Although the history recorded in the rocks forming the High plateaus is one of fascinating geological interest, the easier and more obvious lesson that the region has to offer, more especially to the geographer, has been engraved and etched on its surface by streams and wind-blown sand.
On the High plateaus the rainfall is comparatively small, and the streams originating there mostly ephemeral. But on the mountains to the east and north the precipitation is more abundant and rivers are formed which flow across the plateau region. The Green and Grand Rivers, fed by many tributaries, unite to form the Colorado, which flows southwestward for some 700 miles and discharges its muddy waters into the Gulf of Lower California. This great river
year by year and century after century has deepened its channel through the plateau region where the rainfall is small, more rapidly than the general surface has been lowered by erosion. The main conditions are a broad area of nearly horizontal rocks, raised high above the sea or above the base level of erosion, and an arid climate; crossing this region is the ever-flowing river, which, acting like an endless saw, cuts deeper and deeper into the blocks of the earth's crust which have been raised athwart its course. Resulting from these conditions is a mighty trench or cañon, which is by far the most magnificent of its kind in North America, if not in the world. Not only has the main river sunken its channel into the earth to a depth of more than a mile throughout a large portion of its course, but each tributary stream has been engaged for a long period of time in a similar task. Although most of the streams originating on the surface of the plateaus are ephemeral, they work rapidly when the occasional heavy rains flood their channels. This deepening of the stream channels, while their borders and the intervening portions of the plateau surface suffered but comparatively slight erosion, has produced a wonderful system of deep steep-sided trenches in the borders of which the edges of the dissected rocks are exposed in nearly vertical precipices.
Fig. 19.—Grand Cañon of the Colorado River. After W. H. Holmes.
Aside from the lessons of interest to the geologist and geographer so plainly engraved on the surfaces of the plateaus crossed by the Colorado, the region has a wonderful fascination for the purely æsthetic feelings more or less latent in every human breast. To one traversing the open pine forests, in places clothing the plateaus and inclosing many grassy glades and flower-decked parks, in which timid deer may frequently be seen feeding in the early morning, and emerging on the brink of the Grand Cañon, the scene that meets the eye is marvellous beyond all description. C. E. Dutton, to whom we owe some of the most graphic and inspiring descriptions of natural scenery ever written, states that those who have long and carefully studied the Grand Cañon of the Colorado do not hesitate for a moment in pronouncing it the most sublime of all earthly
spectacles. "If its sublimity," writes Dutton, "consisted only in its dimensions, it could be sufficiently set forth in a single sentence. It is more than 200 miles long, from 5 to 12 miles wide, and from 5,000 to 6,000 feet deep. But it is not magnitude alone that gives this marvellous cañon its prominence; it is the gorgeous and varied colouring of its mighty walls, the endless details in the sculpturing of its battlements and towers, the ever-changing atmospheric effects of its profound depths, and the wonderful stimulus to the imagination with which it feeds the mind. Standing on the brink of the Grand Cañon, the prosaic search for causes and effects for a time at least must be laid aside, and give place to the emotions."
Wonderfully grand as are the scenes beheld in traversing this region of high plateaus, with its magnificent cliffs and profound cañons, one is constantly reminded that it is an arid land. The higher portions of the plateau, it is true, are in places forested, but over vast areas the rocks are bare. Water is everywhere scarce except in the bottom of the larger excavations. Thirsty, and perhaps perishing, the traveller, Tantalus like, looks down on the shining silvery threads of water in the cañons, hundreds and even thousands of feet below, but separated from them by impassable barriers. To the south the plateaus descend to the desolate valleys of southern Arizona, where strange gigantic cactus-plants and scattered clumps of thorny shrubs alone break the dreary monotony of the hot gravelly deserts. Agriculture is there impossible without irrigation, but where the life-giving waters can be utilized, as in the Gila Valley, marvellous productiveness follows.
THE GREAT BASIN