EROSION OF DISTURBED STRATA 841 



ation of the component strata will be produced. The resistant 

 stratum will produce a ridge, one side of which is composed of the 

 steeply dipping surface of the resistant stratum and comparable to 

 the gentle outward slope of the cuesta surface, while the other is 

 formed of the eroded edges of the strata composing the ridge, and 

 is comparable to the inface of the cuesta. (Fig. 213, a b.). Topo- 

 graphic elements of this type are common on the flanks of the 

 Rocky Mountain Front Range where they are familiarly known as 

 "hog-backs." In some cases these hog-backs may, however, be parts 

 of normal anticlines of which the crystalline mountain mass was the 

 original core. The outcropping edges of the component strata will 

 not appear different on the map from those of the normal cuesta, 

 and the phenomenon of overlap is perhaps as frequently preserved 

 in this case as in that of the dissected normal coastal plain. 



3. Erosion Features of the Structural Dome. 



Wherever strata are locally uplifted into the form of a broad, 

 flat dome, as in the case of the Black Hills, a radial arrangement 

 of consequent streams will come into existence, and a series of radial 

 consequent valleys will be incised in the surface of the dome. The 

 birth of numerous insequent streams at the summit of the dome 

 will cause a gradual opening up of a series of summit valleys. If 

 the surface stratum is a resistant one, while the stratum next below 

 is readily eroded, a compound summit valley, drained by tributaries 

 of the various consequent streams, will come into existence on the 

 soft stratum, while the eroded edge of the hard stratum will sur- 

 round this valley as a series of ramparts broken at intervals by the 

 breaches through which the drainage is carried out. The character 

 of the enclosing rampart will be that of a breached circular hog- 

 back with an erosion inface and a steeply inclined outward slope. 

 Continued erosion by the tributary (subsequent) streams will widen 

 the circumference of the rampart by pushing it down slope, and 

 thus increasing the size of the summit valley. If a second resistant 

 layer is discovered beneath the soft layer on which the valley was 

 opened, it may be breached in a manner similar to the first, and a 

 second inner set of encircling hog-back ridges may come into exist- 

 ence surrounding an inner valley opened up on a second soft layer. 

 Several sets of such encircling hog-backs may thus be produced, 

 two sets always being separated by a circular valley which drains 

 through one or more branches in the outer hog-back ring. If the 

 level to which erosion is carried, i. e., base-level, is reached while 



