THE ORIGIN OF LAKES. 221 



than passing notice ; but magnificent examples occur in 

 Greenland, and in some of these the water drains over or 

 under the ice, whilst in others the outflow resembles that 

 from the Swiss lake in being over solid rock. Some 

 interesting lakes of this character are figured in Plates A 

 and C of the first part of the Meddelelser om Gronland. 

 I need only refer here to the famous Parallel Roads of Glen- 

 roy and the great terraces of " Lake Agassiz " in North 

 America, which are considered to be due to glacial dams of 

 this nature. 



In the lowland tracts of river-basins crescentic lakes 

 often result from the formation of "cuts-off," the former 

 sinuosities of the river being barred by river-deposit and 

 giving rise to lakes. Good examples of these are found in 

 the lower reaches of the Mississippi and are described in 

 the nineteenth chapter of Lyell's Principles. Accumula- 

 tions of vegetable matter are frequently sufficient to obstruct 

 the drainage of streams to so great an extent as to form 

 lakes, which are subsequently converted into peat-mosses. 

 Sir A. Geikie records the formation of lakes in North 

 America which have covered thousands of acres and which 

 were due to the formation of a dam of vegetable matter 

 produced by beavers cutting down trees. 



Blown sand is another material v/hich is competent to 

 form a barrier enclosing a large sheet of water. The well- 

 known etangs in the neighbourhood of Arcachon were 

 formed by the gradual advance of sand-dunes from the 

 ocean, converting bays into inland pools, the surfaces of 

 which are frequently at a considerable height above the 

 ocean level. 



In upland districts numerous tarns and lakes are blocked 

 up by barriers formed of landslip material, screes, dry deltas, 

 snow-slope detritus and glacial accumulations. A good 

 example of a lake formed by landslips is that produced near 

 Terranuova during the Calabrian earthquake of 1783. 

 Examples of the other barriers are furnished by various 

 tarns in the Lake District. Goatswater, near Coniston, is 

 blocked by screes; Smallwater, near Haweswater, by snow- 

 slope detritus ; several of the tarns by glacial accumulations, 



