CRITERIA AS TO MODES OP ORIGIN OP SEDIMENTS 355 



lacustrine deposits on the one hand and terrestrial deposits on the other. 

 Terrestrial deposits may in turn be classified into fluviatile, torrential, 

 glacial, and eolian. 



Lakes are characterized by being permanent water bodies as measured 

 by the changes of decades, centuries, or longer periods. Constructional 

 alluvial plains, on the other hand, are largely turned into temporary lakes 

 during the annual season of flood and give thereby some similarities to 

 lacustrine deposition, but are drained during the dry season, and their 

 life is that of the land. 



In the upper part of the alluvial plains the river grades are steeper, 

 the waste coarser, the flooded condition more brief. The marks of sub- 

 aerial exposure are more dominant, as seen especially in the greater oxida- 

 tion of sediments, giving red rocks on cementation, and the greater 

 paucity of fossils. In the lower part of the plains, on the contrary, the 

 grades are flat, commonly less than a foot per mile; the sediments are 

 finer and more impervious to ground waters; the flooded condition is of 

 greater annual duration, and in playa lagoons and swamps may in fact 

 be permanent for a series of years. The rivers, owing to minor climatic 

 fluctuations, will during one series of years entrench themselves and 

 minimize the flooding of the plains. During other series of years they 

 may rise high, flood their plains broadly and for longer periods, depositing 

 an excess of waste- The channels also shift and the natural levees are 

 built upward to above the level of all save the highest floods. In the 

 same geologic section, therefore, river action is characterized by variable 

 relative durations of exposure to flood waters and exposure to the air. 



True lake conditions, then, as distinguished from the temporary and 

 subordinate water bodies connected with fluviatile deposition, are char- 

 acterized on the margins by wave-formed sands and conglomerates, as 

 distinguished from the current deposits of streams. The basins of per- 

 manent lakes must be sufficiently deep to have the greater portion of 

 their bottom below wave base. The shore deposits of lakes will conse- 

 quently, for any one horizon, be essentially marginal, and the greater area 

 of the lake will be marked by clays and fine silts deposited from suspen- 

 sion. Regular, even lamination, giving rise to paper-thin shales, will be 

 typical. Lenses of sand, but especially beds of conglomerate, will be 

 absent. The presence or absence of sand is in itself, however, of doubtful 

 value as a criterion. Conglomerates, on the other hand, when they are 

 of wide-spread occurrence, are of the highest significance. 



For floodplain conditions there are a number of definite criteria de- 

 pending on the texture of the deposit and the climate under which they 

 are laid down. Where the deposits are argillaceous and the climate is 



