852 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



of different deposits. The latter are of plutouic or volcanic origin, and, 

 so far as entering into consideration here, may be comprised under the 

 name of '' dikes". Although a Jarge portion of the erosive work accom- 

 plished is necessarily of the same character in both cases, the require- 

 ments for the production of the first group differ materially from those 

 of the second. Under the definition of ''mural forms", I place such 

 products of erosion which may resemble single walls more nearly than 

 any attempt at architectural design. From the nature of the subject it 

 is evident that hard strata resisting erosion, if placed on end, may for 

 a long time retain their position. By virtue of the stratigraphical dis- 

 turbances they have taken part in, they have acquired positions which 

 are merely rendered more prominent by erosion. They do not owe their 

 present relations to surroundings primarily to erosion, and will, there- 

 fore, not be considered here. 



A. — First Group. 



WHITE ElVEE EEGIOK. 



Near and on White Eiver, within the same sandstone that is so pro- 

 lific in the production of statuesque forms, we find very good illustra- 

 tions of walls caused purely by erosion. The primary formation of 

 valleys there has been discussed above. It may here be added, that 

 tjie gradual transportation of material from between two ridges caused 

 portions of the overhanging sandstones to drop down. Aided by the 

 prevalence of joints or similar iractures, the disruption was more read- 

 ily accomplished, the fresh surface exposed became more uniform in 

 shape. If we carry out the widening and deepening of erosive valleys 

 to such an extent that the ridges intervening between two of them will 

 become very narrow, we may achieve the result of forming walls u[)on 

 their crests. Purely fluviatile erosion could not accomplish this end 

 unless by undermining, and then only if joints of sufiBcient extent should 

 enable the rocks to drop down easily. Where only such erosion can 

 exert its influence, we will often find vertical faces produced by under- 

 mining and subsequent falling down, but the summit of the ridge will 

 be too wide to term it a. wall: it will be a bluff, or even a sloping i^lateau. 



In the vicinity of the White Eiver we have, in fact, a sandstone thor- 

 oughly traversed by joint-fissures. At favorable localities, the early 

 erosion by flowing water has cut narrow, deep channels into the rock, 

 has evidently undermined, and does to-day undermine, certain portions, 

 causing the strata above to break. Before the tension thus produced 

 is relieved by the absolute disruption of the strata, the joints probably 

 open more widely, causing an apparent downward flexure of the beds. 

 Frost, and in part vegetation, rapidly produce a still greater widening 

 of such fissures, and subsequent falls of rock-masses will take place. 

 Eventually, by this means, the production of a wall, several hundred 

 feet long, one hundred to one hundred and fifty feet high, and sixty to 



