OBSTRUCTIONAL LAKE BASINS 127 



in the Sierra Nevada range. Lakes Brienz and Thun are divided 

 by a delta or fan on which Interlaken stands, and other examples 

 are found in the Alps, in the Mississippi region, and elsewhere. 

 Marginal lakes also come into existence when the main stream 

 aggrades its valley, thus damming the tributaries. The lakes along 

 the Red River of Louisiana have been regarded as due to this 

 process, but organic accumulations or "rafts" played an extensive 

 part in the ponding of the tributaries. (Veatch-44.) These 

 organic accumulations are to be regarded as detrital material, just 

 as are the inorganic materials swept together. 



(c) Moraine and drift-barrier basins. These are numerous 

 in all glaciated regions, but are generally accompanied by some 

 deepening of the basin through glacial scour. This is the condition 

 of many of the Alpine lakes and it is also the condition of the 

 Finger Lakes of New York. A typical example of a drift barrier 

 lake now extinct is found in the Upper Genesee valley from Por- 

 tageville southward. This was dammed by drift at Portageville, 

 and the resulting lake overflowed southward for a time, until a 

 gorge was cut through the rock to drain the waters northward. 

 The drainage here was originally southward, but was inverted by 

 depression of the land on the north. Slight glacial overdeepening 

 also occurred. (Fairchild-13 ; Grabau-19.) Drift damming accom- 

 panied by warping is shown in the Great Lakes of North America. 

 It must be clearly understood that only lakes formed in valleys 

 dammed by drift or moraines belong to this class. Lakes formed 

 in depressions in the drift itself, i. e., kettle lakes, are construc- 

 tional, not obstructional. Morainal lakes are generally short-lived, 

 although those blocked by great drift sheets may fairly be called 

 permanent. 



(d) Atmoclastic barriers are either land-slip barriers or dune 

 sand barriers, (i) Land-slip barriers originate when a landslide 

 dams a mountain valley. They are mostly short-lived, for the 

 drainage across the barrier will result in their rapid extuiction. 

 Lake St. Laurent, 10 kilometers long, in the Oisans, western 

 Alps, was created by a land slip in 1181. It existed about 40 years, 

 during which period the farmers of the valley became fishermen, 

 and it was destroyed by the breaking of the drift dam. (Reclus- 

 29:5^^.) In September, 1893, a great landslide, lasting three days 

 and bringing down 800,000,000 tons of rock, dammed the deep 

 valley of one of the upper branches of the Ganges in the Himalayas, 

 150 miles above the city of Hardwar, which lies at the mouth of 

 the valley. The dam made was nearly 1,000 feet deep and the 



