

CORRASION BY GRAVITY STREAMS. 325 



The plan of a recessional form all of whose lines of 

 quickest descent tend to become parabolas, is influenced 

 more or less by such a curve itself. In proportion to the 

 strength of the flow which causes the headward recession 

 of the stream, so will the simplicity of the profiles of cor- 

 rasion be destroyed. In homogeneous structures the plan 

 of the curve of recession will be sharper as the stream 

 action is more decided in strength, but the profiles of 

 corrasion will be flatter. 



Appendix II. — Valley head in heterogeneous structures. 

 Consider the case of a stream channel excavated in a 

 rock mass which is horizontally disposed and which overlies 

 a relatively weak rock series. If the heavy upper struc- 

 tures be homogeneous among themselves the slope of the 

 channel head will be similar to the forms described in Part I. 

 But so soon as the weak underlying series is exposed, 

 more pronounced sapping of the stronger upper rocks is set 

 up. The amphitheatre form is produced in the incoherent 

 structures, which, under the action of weathering, eddying 

 and other agencies, causes an undermining of the heavy 

 overlying mass. The latter will retreat in cliff form, which 

 in plan will be similar to that of the cirque formed in 

 the lower homogeneous structures. 



As an illustration of the tendency of a vertical force to 

 produce these amphitheatres in rock structures whenever 

 a chance offers of a get-a-way for the debris, it may help 

 us here to inquire into the origin of the amphitheatres of 

 the "Red- Wall " limestone as shown in the Grand Canon 

 Section of the Colorado at El Tovar. [See also Davis (d) 

 pp. 178 - 181]. The case in point is chosen because known 

 to geologists the world over. 



Here a mass of limestone several hundreds of feet in 

 thickness is divided into two systems, an upper one con- 

 sisting of relatively thick compact limestone layers not 



