8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



SO deeply cut, and they are being destroyed by current activities. The 

 hills that rose above these old levels were of moderate altitude, 200 

 meters, more or less, and were maintained by the relatively hard 

 limestone fragments involved in the overthrust masses of the Flysch. 

 The old topographic surface thus resulted from differential erosion 

 of a region which had already been disturbed by movement from the 

 northwest. The thrusts had brought up the limestone fragments, 

 but any ridges which may initially have been produced by thrusting 

 had been eroded and replaced by hills adjusted to the the relatively 

 harder rocks. This mature topographic surface is well seen in the 

 Hahnenmoos and surrounding hills above Lenk, and may be recog- 

 nized from that point of view as an obvious landscape of the sum- 

 mits of the Prealpes. 



A mature erosion surface of parallel development to that just 

 described is to be seen in the Hautes-Alpes. I would cite the ice- 

 covered flat of the Plaine Morte as an illustration of its development 

 in the Wildstriibel. This basin, though now the gathering-ground 

 of a glacier, is not due to glaciation ; nor is it due to structure of the 

 rocks. It is an erosion surface of gentle grade, with more or less 

 surviving relief worked out before it was elevated to its present posi- 

 tion. Where it now lies, its destiny is to be cut to pieces by the 

 development of ravines in the mountain mass of the Wildstriibel. It 

 lies a thousand meters above the homologous surface in the Prealpes 

 and is separated from it by the thrust plane on which the Wildstriibel 

 moved northwestward. The thrust plane rises in that direction ; the 

 mass of the Wildstriibel must have been raised in advancing up the 

 incline, and the altitude of the Plaine Morte above its homologue in 

 the Prealpes may, with reason, be attributed to that upward move- 

 ment. 



From these relations I draw the conclusion that the thrusting from 

 the southeast occurred after the erosion cycle had advanced to ma- 

 turity on the overthrust masses that had previously approached from 

 the northwest. 



Since structural and physiographic evidences show that the more 

 recent thrusting was from the southeast, we are led to look for dis- 

 placement of the older structures by the younger. I have already 

 described the folding of the older thrust planes at the base of the 

 Wildstriibel on the northern side of the younger thrust, by which they 

 are there cut off. When we cross to the southern side of that younger 

 thrust any part of the older major thrust which may exist on that 

 side must lie at some higher level in the elevated mass of the Wild- 



