Ch. 2] 



TOPOGRAPHY 



47 



In humid, temperate, and cool climates, soils formed from granite 

 are frequently coarse-textured and acid, and they often have a super- 

 ficial humus layer of the mor type. Under similar climatic conditions, 

 soils derived from diorite and the most basic gabbros are usually 

 deeper and better supplied with calcium and phosphorus. They have 

 mull types of humus layers (Kellogg et al., 1949) . 



Owing to the interplay of soil-forming factors, the systems of soil 

 classification based upon geologic features lack generality. They may 



40 60 

 Feet 



Fig. 4. A slope sequence. Left: sketch showing deep road cuts through chaparral- 

 covered hills of "soft" quartz diorite. Right: exposed soil mantle illustrating 

 depth of soil in relation to slope. 



be valuable in regions of relatively uniform climate and constancy of 

 other soil-forming factors. If comparisons of soils of entire conti- 

 nents are attempted, the geologic systems do not portray the soil rela- 

 tionships correctly. 



Topography (Topofunctions and Toposequences) 



The topography or relief factor is complex, for it includes, in addi- 

 tion to degree of slope, length of slope, shape of slope, and exposure, 

 certain hydrologic features commonly referred to as drainage. 



A pure slope sequence, in absence of ground-water influences, is 

 depicted schematically in Fig. 4. On soft quartz diorite, south of 

 San Francisco, California, the dark soil mantle varies in thickness in 

 relation to slope features. These variations in soil depth are the re- 

 sult of erosion, soil creep, seepage, etc., all being functions of slope. 



Slope sequences become especially marked when capillary rise from 

 ground-water tables influences the soil profile. In humid regions, tern- 



