DEPOSIT. 47 



impossible, so that extensive areas in Utah and Nevada remain absolutely 

 sterile under present conditions (plate 4 b). 



Bare areas due to deposit by wind. — The principal wind deposits are (1) 

 sand, chiefly in the form of dunes; (2) loess; (3) volcanic dust. Of these, dunes, 

 both inland and coastal, are much the most important at the present time. 

 Loess, while covering enormous areas in the valleys of the Mississippi, Rhine, 

 Danube, Hoang-Ho, and other rivers, is not in process of formation to-day, 

 and the prisere developed upon it can not now be traced in the actual course of 

 development. Deposits of volcanic dust are infrequent and localized, and 

 cover relatively small areas. They are unique in the suddermess and com- 

 pleteness with which the area is covered and in their absolute steriUty. 



Dunes are classical examples of deposits which initiate succession. Their 

 wide distribution and striking mobility have made them favorite subjects of 

 investigation by both physiographer and botanist, and there is probably no 

 other initial area and succession of which we know so much. In spite of their 

 characteristic topography, however, dunes affect succession by virtue of insta- 

 bility and water relations, and not by form. This is shown by the inland dunes 

 or sand-hills of the Great Plains. Hills, deep hollows or blow-outs, and sandy 

 plains show the same development, regardless of their differences of form. In 

 all of these the controlling part is played by the sand-catching and sand- 

 binding plants, usually grasses, which act as pioneers. The chief reactions are 

 three, namely, fixation of the sand, gradual accumulation of humus, and 

 decrease of evaporation and increase of holard (plates 1 a, 22 a). 



Dime plants have often been regarded as halophytes, but since Kearney 

 (1904) has shown that this is rarely true of strand species, it seems impossible 

 to distinguish initial dune areas on the basis of salinity. This is borne out by 

 the similarity of the early stages of shore dunes, whether lacustrine or marine. 

 As a result of their location these often differ much in the later stages, and 

 especially in the climax. Inland dimes occur in widely different climatic 

 regions and differ from each other in population as well as from coastal dimes. 

 This is well illustrated by the sand-hills of Nebraska, the "white sands" of 

 southern New Mexico, and the barchans of Turkestan. 



Deposit by ice and snow. — Of these agencies, glaciers have been of much the 

 greatest importance in the past, though their action to-day is locahzed in 

 mountains and the polar regions. The effect of shore-ice, though interesting, 

 is rarely suflacient to produce a distinct result. The influence of snow is often 

 striking and decisive, but it is also pecuKar to mountain regions. Naturally, 

 all of these show a close dependence upon water, as is seen in the water relations 

 of the resulting soils. 



Bare areas due to deposit by glaciers. — From the standpoint of succession 

 there is no essential difference between glacial and fluvio-glacial deposits. 

 This is readily explained by the fact that glacial materials are really deposited 

 in water at the time of general melting. The effect upon the new soil is prac- 

 tically the same as when it is water-laid after being carried for some time from 

 the glacier. In the case of drumlins, indeed, it seems probable that they may be 

 due either to fluvio-glacial deposits or to erosion by an ice-sheet of an antecedent 

 ground-moraine. Hence it seems immaterial whether the deposit is glacial, e.g., 

 lateral and terminal moraines, ground-moraines, or drumlins, or fluvio-glacial, 

 such as valley trains, outwash plains, eskars, kames, or drumlins (plate 7 a). 



