Yol. 62.] TARNS OF THE CANTON TICINO. 175 



Prof. Bonney remarks (Geol. Mag. 1898, p. 20) :— 

 'Supposing a glacier to be descending the Piora valley, we must assume this 

 wall [at the north-eastern end of Lago Pitom] to be already in existence, or it 

 could not acquire any plunging force, and even then the fall seems hardly 

 adequate to produce the erosion of a basin like that of Lake Pitom. Possibly, 

 however, the ice, just at this part, may have been " jammed" ; for the main glacier 

 was probably augmented by another ice-stream, which descended a shallow, but 

 fairly well-marked valley, leading from a gneissic peak lying south-east down 

 to the corresponding corner at the head of the Pitom basin ; while the narrow 

 " gate " by which the water is now discharged towards the Val Pedretto would 

 block the mass of ice above it, and this would produce more than usual friction 

 on the bed of the valley now occupied by the lake and its delta. This basin 

 then, the part which lies below the present contour-line of 6000 feet (in round 

 numbers), is the utmost that, in my opinion, can possibly be attributed to the 

 erosive action of ice. Of this action all the other dominant features in the 

 surrounding scenery exhibit nothing more than superficial traces, and they 

 appear to be due to the usual meteoric agencies.' 



It is this probability of the ice becoming 'jammed' at the 

 western end of Lago Ritom which would, I think, prevent it 

 from excavating to any appreciable extent. In a former paper, read 

 before this Society by Prof. Gregory and myself, 1 we snowed that 

 where the lower layers of a glacier are embayed, their advance is 

 practically checked, and shearing of the upper layers takes place 

 over those below. Under these circumstances the greatest erosion, 

 in the case that we are considering, would be on the surface of the 

 rock-lip near the present exit of Lago Ritom, and it is precisely 

 here where we find the most obvious indication of former glacial 

 abrasion. Such erosion would, therefore, tend to destroy a rock- 

 basin, not to produce one. Again, the presence of the rocky headlands, 

 which have been described as occurring along the south side of the 

 lake, jutting out to the north-west, should have protected the 

 rauchwacke, originally occupying some of the present low bays, from 

 ice coming down from the east ; but not a trace of the rauchwacke 

 is to be found there. So far, therefore, as Lago Ritom is concerned, 

 it does not seem easy to account for the excavation of the basin 

 which it occupies on the supposition that it was ice-eroded. 



If we turn to the other lakes, Tom and Cadagno, the difficulty is 

 increased. It is obvious that these lakes could not have been 

 excavated by a glacier descending the Yal Piora, and no glaciers 

 could have formed on the nearly-vertical cliffs which hem them in 

 to the north. If we postulate ice, other than local accumulations 

 of snow, we must, as shown above, bring it from outside the basin. 

 In this case it must have come over a watershed 8200 feet high, and 

 there is no proof of the country having been swathed in ice to this 

 enormous depth. But, supposing such ice to have poured down from 

 the heights to the west, why, after digging out Lago Tom, did it 

 ascend the slope of rauchwacke between Lago Tom and Lago 

 Cadagno, only to dig Lago Cadagno again, out of the same material 

 and along the strike of the same rocks ? 



Or, again, assuming that the ice poured over the watershed, do wn 

 the precipices which hem in the lakes on the north, it could only 

 have arrived at the bottom in fragments, and could at the most have 

 contributed to a remanie glacier. 



1 Quart. Journ. Geol. Soc. vol. liv (1898) pp. 203 et seqq. 



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