Here we liiul some twenty-five sijiure miles of sand iliines. of 

 .1 beautiful, sniotith. p.ile hrown color, some of them toppinj; six 

 lumdred feet in heif;ht. piled .ij;ainst the feel of the j;reat moun- 

 lams These dunes have a remarkable form, sharp-edj^ed alonn 

 their crests but often with strange double crests or perfect sub 

 sidiary crests on the windward side. They are naked but fi>r 

 some tiny patdies of two vivid green herbs- Lemon Weed 

 il'soriilt'ti lunci'oliitit) and Indian Rice-Grass (Orisopsis sp). These 

 dunes marcel about constantly dianging shape, and their move- 

 ments are said by those who live there and tend them (it is a 

 national monument) to look as if they are clawing at the foothills 

 and trying to climb them. They have swamped lar.ge tracts of 

 tree growth to considerable heights. 



The San Luis Valley was once the bed of a sea .Alter being 

 elevated to a height of some seven thousand feet, parallel cracks 

 appeared along what are now its sides, and while ii sank, great 

 blocks of territory to either side rose almost a mile high to form 

 the present mountains Then the south end of the valley was 

 blocked by an outpouring of volcanic material and it was vir- 

 tually sealed in and dried out. As a result, its fine surface soils 

 were whipped olT by the powerful prevailing winds from the 

 southwest. But these winds, while rushing up and topping the 

 Sangre de Cristos at most places, were sufficiently stalled above 

 this point to drop their loads of dust. Thus the dunes were 

 formed. 



THE Gl.ANTS CAUSEWAY 



At the north end of the Colorado Block you can trek out onto 

 one or another of a series of great mountainous fingers over- 

 looking the Wyoming Basin, or you can turn sharp left to one 

 of nature's grander achievements. The best route is down the 

 Yampa River valley, leaving the mighty Park Range on the east 

 and going down between the Elkhead Mountains to the north 

 and the White River Plateau to the left. This takes you through 

 a marvelous succession of massed spruce, then alpine meadows, 

 and then through pine forests and a lush park zone, down to a 

 lovely valley prairie in which are some remarkable volcanic 

 plugs towering up like black teeth. Following the river down 

 farther, you debouch onto an arid scrub-covered plain between 

 low gray hills on the left and a rampart of saw-toothed moun- 

 tains on the right. The latter soon begin to rise precipitately into 

 crazy uptilted ridges of bare rock of brilliant colors, one behind 

 the other. If you could climb onto these and scramble westward, 

 you would presently be confronted by one of the most terrifying 

 gorges to be found anywhere, at a point appropriately called 

 Split Mountain. Through this the Green River flows to join the 

 Yampa. Beyond lies a formidable oblong block of rugged wild 

 mountains known as the Uintas. which form a natural causeway 

 of gigantic dimensions leading to the western or Utah Block. 

 Hundreds of lakes nestle among the peaks of these mountains. 

 The Utah mountains reach from the southeast corner of Idaho 

 to the southern border of the state. They are formed of a high 

 steep spine facing the Great Basin, inside which parallel ranges 

 step down toward the Colorado Plateau, with several upland 

 plateaus between them, the most notable of which is the Wasatch. 

 Here are peaks over twelve thousand feet high, but on the whole 

 they are more subdued than the Colorado Block. They are clothed 

 in much the same types of forest but here the more southern 

 species of conifers make their presence obvious. Among them 

 are Rocky Mountain. Utah, and One-seed Juniper; some Douglas 

 Fir higher up; Alpine. Corkbark. and White F'r. Blue. Engle- 

 mann's. and White Spruce; and Bristle-Cone. Whitebark. Ponde- 



rosa. Lodgepole. and Pinon pines Many tanyonb have cut into 

 this bloi-k, the best known being Bryce Canyon in the south, 

 which is filled with weird pinnacle like formation* and 1» 

 •painted" a vivid combination of colors Thi.s and other canyons, 

 sudi as Zion. cut north into a great wall of rock known as the 

 White ClifTs. which faces south onto the Kanab Plateau. This 

 plateau rises slowly to the south and then pitches over the Ver- 

 milion Cliffs, after which it rises again steadily to the northern 

 rim of the Grand Canyon. This plain Is heavily forested to the 

 south; its eastern end is cut off by the barrier of the Kaibab 

 Plateau at eight thousand feet, its western end by what itrc 

 called the Hurricane Cliffs; and beyond this lie the narrow ridges 

 of the Virgin Mountains, which form another great natural 

 causeway leading from the Utah to the Arizona Block. This 

 would seem to be the logical place for the Colorado to have 

 diainod from the plateaus but instead it cut the Grand Canyon 



THE GREAT UNEXPLAINED 



Quite apart from being a world wonder and almost breath-taking 

 in its immensity, the Grand Canyon presents a great mystery. It 

 is four to five thousand feet deep from its more or less horizontal 

 rims (which are the two edges of a bisected plateau), triangular 

 in section, from four to eighteen miles wide at the top. and 217 

 miles long. It cuts through a large number of horizontal strata 

 of various ages and colors that give it much of its natural beauty. 

 Most of these are comparatively soft and some are hardly com- 

 pacted at all, but in its depths the river has carved down through 

 considerable thicknesses of extremely hard Pre-Cambrian rocks. 

 Now rivers cut deepest and fastest when they are running at 

 maximum speed and carrying an overload of silt, and they run 

 fastest of all when they are pitching down the steepest slopes. 

 When their seaward descent is gradual they do very little dig- 

 ging, and if they then hit really hard rock in any quantity they 

 pond behind it and cascade over it in the form of falls or rapids; 

 their waters by then are clean, having dropped their load of silt 

 in the pond, and so they do little if any cutting back. But the 

 Colorado River appears not to have learned its geological or 

 even its mechanical lessons very well, for it seems to have done 

 everything quite otherwise. 



This river is the only outlet for the drainage of an enormous 

 area fed by rivers descending from the snows of the Colorado 

 and Utah mountains, plus what rainfall there is on the Arizonas. 

 It also drains much of the Wyoming Basin via the Little Snake 

 and Green rivers. It is a fairly big river, and it carries a great 

 deal of silt even during the long periods between the annual 

 melting of the snows. Thus in normal circumstances it might be 

 expected to do considerable digging and ditching. In its present 

 condition, however, it cannot and does not. all the silt being 

 derived from gutter erosion at the very heads of its smaller 

 tributaries, and itself being an old and rather sluggish river, and 

 one which even "meanders." This latter is the sign of old age 

 and mechanical weakness in a river that simply has not got the 

 strength to cut a straight channel for itself. Yet it has obviously 

 cut a five-thousand-foot canyon, not only through uncompacled 

 strata of old dried-out muds, clays, and sands, but also for con- 

 siderable distances and to great depths through some of the 



Overleaf: Although by no means the largest canyon or the 

 greatest hole in the earths surface, the Grand Canyon of 

 the Colorado River in Arizona is a natural phenomenon of 

 endless mystery. 



