EROSION. 41 



degrees, which present wide conditions for ecesis. For this reason it offers 

 material of the first importance for reconstructing the course of succession 

 and relating the various stages. Superficial erosion to varying depths is 

 likewise a ready source of developmental clues. When produced artificially 

 under control both processes fiu^nish an invaluable e3q)erimental method of 

 studying succession by denuded quadrats and transects (plate 5 a). 



Bare areas due to water erosion. — ^The important areas laid bare through 

 erosion by water are: (1) guUies, ravines, and valleys; (2) sand-draws; (3) 

 washes; (4) flood-plains and river islands; (5) banks; (6) lake-shores and sea- 

 shores; (7) crests and slopes; (8) bad lands; (9) buttes; (10) monadnocks. In 

 some of these, such as stream-banks, the erosion is chiefly or wholly lateral, 

 and hence more or less local and fragmentary. In others, e. g., washes and 

 flood-plains, the erosion is superficial and general, and is often intimately 

 associated with deposition. The majority of them are the result of inter- 

 action of both methods, as illustrated in the production of a gully or ravine or 

 a sea-shore. Bad lands and beaches represent, perhaps, the ejctreme condi- 

 tions of erosion, in which colonization is all but impossible. In all, the success 

 of initial invasion depends upon the kind of surface laid bare and the water 

 content as determined by the surface, the slope, and the climatic region. The 

 form and nature of the area itself are important only as they affect these 

 factors (plate 2 a). 



Bare areas due to wind erosion. — ^The most characteristic areas of this sort 

 are wind-denuded areas of dimes and sand-hills, particularly the well-known 

 blow-outs. Related to these are the strands from which the dune-sand is 

 gathered by the wind, and the plains of rivers, lakes, and glacial margins from 

 which sand-hills and loess deposits have been formed by wind action. Wind 

 is a powerful factor in the erosion of strands, but at the present it is of slight 

 importance in flood-plains and lacustrine plains as compared with its action 

 in Tertiary and Quaternary times. The abrasion and removal of material 

 from exposed peaks, ridges, and slopes of rocks is constantly going on, but it 

 does not often assmne such striking proportions as are found in the characteristic 

 mushroom rocks found in the Rocky Mountains. It plays some part, and 

 often a controlling one, in the Uchen and moss stages of the rock succession 

 (plates 1 A, 3 b). 



Bare areas due to gravity. — Many areas owe their origin to the action of 

 gravity on material freed by weathering, or in some cases by water erosion. 

 In the case of mountains, relatively large areas are exposed by exfoUation, 

 crumbling, or slipping. In certain mountain regions with heavy snowfall, the 

 effect of gravity on the snow-fields produces numerous characteristic snow- 

 slides in which the ground is often swept bare. Crumbling and sUpping are 

 also universal processes on the steep slopes of crests and hills and along stream- 

 banks and lake and sea shores everywhere. From their hardness, instability, 

 or dryness, and the steep or vertical faces, such areas are among the slowest 

 to be invaded as a rule. In consequence, they often permit the persistence 

 of initial stages or their reciu-rence long after they have disappeared elsewhere 

 (plate 5 b). 



Bare areas due to ice action. — ^At the present time, the effect of ice in pro- 

 ducing bare habitats is confined to wind-exposed shores and to the margins 

 of glaciers. In the latter case the final condition of the area is naturally due 



