56 



GEOMORPHOLOGY: 



1^ PARALLEL DUNES 

 longitudinal transverse 



ri I . * ^ . Ji . 



SEIF 



ml 



^^ prevailing wind directions 



Figure 4.18 The kinds of dunes as produced by different prevailing 

 winds. 



the form of alluvial plains, gullies, canyons, deep 

 ravines (called waddles), and temporary alkali lakes. 

 Although flood waters sculpture such features, the 

 features are modified by wind action. In the greatest 

 modification, wind removes fine particles, usually 



: deflation ' 



desert pavement aeolian deposits 



Figure 4.19 Features caused by wind erosion. Such land forms 

 usually ore best developed in orid climates. 



leaving only the coarsest sand and rocks. This ac- 

 cumulation of rocks forms the characteristic substrate 

 of arid areas and is called desert pavement. (Contrary 

 to popular impressions, sandy areas and dunes do not 

 constitute the predominant land forms of deserts.) 

 The desert pavement frequently is stained shiny 

 brownish to blackish by iron-manganese compounds 

 (and perhaps polished by the winds), forming desert 

 varnish. In addition to desert pavement, deserts also 

 have the various features already mentioned as being 

 typical of wind erosion. 



ORGANISM ACTIVITIES 



Organisms using products within their environ- 

 ment may construct land forms that may or may not 

 be considered destructional. The most pronounced 

 landscape effects are accomplished by corals, animals 

 that produce reefs in certain parts of the world; how- 

 ever, certain other organisms, plant and animal, also 

 modify landscapes. 



PLANTS 



Vegetation is a primary agent in the process of fill- 

 ing in lakes. This organism activity occurs in a very 

 orderly sequence of plant growth forms, including 

 submerged, floating, and emergent plants. Further 

 details of the phenomenon of plant succession in 

 water are left for later discussions of ecology. All 

 that need be mentioned here is that each successive 

 growth form accomplishes greater filling of a body of 

 water, and succession ends with an essentially flat 

 area of land having the characteristic vegetation for 

 the locality. 



Trees and shrubs sometimes accumulate in a 

 stream to form a conspicuous raft or dam. A raft is a 

 semi-floating structure that is only moderately ef- 

 fective in hampering stream flow. A dam is more an 

 integral part of the landscape and is efiective in cre- 

 ating an upstream body of water. 



ANIMALS 



The most prominent land forms produced by ani- 

 mals other than corals are those formed by beavers 

 and other rodents, termites, ants, and earthworms. 

 Beavers cut woody vegetation and use it to build 

 dams that trap streams, thereby causing deeper and 

 quieter waters where alluvium collects and successive 



