484 BOUNDARIES OF THE SEA 



pent! \-ery much on what classification of en\'ironments is followed, 

 and there is considerable danger of reasoning in a circle. If the 

 classification of environments is based mainly on grain sizes, the 

 transporting mechanisms are bound to come out on top in the 

 quest for the chief determining factor. If based on animal remains, 

 then living conditions would be the more important item. 



I shall attempt to show that differentiation between environ- 

 ments by major topographic features is the most useful for epi- 

 continental sediments because sources, transportation, and local 

 conditions depend in large measure on the configuration of the 

 crust. Topographic factors of importance are: depth, bottom slope, 

 extent, topography of adjacent land and of the adjacent sea floor, 

 degree of communication with the open sea, and connection with a 

 source of freshwater. The principal nontopographic factor is 

 climate (precipitation, evaporation, wind, temperature). 



It would leal too far to discuss all these factors separately and in 

 detail, but a few may be considered. Depth is of importance in that 

 it governs the penetration of light and therefore influences plant 

 growth and in that it limits the degree to which waves can act on 

 the bottom and determines whether periodic emergence can occur. 

 Slope is the main factor in slumping of bottom sediment and the 

 setting off of turbidity currents. It also affects the action of waves 

 and currents. Extent is significant because of its relation to the 

 length of fetch for waves. Coasts will be subject to greater attack 

 and the bottom w ill be churned up more intensively with increas- 

 ing size. 



Mountainous land, in contrast to low-lying country, will not 

 only influence local climate, but will also deliver more and coarser 

 sediments of local origin. On the other hand, there is less chance 

 of the all-important influence of a large ri\'er mouth with its vast 

 supply of fine sediment derived from a continent-wide area. The 

 topography of the adjacent sea floor is in many cases of the utmost 

 importance. Thus a neighboring basin can trap sediment and 

 result in winnowing of its surroundings. The nondepositional edges 

 of the continental shelf are large-scale examples of this. Sediments 

 can be provided to a slope from nearby shallow water. Sliding and 

 turbidity currents may carry them deeper. A depression, even in 



