186 GUIDEBOOK OF THE WESTERN UNITED STATES, 
dips only slightly toward the north, and as it has a thickness of 
about 3,000 feet, the valley which it occupies and which has been 
formed by its erosion is of considerable width. To the north the 
rocks above the Mancos shale cap the Book Cliffs, which were so 
named becavise the beds of rock when seen from a distance suggest 
the edge of a book lying on its side. To the south the underlying 
variegated sandstone of the Gunnison formation makes the slope 
that leads up to the great red cliffs on the Uncompahgre Plateau. 
The traveler may see these rocks, as already stated, soon after leav- 
ing the station at Grand Junction, and they are generally in sight 
on both sides of the road as far as Mack. 
The peculiar shape and structure of the Book Cliffs (see PI. 
LXVIL, p. 157) gives them a striking resemblance to architectural 
features. In their lower part they are composed of shale, which is. 
capped by heavy beds of sandstone that lie almost flat. Nearly 1,000 
feet of shale is exposed, and where it is not protected by blocks of 
sandstone that have fallen from the ledges above it has been cut by 
the rain into innumerable branching ravines separated by low ridges. 
Viewed from a distance when the sun is low enough to cast a shadow 
on one side of these dividing ridges the sculpture is marvelously 
accurate and sharply defined, resembling the venation of a leaf. 
The slope is steep, nearly 45°, and the profile of the slope and the 
cliff above is well shown in Plate LX VITI (p. 157). 
The cliffs on the south are composed of great beds of red sandstone 
or white sandstone stained red by the overlying shale. At first sight 
these beds appear to lie so nearly flat that if they were extended 
they would reach entirely across the river valley and would lie far 
above the head of the traveler. When they are studied closely, how- 
ever, they may be seen to bend down sharply as they approach the 
river, and in reality they pass under the stream instead of far above 
it. The bend in the rocks may be seen by looking back after the 
train has gone a mile or so beyond the station. 
In this valley, as in most other irrigated parts of the West, the 
railroad does not traverse the aréa that is most highly cultivated, 
and the traveler may think that a large part of the valley below 
Grand Junction consists of land so highly impregnated with alkali 
as to be unfit for farming, but here and there he may catch a glimpse 
of the terrace or bench lands, which support the finest ranches in 
the valley. Along the railroad he may see some good ranches and 
orchards, and in striking contrast to them he may see in many places 
remnants of the original growth of sagebrush which covered the 
whole valley before it was irrigated and cultivated. This valley 
is the most arid part of Colorado, for, according to the records of 
the Weather Bureau, its annual rainfall is only 7.7 inches. The 
