99 



abundant grasses, CciicJinis caroiiiiianiis and Paspalitm setaceum, are 

 not injured by burial to a slight depth, but are easily undermined. 

 They find their optimum conditions on deposits of this type, where 

 their fruits are easily buried and where the annual deposit of sand 

 is not sufficient to injure them. If the deposition becomes too rapid 

 and the fruits are buried too deeply, the sand remains entirely bare 

 (PI. V). The best observed example of this condition is a large 

 dune just south of Keithsburg, in the Oquawka area (Pb X, Fig. 

 i). The top of the dune is here entirely bare. At its base along the 

 lee side is a zone of sparse vegetation consisting of HcUanthus lenii- 

 cularis, Euphorbia coroUata, CciicJinis caroUnianus, Cyclolouia atrl- 

 plicifolium and Lcspcdcza capitafa. A line of dead plants of HcUan- 

 thus shows that the annual forward movement of sand is about 

 15 feet (5 m.). 



None of the four most efficient sand-ljinders is abundant in the 

 Oquawka area, either in the bunch-grass or on the deposits. The 

 blowouts there are mainly broad and shallow, with similarly broad 

 flattened deposits, spreading- out fanwise over a large area. They 

 are occupied especiahy by Cenchnis caroliiiiaiius and Paspaluiii scta- 

 ceiiin, with most of the species of the blowsand association. With 

 these are a few additional species, such as Sporoboltis cryptandrus, 

 Leptoloma cognatum, Patiicum pseiidopuhescens, Boutcloua hirsuta, 

 and Lespcde.ca capita ta. These five species do not occur on the reg- 

 ular lee slopes, and represent the deposit association in the narrower 

 sense. The Havana dunes resemble those of the Oquawka area 

 and have in general the same vegetation. Paspaluiii sctaccuin, Ccii- 

 chrits carolinianus, and Sporoholus cryptandrus are the usual species. 

 Deposits of this type occur rarely in the Hanover area also, and 

 then generally in connection with secondary l^lowouts which have 

 developed on parts of other larger ones. They are especially char- 

 acterized by the abundance of Diodia teres. 



The second type of deposit is marked 1)y the presence of effect- 

 ive sand-binders, and is best developed in the Hanover area. Usu- 

 ally Paniciiin ■virgatuui or Tephrosia virginiana appears immediately 

 on the youngest deposits and begins at once the building of the dune. 

 They may appear at a later stage, but in either case the result is the 

 same. Dunes may be held by Panicuin or Tephrosia or by both to- 

 gether. They seldom appear in association with Rhus, probably be- 

 cause the rate of increment of a Rhus dune is too rapid to permit 

 their growth there. Associated with them are a large number of 

 annuals, such as occur also on the lee slopes. They are most nu- 

 merous on the Tephrosia dunes, which are of relativelv slow growth, 

 and least numerous on the ra])idly growing RJuis dunes. Tf the 



