220 SCIENCE PROGRESS. 



(ii.) Lakes in depressions zohich have been blocked by 

 some material. — The variety of materials which by dam- 

 ming up valleys give rise to lakes is very great, and a much 

 larger proportion of lakes than is commonly supposed 

 probably owe their existence to the formation of a dam 

 across a valley. In the case of such blocked valleys, the 

 water which accumulates behind the dam to form a lake 

 may eventually find its way over the dam, and if this be 

 composed of incoherent material, as frequently occurs, the 

 dam is readily worn away and the lake is a short-lived one 

 and is rapidly drained, its position being often marked by 

 a peat-bog, overlying the deposits which were laid down on 

 its floor prior to its destruction. The number of peat-bogs 

 occurring behind dams, as compared with existing lakes in 

 such a region as that of the English Lake District, proves 

 that the surviving lakes are a mere fraction of those which 

 once existed. But other dammed-up valleys are blocked 

 by dams, of which the lowest point is at a higher level than 

 that of a col uniting the valley with an adjoining one, in 

 which case the lake fills up to the level of this col and 

 drains through it, oftentimes over solid rock ; lakes of this 

 character are much more durable than those which dis- 

 charge over incoherent dams, and are likely to survive 

 when the latter are destroyed. I have elsewhere attempted 

 to prove that a large number of the tarns and lakes of 

 English Lakeland were formed in this way, and that their 

 formation was coincident with deviations of the drainage 

 system, often of considerable importance {3). 



Passing on to consider the various kinds of dam which 

 give rise to lakes, we may notice in the first place lakes 

 held up by ice, whether in the form of an ice avalanche or 

 a glacier. The lakes formed by ice avalanches are neces- 

 sarily of an ephemeral character and are mainly interesting 

 on account of the damage wrought by floods when they 

 burst. The best known instance of a lake of this character 

 is that of the sheet of water produced by the blocking up of 

 the Dranse by the Getroz Glacier, which caused the disas- 

 trous flood in Rhone Valley in 1818. Of glacier-dammed 

 lakes, the Marjelen See is too well known to require more 



