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Show .. 210 EXPLORATION OF THE O.ARONS OF THE OOLOitADO. evidences that the lines of cliffs themselves have been carried back for great distances as cliffs by undermining, which is a process carrieu on only in an arid region. The evidence is of this character. I have stated that tho drainage of the inclined plateaus is usually from the brink of the cliffs backward; i. e., the water falling on the plateau does not find its way immedintely over the cliffs, but runs from the very brink or edge of the plateau back toward the middle or farther sido, which is usually found against the foot of another line of cliffs, and here the waters are turned toward some greater channel, which runs ag~inst tho dip and cuts through the cliffs. Now the water-ways at the heads of these streams that have their sources near the brink of the cliffs would always bo small, shallow, and ramifying into many minute branches if the line of cliffs were a fixed or immovable line, but we often find that the cliffs have been carried back by tho undermining process until all these minute ramifications have been cnt off; and we find canons opening on the faces of the cliffs, the waters of which run backward as above <.lescribed. Let us suppose that we have a line of cliffs with an escarpment facing the south. The rain, falling on the escarpment and in the region south of the cliffs, would run toward the south or alor1g the foot of the cliffs until it reached some more important water channel; the rain falling on the plateau, from the brink of tho cliffs backward, would run toward the north and the waters falling on this upper region would excavate channels for the' mseh·es, and, undor proper conditions, cafions would be cut. As the cliffs are undermined and this line carried back into the plateau, the area with a southern drainage would be increased, the area with a northern drainage correspondingly diminished, and, when the process had continued for a suflicient length of time, wo would find the southern edgo of the plateau carried away by this undermining process, until all the heads of tho streams were cut off and until the line had reached the canons. Gradually, during the progress of erosion, tho excavation of the bottom of tho canons would cease, as the supply of water running through them ~ould be ~ut off, and such canons would have to be considered as comparatlvcly anctent. Such facts are frequently observed in this canon and cliff country. From such considerations, it seems that we may safely conclude that GILBERT ON THE GLAOIAL EPOCH IN THE "GREAT BASIN." 211 the cliff topography has prevailed in that region for a long time. There aro evidences also that there were canons here before the present canons were carved. The facts in relation to this matter can be better stated when we come to discuss the geology of the region. Mr. G. K. Gilbert, a geologist of Lieutenant Wheeler's corps, in a paper communicated to the Philosophical Society of Washington, in 1873, deduced a similar conclusion from an independent series of facts observed in Western Utah. The basin of Great Salt Lake, a portion of what Fremont designated the "Great Basin,'' has now so dry a climate that its waters gather in its lowest parts and evaporate and have no outlet to the sea. In a former period, however, there was more rain, the valley was filled with water to its brim, and in place of the Salt Lake Desert, there was a broad and deep fresh lake, discharging its surplus into the Columbia River. The epoch of this lake Mr. Gilbert finds reason to consider identical with the Glar.ial Epoch,· and it was of limited duration. Among its vestiges are deposits of fossiliferous marl, which are conspicuously contrasted with the gravels and sand that now slowly accumulate in the same region, borne by the intermittent streams that descend from the mountains. Where the beds are superposed, the marls testify to a moist climate and the gravels to a climate so dry that the basin was never filled with water. But above tho marls are found only scattered and thin deposits of gravel, while below them the gravel beds are omnipresent and of great depth, and hence it was reasoned that the arid period that preceded the Glacial Epoch was many times longer than that which has followed it. , Even during the Glacial Epoch, Mr. Gilbert considers that "the Atlantic slope, and the region of the Great Basin, were contrasted in climate, just as now. The general causes that covered tho humid east with a mantle of ice, sufficed, in the arid west, only to flood the valleys with fresh water, and send a few ice streams down the highest mountain gorges."* RECORDS OF MORE ANCIENT LANDS. rrhe summit of the I{aibab Plateau is more than six thousand feet above the river, and I have already mentioned that tho summit of the plateau is also the summit of rocks of Carboniferous Ag-e. Thoso bods are about three • Bullotin Phil. Soc., Wnshingtou, 46t.h meetiug, April 26, 187:1. |
