95 



An interesting" phenomenon caused by the dead stems of Aristida 

 tuberculosa is frequently observed in the spring and early summer. 

 The dead culms of the preceding- year lie flat on the sand iDUt remain 

 firmly attached at the base. When blown by the wind they swing" 

 around in arcs of a circle and the tips scratch concentric curves in the 

 sand. The maximum diameter of these wind circles is about three 

 feet (8 dm.), and the average arc about 60 deg'rees, althoug^h some 

 complete circles were observed. 



THE DEPOSIT ASSOCIATION 



The chief difference in physical environment between the lee 

 slope and the deposits is the nature of the movement of the sand. On 

 the deposits sand is being added by the wind more rapidly than it 

 is being moved aw^ay, so that there is a gradual increase in height. 

 This soon leads to the development of a ridge, its size depending 

 naturally on the size of the basin which furnishes the sand. When 

 sand is piled up by wind alone, unimpeded by obstacles of any sort, 

 it is distributed rather uniformly over a considerable area. The 

 resulting dune has a very gentle windward slope and a slightly steeper 

 face. According to Cowles these slopes are about 5 degrees and 30 

 degrees respectively {iSgg: 191). Sand can not accumulate to a 

 g'reat depth because of the full exposure to the wind, and the dune 

 is sometimes so flat that it almost escapes attention. The blowouts 

 in the Oquawka area are particularly notable for their broad, flat 

 deposits, which are usually not more than three or four feet ( i m. ) 

 above the g'eneral level. Their vegetation differs but little from that 

 of the lee slopes of the blow-out. 



For building up the steeper dunes, so characteristic of the blow- 

 outs in the Hanover area, the wind alone is not sufficient. There 

 must be an obstacle of some sort which will cause the wind to drop 

 much of its load of sand at one spot, and which will also prevent its 

 removal by other winds from the same or different directions. This 

 obstacle must grow up with the dune, othewise it would eventually 

 be covered and its efficiency destroyed, and it must last through the 

 winter, when the wind is strongest. All of these conditions are met 

 only by plants, a few species of which become, because of their 

 growth habits, the chief dune-builders of the region. Cowles has 

 shown very clearly the necessary characteristics for a good dune- 

 forming plant {i8gg: 175-190). They are (i) a perennial life, 

 (2) the ability to spread radially by rootstocks (with certain ex- 

 ceptions), (3) the power of growing out into the light when buried 



