98 



As the thicket becomes larger, portions of it may cUe away and leave 

 unprotected areas between smaller thickets. These bare spots are 

 usually one or two feet (3-6 dm.) below the general level of the dune, 

 indicating the erosion of the sand after the death of the sumach. 

 The steepest and highest dunes are invariably held by the sumach. 

 The highest ridges along the Mississippi river, which will be de- 

 scribed later, are usually crowned at their very summits by scattered 

 patches of sumach, whose large size and irregular outlines bear wit- 

 ness to their great age. 



Ccanothus ovatus behaves in a way similar to Rhus, but is much 

 rarer. It is more susceptible to injury by burial and does not pos- 

 sess so great a capacity for unlimited growth above the accumulating 

 sand. Ceanothiis dunes have been observed only along the Mississippi 

 river in the Hanover area. 



Besides these plants which are of chief importance, a few other 

 perennials or grasses may locally aid in building up dunes. They 

 are, however, generally temporary in their nature and persist only 

 during the summer when the plants are growing. Their size de- 

 pends upon the habit of the plant, but seldom exceeds a foot in height. 

 The larger dunes of this type are formed by Buphorhia coroUata, 

 Stipa spartea, Sporoholus cryptandrus, and Paspaluiii setacenm. 

 Even annuals, if growing in close patches, may accumulate an inch 

 or so of sand around them. Buphorhia Geyeri and Mollugo verticil- 

 lafa, the two common prostrate species of the deposits, do not ac- 

 cumulate sand, but their flat close mats prevent erosion if it is not too 

 rapid. They are sometimes seen growing on plateaus a half-inch 

 (1-2 cm.) in height, and corresponding with the shape and size of 

 the plant. Mollugo reaches its largest size on the deposits, forming- 

 mats sometimes two feet (6 dm.) across. 



Two general types of deposits may be distinguished ; those with 

 and those without effective sand-binders. Examples of the latter class 

 are broad and low with gentle slopes, and scarcely differ in vegetation 

 from the neighboring blowsand association, of which they may be 

 considered an extension. When the blowout is young and small the 

 annual increment of sand is but a few inches thick and affords opti- 

 mum conditions for the burial of seeds of the annuals. The young 

 deposits are accordingly covered with a dense growth of these 

 plants, and under such circumstances may be regarded, as far as the 

 vegetation is concerned, as extensions of the lee slope. If efficient 

 sand-binders do not appear on the deposits with the subsequent 

 growth of the blowout, the vegetation remains essentially the same, 

 except for the addition of various species of perennials. The most 



