and Lake Basins of Northern Italy. 

 III. The Lake Basins. 



40£ 



The following table gives the altitudes and dimensions of the 

 principal six Italian lake basins, which, it will be observed, lie in 

 a zone similar to that of the five Swiss lake basins north of the Alps. 



It will be seen that not one of these lakes reaches the altitude of 

 300 metres, which, as previously shown, is that of the marine shell- 

 bearing deposit a few kilometres north of Como, nor the altitude of 

 the Fino deposits (370 metres) on the second moraine wall south of 

 Como. The sea in the Po Valley must therefore have communicated 

 with the fjords at a level much higher than that of the present lakes. 



The question of how and when the present lakes were formed is 

 obviously and closely allied with the retreat of the sea from the Po 

 Valley. Desor l held that from the time of the glaciers coming into 

 contact with the sea the littoral of the Lombardy and Piemont plain 

 was gradually raised ; but this would only account for the high level 

 of the Pino and the other marine shell-bearing deposits. It appears 

 much more likely that a simultaneous lowering took place on the 

 one hand in the floor of the Po Valley, 2 while the sea gradually 

 receded, and on the other along the base of the Alps, which converted 

 the fjords into lakes in addition to the fjords being choked by the 

 moraine bars of the retreating glaciers. The formation of the lakes 

 would, therefore, be due to the same two concurrent causes which 

 operated in the formation of the lake basins north of the Alps — 

 a zonal flexure and a moraine barrier behind which the river valleys 

 or fjords became lakes during the recession of the glaciers. 



The cross-section of the triple moraine walls south of Como shown 

 in Sheet No. 2, Fig. II, which Rutimeyer gave for another purpose in 

 his memoir already quoted, exhibits a striking reverse dip from the 

 outermost wall to Lake Como of no less than 165 metres. The third 

 moraine wall, which rests upon considerably bent strata of Molasse, 

 thus constitutes, in my view, the anticline of the flexure whose 

 syncline to the north follows the deepest points of the lakes, while 

 the syncline to the south lies along the lowered floor of the Po 

 Valley. The conditions in respect of Lake Maggiore and of Lake 

 Garda are precisely similar, the reverse dip to the former being 150 



1 Pay sage morainique, p. 72. 



2 The deposition of the rich alluvia which cover the floor of the Po Valley, 

 and to which it owes its wonderful fertility, began with the retreat of the 

 glaciers when the rivers resumed their erosive energy, and continued throughout 

 the long interglacial period after the maximum glaciation. 



