EROSION AND THE SUMMIT LEVEL OF THE ALPS 35 



of drawings, which are all furnished with scales so that definite 

 measures may be made of altitude of upheaval, depth and breadth 

 of valley erosion, and so on; and the accompanying text sets many 

 questions which lead to explanatory and quantitative answers. 

 The first drawing of a plateau shows it at an altitude of 2,550 

 feet, with a narrow canyon already eroded across it, 750 feet deep 

 at the background and 1,700 feet deep in the foreground, where 

 it is about 1,900 feet wide at the plateau level. The second drawing 

 shows the plateau 4,200 feet high, and the canyon 2,700 and 3,200 

 feet deep at background and foreground, and over 5,000 feet wide 

 at the foreground top. In the third drawing the plateau is 4,700 

 feet high, and the background and foreground depths of the canyon 

 are 3,500 and 4,000 feet, its width at the foreground top being 

 8,500 feet. The fourth drawing shows the plateau height 

 unchanged, the canyon depths 4,000 and 4,100 feet, with its width 

 at foreground top roughly 15,000 feet, and a flood plain about 1,000 

 wide in its floor. The corresponding questions bring out clearly 

 the idea of progressive erosion during the uplift, as indicated by 

 the increasing altitude of the plateau and increasing dimensions of 

 the canyon in the first three drawings; and of continued erosion 

 after upheaval has ceased, as indicated by the greater dimensions 

 of the canyon in the fourth drawing than in the third, although the 

 plateau has the same altitude in both. Later stages of erosion on 

 the still-standing mass are pursued in smaller diagrams to eventual 

 peneplanation. 



The exercise on mountains opens with a drawing of a faintly 

 undulating lowland. The second drawing shows the greater part 

 of the lowland warped up into an arch which curves around from 

 east-west in the right foreground to south-north in the left back- 

 ground, and has altitudes of from 4,000 feet to 7,000 feet along its 

 crest; the arch is cut across where its height is between 5,000 and 

 6,000 feet — that is, aside from its lowest summit — by the canyon 

 of an antecedent river; other parts of the arch are well incised by 

 the revived streams of the lowland or by new streams consequent 

 on the slope of the arch; but these valleys occupy less space than 

 the undissected flats and slopes of the upland. The third drawing 

 shows only the east-west part of the arch with its crest now raised 



