...M.woNsl SALT LAKE CITY TO GRAND JUNCTION. 399 



eroded clay beds below, the sandstones are left behind forming in 

 curving lines of precipitous cliffs on either side. 



At Price the road emerges into the great open nionoclinal valley 

 in the clays of the Middle Cretaceous, which extends eastward 

 to and beyond Grand Junction, a distance of nearly 200 miles (322 km.). 

 The Laramie sandstones, dipping gently 7° to 10° north, retreat to the 

 northward, and gradually merge into the great mural escarpment of 

 the Roan or Book cliffs, which, with an average height of over 2,000 

 feet (010 in.), stretch entirely across the Colorado basin, in a curve con- 

 vex to the south, connecting the plateau system on the east Hank of 

 the Wasatch uplift with that on the west of the Rocky ."Mountain sys- 

 tem of Colorado. Through this whole distance the Laramie Cretaceous 

 forms a continuous outcrop. Above it, sometimes forming second lines 

 of cliffs at a little distance back, and sometimes capping the main 

 escarpment, rest successively the coarse reddish or roan-colored sand- 

 stones of the Wasatch Eocene, and the drab calcareous shales of the 

 Green Biver Eocene. The latter are characterized by the thinness of 

 their strata and the great definition of their bedding lines, so that their 

 cliffs resemble the leaves of a book, whence the name "Book Cliffs."' 

 Throughout the interior of the basin these Eocene beds rest in parallel 

 transgression upon the Laramie Cretaceous, but along its periphery, 

 especially on the flanks of the Uinta mountains, which form its north- 

 ern border, they overlap its upturned edges, high up on the Hanks of 

 the range, to a contact with Jurassic, Triassic, and even Carboniferous 

 rocks. 



The erosion which has formed these cliffs is peculiar to a practically 

 rainless region surrounded, as is the Colorado basin, by high moun- 

 tain masses. It is produced by the undermining of rock faces in the 

 softer strata beneath by sudden floods, resulting from violent showers, 

 popularly known as cloud-bursts, starting at long intervals and under 

 favorable meteorological conditions from the surrounding elevated 

 regions ahd spreading out locally over different portions of the arid basin 

 region. These In a few moments change dry stream beds into boiling, 

 muddy torrents, which carry away an enormous amount of material that 

 dry disintegration, aided by great diurnal variations of temperature, 

 had already loosened. Thus the Book cliffs, which, owing to the 

 northerly dip of their component strata, are eroded almost entirely on 

 their southern face, have retreated most rapidly at either extremity, 

 owing to its proximity to the mountains, and now stand in a curve 

 convex to the south, which in the center of the basin is 40 miles (G4 

 km.) farther south than its western end. 



The Cretaceous in this region consists of a series of peculiarly resist- 

 ing quartzitie sandstones (Dakota) at the base, succeeded by several 

 thousand feet of clay shales, with a few thin sandstones and limestones, 



