OBSTRUCTIONAL LAKE BASINS 



125 



on the north and raised its level 2,000 feet, until it began to over- 

 flow southward across the gneiss rim into Lake Tanganyika. The 

 watershed on the north has been raised by these (in part) still active 

 volcanoes to something over 7,000 feet, while individual peaks of 

 the barrier rise to great altitudes, the peak of Karisimbi reaching 

 14,000 feet and being often snow-capped. (Moore-26:5'7.) 

 (Fig. 21.) The valley of Lake Kivu is of tectonic origin. The 

 extinct Tertiary Lake Florissant of Colorado was due to a lava 

 dam. Lakes of this type are usually long lived. Navahoe Lake of 

 Utah is held by a flow of scoriaceous lava. 



3. Chemical (Hydrogcnic ) or tufa barrier basins are probably 

 always of small size. Some of the smaller lakes of the Yellowstone 



Fig. 21. Sketch of Lake Kivu and the Mfumbiro Mountains of Africa. The 

 lake lies in the great rift valley of Africa, and is dammed by 

 volcanoes of recent origin. (After Moore.) 



region appear to owe their existence, in part at least, to damming of 

 valleys by hot springs or geyser deposits. The Plitvicer seas of 

 Croatia are small lakes in a Karst valley dammed by tufa deposits. 

 The same may be true of Lac de Brenets of the French Jura, which 

 has a depth of 31.5 meters. (Penck-28: 288.) 



4. Ice barrier basins. Of these the Merjelen See in Switzer- 

 land is the best known existing example. An example of one now 

 extinct is seen in Glen Roy (Agassiz-i :?j) in the Highlands of 

 Scotland. Here only the old wave-cut shore-lines remain in the 

 "Parallel Roads," marking the successive changes in level of the 

 lake which was held up by the glacier occupying the valley to which 

 Glen Roy is tributary. Ice barrier lakes were abundant toward 

 the close of the Pleistocenic ice age, when the continental glacier 

 blocked northward draining valleys and converted them into tem- 



