar marine conditions obtained during the deposition of all the succeeding systems of strata down to the close of Cretaceous times. Throughout all that long succession of ages the sea would appear never to have been deep, although during the early part of the Carboniferous Period it was probably deeper than in subsequent times. When we consider that the marine sediments reach a united thickness of over 15,000 feet it may at first sight appear impossible that so thick a mass of materials could accumulate in a shallow sea. The explanation, however, is simple enough—subsidence kept pace with sedimentation. Slowly and gradually the bed of the sea went down—slowly and gradually it was silted up by sediments derived from the adjacent land.
At last, towards the close of Cretaceous times, certain new crustal movements began—elevation ensued, and the sea finally retired from the district. An extensive lake now occupied the site of the plateaucountry for a prolonged period, during which sediments were washed down as before from the neighbouring uplands, and gathered over the level surface of the Cretaceous marine strata until they had reached a thickness of 5000 feet or more. As these deposits appear likewise to have been laid down chiefly in shallow water, it may be inferred that the slow subsidence of the area which accompanied the accumulation of the underlying marine strata was repeated during the lacustrine period.
The whole region, it will be understood, had been elevated at the close of Cretaceous times; but the movement was differential, the greatest rise having been experienced by the uplands surrounding the lacustrine basin. Eventually the river, escaping over the lower lip of that basin, deepened the outlet and succeeded in draining the lake, which was then replaced by an alluvial plain. At this stage the nearly level surface of the drained lake-bed sloped gently from east-north-east to west-south-west, and thus determined the direction of the primeval Colorado River and its larger tributaries, which headed then as now in the high lands overlooking the basin. When these waters first began to wander across the alluvial plain, the slope of the surface and the inclination of the underlying sedimentary strata doubtless coincided. But these conditions were ere long disturbed by successive movements of elevation, and the prevalent horizontality of the strata was modified. Here and there the beds were bent or flexed, and traversed by great fractures along which the strata became vertically displaced for thousands of feet. Yet, strange to say, none of these earth-movements succeeded in deflecting the main drainage of the district. The Colorado and its chief affluents continued to flow in the courses they had attained at the final disappearance of the great lake. It is clear, therefore, that the bending and dislocation of the strata must have proceeded very slowly, for the rivers were able to cut their way across both flexures and faults as fast as these showed at the surface.
Before the great lake had vanished some portions of the older marine strata had been elevated, and formed part of the land surrounding the basin. Here they were for a long period exposed to the erosive action of epigene agents, and must have suffered much loss. But all such denudation sinks into insignificance when we consider the magnitude of the erosion which has taken place since the great lake dried up. Fortunately, owing to the simple geological structure of the Grand Canon district, the amount of that erosion can be readily estimated. According