ORIGIN OF TOPOGRAPHIC FORMS. 



11SI 



ure 13, Nos. 3 and 4. Suppose No. 3 to represent a cross sectii m i if a cany* >n, 

 the upper bed of the plateau being hard, succeeded by soft and hard beds 

 iu alternation, as is seen in the ({rand 

 canyon of the Colorado, PI. xm. The 

 course of the stream in forming this 

 canyon is shown by the light lines in 

 tin- figure. It cuts first a canyon with 

 steep sides in the upper hard bed, 

 an operation which perhaps consumes 

 much time. Then reaching the softer 

 bed below, it burrows rapidly into it, 

 at the same time undermining- the bed 

 above, which from its weight breaks 

 away, leaving cliffs. A similar opera- 

 tion carries it through the next hard 

 and soft beds. Thus a succession of 

 cliffs and terraces is formed. The 

 presence of cliffs in a canyon wal 

 generally indicates that the bed be- 

 neath the Cliff is more easily eroded Fig. 13-.Cros 8 ~sectton s of canyons. 



than that above it. The fragments of the cliff falling upon the slope of the 

 soft bed below form what is known as a talus. 



The above is a common case in a plateau region, since the surface bed 

 is usually hard. Where the surface consists of a soft bed. No. 4. Fig. l.'i, 

 represents the condition of the canyon walls. The undulating surface of 

 the soft bed slopes down to the cliff produced by undermining the hard bed 

 beneath. Otherwise the case is similar to that described above. 



A third case is afforded by the Black canyon of the Gunnison in Col- 

 orado, where a hard sandstone forms the surface of the plateau, underlain 

 by granite. A section is represented by No. 5 in Fig. 13. The sandstone 

 stands at an angle of about 30°, beneath which are the walls of the granite 

 canyon, which are somewhat steeper, the angle of slope being perhaps 40° 

 to 45°. There is no undermining and consequently there are no vertical 

 cliffs. 



