3 i 4 PREHISTORIC EUROPE. 



to the upper reaches of the valleys, as it is to-day. While the 

 river Serio sank to a lower level, the Komna, escaping from the 

 Val Gandino, gradually cut its way across the dam or bar, and 

 thus reduced the level of the Leffe lake, until eventually, partly 

 by this process of drawing off the water, and partly by the 

 deposition of fine silt, the lake became restricted in size, and 

 vegetation ere long crowded its margin. The beds above the 

 lowest lignite indicate a rising of the lake, consequent, probably, 

 upon a corresponding increase in the volume of the Serio. The 

 next bed of lignite points to a pause when the surface of the 

 lake was again encroached upon by the growth of vegetation. 

 Once more, however, the waters began to rise, and freshwater 

 clays were deposited upon the surface of the now submerged 

 peat. But as the climate deteriorated, and a glacier again filled 

 the upper part of the Val Seriana, the former conditions returned. 

 Great torrents derived from melting snow and nev4 poured 

 down the Val Gandino, tearing up the lacustrine sediments, 

 and eventually burying them under great sheets of shingle and 

 gravel. The Serio at the same time may have partially choked 

 up the gorge of the Komna with its shingle and gravel, but 

 it did not succeed in again converting the Val Gandino into a 

 permanent lake. 



The phenomena exhibited in the Val Borlezza are quite in 

 keeping with these conclusions. The lake of Borlezza unques- 

 tionably owed its origin to the blockage caused by the glacier of 

 the Val Camonica and that arm of it which crossed over into the 

 Val Cavallina. The bottom-beds of the lacustrine series are 

 composed of true glacial clays and silt, containing in the deep 

 cutting below Pianico scattered glaciated stones, which become 

 more numerous towards the base of the section, where the sandy 

 mud or muddy sand is most abundantly charged with them. 

 All these deposits must have gathered there while the mouth of 

 the valley was barred by the glacier. Eventually, however, 

 that glacier melted away, and left behind it an embankment of 

 morainic detritus, which for some time would continue to act 

 as a bar ; but the lake, as it overflowed, would sooner or later 



