ORIGIN OF VALLEY-LAKES. 79 



same time made more accessible to the surface-water and con- 

 sequently rendered more yielding, might very well be elevated in 

 places where the mechanical action of the valley-stream was too 

 sluggish to enable it to wholly counteract any upthrust by the 

 deepening of its own channel. In this way I conceive that some 

 of the smaller valley-lakes may have been formed. 



(4) Some valleys occupy lines of faulted dislocation — such, for 

 example, as that which runs northward from the Inn Thai, and coin- 

 cides with the gorge of Jenbach and the Achen Thai. The evidence, 

 which I recorded on the spot during three ascents of the surround- 

 ing mountains, and which has been given by me elsewhere*, is 

 confirmed by a reference to Yon Hauer's map of Tyrol. In this line, 

 at nearly its highest part, lies the Achensee, the greatest depth of 

 which (2500 feet) is said to be just off the eastern shore. Here is 

 a connexion of a lake with a fault. 



(5) The function of moraines in forming dams across valleys is 

 too well known to require that I should occupy much space here 

 with the consideration of it. As, however, one's acquaintance with 

 the Alps increases, one is greatly impressed with the frequency of 

 such occurrences, notable examples of which occur in the Oetz-Thal, 

 in the cases of the Obersee and Toplitsee, and many others which 

 might be mentioned, where a terminal moraine has blocked up the 

 valley. In some cases, as with L. Garda f and the L. of Llanberis, 

 such moraine-material would appear to have been redistributed, 

 and so partly stratified by marine action. Credner % has already 

 pointed to the connexion of the outlying lakes of the Alpine region 

 in S. Bavaria — the Ammersee, Starnbergersee, and Chiemsee — with 

 moraines. Huge lateral moraines, left by the glaciers on the sides 

 of valleys, such as may be observed at the head of the Eofen Thai 

 below the Hospiz, and at Carthaus in the Schnalser Thai, may 

 descend and block up a valley if once permeated by water, which 

 increases its weight and at the same time diminishes the internal 

 friction of its materials as well as their friction against the moun- 

 tain-side. Such a movement of a great moraine is now actually 

 going on at Petar, in the Unter-Engadin§. 



(6) The last point leads to the consideration of Bergstiirze, of 

 which there are some notable instances, e.g. the formation of L. 

 d'Alleghe and L. Derborence during the last century only, and the 

 catastrophes of Goldau and Elm within the present century. The 

 extent of the part which these have played in Alpine physiography 

 can only be faintly understood from what is observable in such 

 worn-down stumps of more ancient mountains as we meet with in 

 Wales, Cumberland, and Scotland. " We find," says Heim, " no 

 Alpine valley without such heaps of mountain debris and tradi- 

 tions having reference to them. A still older series belongs to 

 a time extending far back beyond the range even of tradition. 

 The largest and, perhaps, the oldest we can recognize is that of 



* Geol. Mag. Nov. 1882. " The Triassic Deposits of the Alps." 

 t Cf. Mr. John Ball, F.K.S., Phil. Mag. 1863, vol. xxvi. p. 499. 

 | Elena, d. Geol. p. 663. § Vide Heim, ' Ueber Bergstiirze.' 



