DESTRUCTIONAL LAKE BASINS 123 



of 20 feet and a depth of 40 feet. Waterfall hollows also are 

 limited in size. At Niagara a hollow nearly 200 feet deep, over 

 three miles in length, and a thousand feet or more broad has been 

 gouged out by the Horseshoe Falls. When Niagara goes dry, this 

 will remain as a lake. A typical example of an existing lake of 

 this type is Lake Thaxter of the St. Croix Dalles region in Minne- 

 sota. Along the course of most rivers are deeper reaches which 

 represent stronger scour, and these may be converted into lakes. 

 This is the case in some of the branches of the Nile, where, during 

 the dry season, these pools contain water. In old river courses 

 like that of the Oxus, an affluent of the Caspian, and the old course 

 of the Huang-ho, such lakes are sometimes a characteristic feature. 



4. Glacial rock basins. WHien a glacier gouges out the floor of 

 its valley above the mouth, so as to leave the frontal rim actually 

 higher than some other parts of the basin, the conditions for the 

 formation of a destructional lake basin are furnished. Such lakes 

 are common in areas of recent glaciation and many of the lakes 

 in the region of the Pleistocenic ice sheet invasion owe their exist- 

 ence partly to this cause. The Finger Lakes of New York are 

 believed to be in part glacial rock-basins, and in part due to ob- 

 struction. 



5. Deflation basins. These result from the removal, by wind, 

 of sand and dust from a disintegrating surface, in such manner as to 

 leave a complete rock rim surrounding the area of removal. This 

 area will be deepened to the extent determined by the power of the 

 wind to remove disintegrated material. Desert basins, in so far as 

 they are not tectonic depressions, owe their character to this agent, 

 since no other force (excepting man) can remove soil or sand from 

 the enclosed basin. Playa lakes may occupy temporarily the lower 

 depressions of these basins. Hollows excavated by wind carrying 

 sand, though insignificant, should nevertheless be mentioned. 



6. Artificial excavations. These are included for completeness 

 sake, but need no further comment. Mud wallows, on the other 

 hand, may be briefly discussed. In desert areas where elephants 

 and other large creatures wallow about in pools, they produce hol- 

 lows several meters deep. Rains washing down the sandy sides of 

 these hollows widen the area afifected. Thus if the hollow left after 

 the filling is still i meter deep its diameter may be 25 m. Continued 

 wallowing will result in the formation of a depression 5 meters 

 deep and 120 to 150 meters in diameter, such as are found in abun- 

 dance at the present time in the drier districts. Their formation by 

 animal erosion is often observed in the Kalahari and other deserts. 



Where salty marl surfaces are covered by sand, these retain 



