326 E. C. ANDREWS. 



prominently split up by vertical joints, and a lower one 

 consisting of a great number of thin layers of limestone 

 fairly well provided with vertical joints ; that is, the upper 

 portion of the Red Wall limestone at this spot appears to be 

 a stronger structure than the lower portion. Under these 

 comes a relatively weak series of shales and sandstones of 

 greenish tint. 



Heavy rain storms in very recent times have produced 

 large alluvial fans under the limestone cliffs. Along the 

 Bright Angel Trail, however, an exceptionally heavy storm 

 of more recent date has carried the fans away almost in 

 their entirety. In this process it has exposed a basin of 

 corrasion in the incoherent lower green shales and sand- 

 stones. By the formation of this basin a slope of repose 

 has been induced in the weak series. Such sapping action 

 has produced amphitheatres with varying angle of slope. 

 Now this tendency to form amphitheatres in the retreating 

 shales is carried on by gravitative processes unaided by 

 streams to a certain point ; the sapping process reaches 

 the limestone and there by reason of its strong vertical 

 joints and abundant stratification planes, it carries the 

 " gravitative curve" into their mass but with steeper angle 

 than that possessed by the curve in the lower and weaker 

 sandstones. 



Upon reaching the stronger layers above, the propagation 

 upwards of the sapping movement is progressively weakened 

 and the higher portions actually overhang while the upper 

 curve rapidly flattens (Fig. 12). In this way the reversed 

 "gravitative curve" is produced. Above this reversed 

 curve the Red Wall limestone has the appearance of a cliff 

 and in plan it " curves sympathetically" with the contours 

 of the general amphitheatre of recession. This is the 

 expression of forces which act vertically on material form- 

 ing the walls of a deep cut, when such cut is either closed 



