74 MAN ON THE LANDSCAPE 



balds, galls, slopes, bad lands, buttes, scours and bars. 



The invasion of such bare areas by pioneer plants depends in 

 considerable part on the type of surface, its stability for a period of 

 time, the water supply, degree of slope, and insolation. Steep slopes 

 in Palestine eroded down to bare rock are very discouraging to life, 

 as are broken cliff faces in the Cascades, and the rocky bottoms of 

 gullies. 



Wind erosion creates dunes, sand hills, and blowouts. The fine 

 material is lifted and carried away. The once good loam of the 

 dust bowls has lost its light humus, fine clay, and much of the 

 mineral-rich silt by this process. Of the original topsoil only sand 

 remains over large areas ; and it may be on the move, slowly. 



Ice denudes narrow strips along streams and lakes, along glacial 

 beds and margins, and leaves bareness where the ice has retreated. 



Gravity causes slipping and slumping of wet clay slopes, hill 

 crests, stream banks, shores. Land and snow slides denude the 

 areas involved, cliffs break off, exposing bare surfaces. 



Deposits of barren material result from all the above erosional 

 actions, also from volcanic and ground water actions. Surface 

 waters deposit flood plains, deltas, sand bars, reefs, alluvial cones 

 and fans, beaches, spits, and channel deposits. Ground water, rising 

 as springs and geysers, deposits lime, silica, or salts. Wind blown 

 soil deposits are called loess (if fine), or dunes (if sand), or volcanic 

 dust. Glaciers in the past have laid down deposits over vast areas, 

 the materials ranging from great rocks to the finest rock flour or 

 clay. Gravity produces the talus slopes below crumbling cliffs. Vol- 

 canoes emit cinders, rocks, lava, dust, ash, mud, sinter. Volcanic 

 lava deposits are very resistant to environmental evolution. Earth- 

 quakes, and the possible (but rare) rapid uplift or subsidence, may 

 cause bare areas. 



In all these cases, a succession of life forms usually begins at 

 once. 



Climatic Causes. Destruction of existing vegetation may result 

 from drought, wind, hail, snow, frost, lightning, or evaporation. 

 Drought is most destructive in areas where water is a critical factor, 

 as on the Great Plains. Repeated hailstorms have forced abandon- 

 ment of certain plains areas. Elsewhere great damage is often done 

 by hail to crops, meadows, broadleaf forest and scrub. Snow damage 

 is serious only in polar and alpine regions. Elsewhere it may benefit 

 vegetation by protecting it from freezing and by serving as a water 

 supply. Lighting as a cause of fire is an important factor in national 

 forests. In unprotected or heavily populated forest regions, fires 

 started by people make lightning a lesser evil, comparatively speaking. 



Evaporation, perhaps speeded by wind, may dry up ponds which 

 have an unreliable water supply source. The pond life dies. A land 

 succession begins, only to die in turn if the water returns. Flooding 

 of depressions and lowlands may persist long enough to destroy the 



