124 PRINCIPLES OF STRATIGRAPHY 



the moisture to which the buried salt surface is relatively impervi- 

 ous. Efflorescences of salt on the surface attract herds of animals 

 who stamp the soil into powder, which then is carried away by 

 wind. Water may gather in the resulting shallow depression, and 

 this will attract more animals, and by their trampling of the soil 

 and the deflation of the material large hollows are formed, which 

 will be permanent lakes when the ground water level is reached. 

 Such lakelets abound in the northern Kalahari. Where the surface 

 of the Kalahari is formed by a porous limestone with efflorescence 

 of salt, it is broken up by the hoofs of animals and blown away by 

 wind. As water accumulates in the shallow depressions, the ani- 

 mals coming to drink trample the rock into fragments. The finest 

 particles are separated out by the water and settle as a layer of fine 

 mud which, on the drying of the pool curls up, is powdered by the 

 hoofs of the animals and blown away by the wind. 



In a region where, through increasing aridity, vegetation dies 

 out, the roots are grubbed after by animals, which thus begin the 

 formation of a hollow, later subject to enlargement. 



D. Obstnictional or barrier basins. 



These in all cases arise by an obstruction or dam placed across 

 a valley which, but for this obstruction, would have free drainage. 

 The principal types are the following. (Davis-7.)* 



1. Tectonic barrier basins. These are brought about by warp- 

 ing, by folding, or by faulting. Warp barrier basins are produced 

 by the deformation of an entire valley area by warping so as to 

 reduce it from its continuous slope to a depression. Lake Ontario 

 owes its existence in part to such warping, which has carried the 

 upper part of the ancient river valley below sea-level, leaving the 

 lower part considerably above it. (Grabau-17 :^p.) Valleys 

 dammed by folding are not uncommon in the Alps. Laggo Mag- 

 giore. Lake Lucerne and others have been classed here. 



2. Volcanic barrier basins. These are formed by the growth of 

 volcanoes so as to cut off a preexisting valley, or by the damming 

 of such a valley by a lava flow. Lake Kivu in the great rift valley 

 of Africa is a good example of the first type. Formerly this 

 drained northward into the Albert Edward Nyanza and the Albert 

 Nyanza, with the valleys of which its own is continuous. A 

 group of modern volcanoes, the Mfumbiro Mountains, dammed it 



* Davis includes as obstructional many of those here classed as constructional, 

 especially the detrital section. 



