SAND DUNES 563 



The chief agent in holding the dunes and making the spread of vege- 

 tation possible is the bunch grass {Andropogon scoparius), though 

 the sand grass {Calamovilfa longifolia) and the needle or silk grass 

 {Stipa coiiiata ) are also important as binders. The most charac- 

 teristic plant next to the bunch grass is the dagger weed {Yucca 

 glauca), which often occurs in great abundance. At the present 

 time hollows or "blow-outs" are more characteristic of these sand 

 hills than new dunes. These blow-outs begin where, from over- 

 grazing or fires, the vegetation is destroyed, and w^ill increase in 

 size until they are from 300 to 900 feet in circumference with sides 

 of bare sand sloping inward at an angle of about 30 degrees to th? 

 bottom, which may be from 20 to 75 feet or more beneath the rim. 

 (Pool-38.) In South America continental dunes are best devel- 

 oped in the great Atacama desert of Chile, west of the Andean 

 chain. Finally many of the deserts of Central and West Australia 

 are characterized by dunes of great extent. 



TJic Forms of Dunes. 



Three main dune forms may be recognized, of wdiich all others 

 constitute more or less divergent modifications. These are: ist. 

 the conical hill ; 2nd, the long ridge, generally occurring in parallel 

 groups, and, 3rd, the crescent-shaped dune or barchan. The cres- 

 cent or barchan type is most characteristic of the inland desert 

 regions. It presents a gently convex surface to the wind, while the 

 lee side is steep and abrupt. The horns of the crescent mark the 

 lateral advance of the sands. Its wide distribution and all but uni- 

 versal presence in the sandy deserts of all continents make this 

 type the normal one for sand hills formed on an open area. In- 

 deed, it is probable that in very many cases the linear ridges are 

 mere modifications of this type, formed by the lateral confluence 

 of many simple barchans. This is often shown in the wavy form of 

 the crest line, as in the case of the dunes of the Transcaspian 

 desert. 



On the coast, barchans, though present, are less characteristic. 

 Here we find the linear series more commonly developed, because 

 a nearly constant and uniform supply of sand is furnished by the 

 parallel coast, and because the sloping coast itself exerts a directing 

 force. Coastal dunes never have a symmetrical cross section, the 

 windward side generally having a gentle slope of from 10° to 20°, 

 which may in places be somewhat convex, while the leeward slope is 

 generally much steeper, being often as high as 30°. 



