2o6 NATURAL SCIENCE. m^^v. 



than middle Tertiary times. A geologist is at first staggered by the 

 proofs of enormous denudation evident everywhere in a country 

 apparently devoid of water ; but the mystery is solved when he has 

 the fate to be overtaken by one of the erratic storms which are a 

 characteristic feature of the region. The whole surface of the 

 plateau is covered with running water ; the accumulated detritus of 

 the rocks is hurried along by boiling torrents where an hour ago 

 were only dry gullies ; and the larger streams abundantly justify the 

 name which several of them bear — Rio Puerco, the dirty river. The 

 great diurnal variations of temperature have already disintegrated 

 the surface of the rocks ; there is no vegetation to receive the direct 

 impact of the rain, and no soil to impede the transport of the material ; 

 the rain has thus an enormous power for degradation, a power which 

 is, indeed, a direct consequence of the dryness of the climate. 



It is when a tract of homogeneous, horizontally-bedded rocks is 

 subjected to action of the kind described that the typical " Bad 

 Lands " are seen. The country is studded with thousands of bare 

 hills, following one another in close succession as far as the eye can 

 reach, and divided only by a network of dry channels. When harder 

 strata alternate with softer, the simplicity of contour of the hills is 

 modified by projecting ledges, and we have what Major Powell has 

 called " Alcove Lands." The alternation of ledge and talus, or, on a 

 larger scale, of cHff and slope, is strongly marked, and the emphasis 

 with which the stratification thus expresses itself in the surface- 

 configuration is again a consequence of the dry climate, which makes 

 the impact of the rain so formidable a factor in erosion. In a country 

 like England the soil holds the water, which is enabled to effect 

 disintegration in the rocks beneath, whether hard or soft, and the 

 differences of existence are minimised. In an arid region the soft 

 rocks are destroyed rapidly, leaving the more resisting beds in strong 

 relief, and the only limit to this is fixed by the undermining of the 

 ledges, which removes their support. 



The traveller on the Rio Grande Western Railway over the two 

 hundred miles from Castle Gate to Grand Junction traverses a 

 magnificent panorama of bad lands and alcove lands, the stratification 

 being here, as is often the case, accentuated by brilliant contrasts of 

 colours in the naked rocks. During most of the journey he has in 

 sight on his left a range of steep cliffs 2,000 feet high, known as the 

 Book Cliffs. These are an example of a feature common in the region 

 — the escarpment cliff, or " cliff of erosion," dividing a higher from a 

 lower plateau. Such abrupt escarpments characterise gently inclined 

 formations of alternating hard and soft characters, the cliff being 

 formed by a hard rock resting usually upon a soft one. There may 

 be two or three parallel escarpment cliffs built of successive forma- 

 tions, as in the plateaux traversed by the "Terrace Cafions " of the 

 Green River. The surface of each platform is approximately a 

 bedding plane, inclining gently towards the cliff which overlooks it. 



