NO. 31 GEOLOGICAL STRUCTURE OF THE ALPS — WILLIS 7 



miliar with folded thrust planes as they occur in the Appalachians and 

 have been described by Keith, I have never elsewhere seen such clear 

 and convincing examples as are to be found here. The anticline of 

 the Iflfigental, which shows a superb arch of Jurassic limestone over- 

 lying the Eocene Flysch, is without an equal in my experience for 

 perfection and simplicity ; and the folded thrust planes of the foot- 

 hills north of the Wildstriibel (Oberlaubhorn and Laufbodenhorn), 

 though more intricate, are no less clearly defined. At the immediate 

 base of the Wildstriibel the disturbance of the thrust planes is more 

 intricate and nlinute. They are affected by shearing as well as by 

 folding ; and tracing these minor secondary structures directly to the 

 great overthrust on which movement was from the opposite direction, 

 one cannot doubt that they were produced by that movement. Hence, 

 the thrust from the southeast is younger than that from the north- 

 west. 



A method of investigation which has been more widely adopted 

 by American geologists than by those of Europe, is based upon the 

 relation that may exist between structures, such as folds or thrusts, 

 and the features developed by erosion. I know many mountain 

 ranges whose internal structures are older than the erosion of the 

 surface. I know others whose relief is an effect of relatively recent 

 folding or thrusting. On examining the Prealpes and Hautes-Alpes 

 with this relation in mind, I found a clear distinction between the 

 movement from the northwest, which is older than any recognizable 

 erosion of the region, and the movement from the southeast, which 

 is younger than the earliest erosion cycle that I could recognize. 

 The succession of events was: (i) movement from the northwest; 

 (2) erosion to a mature stage, with moderate relief; (3) movement 

 from the southeast, resulting in dislocation of the older structures 

 and also of the mature erosion surface, together with the elevation 

 of the Hautes-Alpes ; (4) erosion of the younger features of topog- 

 raphy as they now exist. 



The observations which I made on these points were as follows : 

 In the Prealpes I detected features pertaining to two different 

 cycles of erosion. The more recent is represented by the deep 

 valleys and slopes of the present stage. The other is an older, mature 

 surface, which may be seen in the long ridges and summits of the 

 Prealpes. It is extensively and generally dissected by the later 

 valleys, but there are many flat areas which are not the result of any 

 structure or of any recent condition of erosion. They represent the 

 lower levels of the mature surface in which the present valleys are 



