GEOLOGICAL IMPORTANCE OF SEDIMENTATION 441 



The result as observed in nature is an added scour in regions 

 of high tides by which the bottom is cut away, tidal marshes at the 

 heads of bays or other suitable places are built with great rapidity, 

 and these are dissevered from each other by tidal channels draining 

 the marshes, which in the more striking cases may be sufficiently 

 deep and wide even at low tide for the purposes of commerce. In 

 time, if subsidence does not take place, the tidal marshes become 

 raised by accretion through aeolean and organic action until reclaimed 

 from the sea, and the littoral zone is diminished as before to a cer- 

 tain stable width. 



CONDITIONS FOR PRESERVAL OF THE SEDIMENTARY RECORD 



It has been seen that in the making of the sedimentary record but 

 an insignificant portion would be contributed by the littoral zone. 

 There are still, however, two factors to be considered — that of the 

 preserval of the record, and that of its ultimate exposure to observa- 

 tion through partial erosion. 



In order to become part of a permanent geological record, the 

 sedimentary structures must be preserved without obliteration, first, 

 until buried and lithified, and, secondly, indefinitely protected from 

 erosion until some new cycle of activities proceeds to destroy it and 

 while so doing transitorially exposes it to observation. 



PRESERVAL OF THE CONTINENTAL AND MARINE RECORDS 



In regard to the river sediments, slow subsidence of the region or 

 elevation of an adjoining region is necessary for their continual 

 formation. Each layer of sediment from the flood waters is laid 

 down upon the previously dried and hardened layer, and there is 

 therefore not much tendency to erase the record made on the previous 

 surface except in the lines of the channels. Soil beds, swamp deposits, 

 mud-cracks, and ripple-marks are consequently abundantly recorded 

 in fluviatile formations. Unless, however, subsidence carries at least 

 the basal portions of the deposits below the ultimate base-level of 

 erosion, the formation will be finally destroyed, as is illustrated by 

 the present erosion of the Tertiary river deposits of the Great Plains 

 facing the Rocky Mountains. In river deltas, however, the proper 

 conditions arc observed to occur. Here the upper limits are but 



