258 THE FORMATION 
covering on their faces, while the forces which produce disintegration oper- 
ate from above or below. 
317. Succession by binding aeolian soils. Dunes (thinia) are classic ex- 
amples of the reaction of pioneer vegetation upon habitats of wind-borne 
sand. The initial formations in such places consist exclusively of sand- 
binders, plants with masses of fibrous roots, and usually also with strong 
rootstalks, long, erect leaves, and a vigorous apical growth. They are al- 
most exclusively perennial grasses and sedges, possessing the unique prop- 
Fig. 66. Thicket formation (Quercus-Holodiscus-driodium), stage V 
of the talus succession. 
erty of pushing up rapidly through a covering of sand. They react by 
fixing the sand with their roots, thus preventing its blowing about, and also 
by catching the shifting particles among their culms and leaves, forming a 
tiny area of stabilization, in which the next generation can establish a foot- 
hold. The gradual accumulation of vegetable detritus serves also to enrich 
the soil, and makes possible the advent of species requiring ‘better nourish- 
ment. Blowouts (anemia) are almost exact duplicates of dunes in so 
far as the steps of revegetation are concerned; while one is a hollow, and 
the other a hill, in both the reaction operates upon a wind-swept slope. . 
Sand-hills (amathia) and deserts (eremia) show similar though less marked 
