4 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



sediments, because their constituents have settled under the influence 

 of gravity, albeit only a very small distance. For more detailed ac- 

 counts of sedimentation see the works of Gilbert (1890), Walther 

 (1894), Grabau (1913), Twenhofel (1932, 1939), Boswell (1933), 

 Hatch, Rastall, and Black (1938), Trask (1939), Shrock (1948), and 

 Pettijohn (1949). 



Sedimentation consists of five fundamental processes: (1) weather- 

 ing, (2) erosion, (3) transportation, (4) deposition, and (5) diagenesis 

 or consolidation into rock. 



WEATHERING 



Most of the constituent particles of sediments are derived from rocks 

 or earth materials that have been more or less weathered. The original 

 constituents of sediments, of course, can come from unweathered rock, 

 but weathering and concomitant solution are the dominant processes 

 by which rock or soil constituents are transformed into a state whereby 

 they can be eroded and transported to some place of deposition. 

 Weathering is essentially a process of soil formation; for details the 

 reader should consult standard references on the subject such as: 

 Baver (1948), Clarke (1924), Goldich (1938), Goldschmidt (1937), 

 Jenny (1941), Leith and Mead (1915), Polynov (1937), or Reiche 

 (1945). 



Weathering consists of two fundamental processes: (1) mechanical 

 disintegration and (2) chemical solution. Living organisms, particu- 

 larly microorganisms, also play an important role, but in the last 

 analysis organic action is essentially a question of physical disruption 

 or chemical solution of rock and earth constituents. 



Five fundamental factors, as Jenny has pointed out, influence weath- 

 ering: (1) parent rock material; (2) climate, particularly the tempera- 

 ture and rainfall; (3) physical environment in which the weathering 

 takes place, especially topography or shape of the land surface; (4) 

 length of time the processes operate; and (5) action of organisms. To 

 this might possibly be added a sixth factor — the dynamics of the 

 environment, that is, the ratio of the rate of weathering to rate of re- 

 moval of weathered particles by erosion. If erosion is rapid, soil-form- 

 ing processes cannot proceed very far toward maturity before the con- 

 stituents are removed. Moreover, these principal variables that affect 

 weathering and soils to a considerable extent are mutually inter- 

 related. 



Mechanical weathering is largely a question of thermal expansion 

 and strength of the mineral constituents. Mechanical disintegration 



