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IV.—On the Origin of the Engadine Lakes
Published online by Cambridge University Press: 01 May 2009
Extract
Any geologist who, in these days, is still bold enough to affirm that the origin of Alpine Lakes generally is due to glacial erosion, must surely derive considerable comfort from a visit to the Upper Engadine; for the four lakes which adorn that valley appear, at first sight, to constitute so many typical examples of the erosive power of the ancient Inn glacier, and therefore a striking confirmation of that time-honoured theory. A closer and more extensive examination of the physiography of that exceedingly interesting district, leads, however, as in so many similar cases, to a very different, and precisely opposite conclusion.
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page 450 note 1 Swiss Alpine Club, 1879–80.—When Prof. Heim attributes the receding of the valley and of the divide to the backward erosive action of the Meira, it is not very clear whether he refers to the principal torrent south of the old divide, or to the Meira of the present day, the former tributary or one of the sources of the Inn. If the former, it would not be the Meira at all; if the latter, the Meira assuredly could not run and erode to the south until the old divide had disappeared. In the metaphorical phraseology in which he delights, that distinguished geologist speaks of the “tremendous fight between the Meira and the Inn which ended not only in the complete discomfiture of the latter but also in the Orlegna and Albigna forsaking their old allegiance.” It is difficult to see how there could be a fight, because—to follow up the metaphor—the Inn was simply a passive looker-on, and the Meira and the other two rivers turned naturally to the south when an opening was made for them, giving them a greater fall.
page 451 note 1 From measurements made by the writer the combined drainage area of the Meira, Orlegna, and Albigna within the old (Vicosoprano) divide, is about 120 square kilometres (44 square miles); that of the four lakes from the present (Maloja) divide to the lower end of St. Moritz lake is 140 square kilometres (51 square miles); and that of the present Inn torrent rising in the lake Lunghino, is 16 square kilometres (six square miles). Taking the rainfall in those Alpine valleys at 2·5 metres per annum, less 25 per cent, for absorption and evaporation, or in round figures at 2 metres (6·56 feet), the quantity of water of the three drainage areas respectively worked out as follows:
The old Inn carried therefore, at the present outflow of the St. Moritz lake, about 1000 cubic metres or tons per minute, which quantity the deflection of the Meira, Orlegna and Albigna reduced to about one half, whilst the present Inn at Maloja only carries about one-sixth of the volume of the old river, the erosive power being reduced in the same proportion.