SECT. 2] 



MICROTOPOGBAPHY 



467 



spread over a depth of 5 cm giving a mean concentration at that depth of about 

 0.05 g/cm3 for a concentration of surface material of 1 g/cm^. 



In this analysis the problem has been treated as if the disturbance took place 

 on one horizon only. In fact the sediment is slowly accumulating and the 

 surface of activity is slowly rising. This means that the material of a narrow 

 stratum will be diffused both upwards and downwards resulting in a general 

 blurring of the stratification. Clearly many factors are responsible for the extent 

 of disturbance by organisms on the bottom. The ecological conditions will 



Stn. 3734 

 5340 m 



Stn. 3737 

 5280 m 



10 



DEPTH 



1 

 10 O 



BELOW SURFACE 

 /, cm 



Fig. 38. Disturbance of the surface sediment by bottom-living animals at three stations 

 in a deep-ocean basin, measured from bottom photographs and core sections. 



Abscissa — Depth {z cm). 



Ordinate — Mean concentration of surface material at depth ;: for unit mass per 

 unit area on the surface ('"/'"o f'^i ~^)- 



holes l^:':-:-:-:-:-:-! mOunds 



tracks I I galleries 



determine first of all the faunal populations. The extent of the disturbance due 

 to their activity will depend on the hardness and composition of the bottom, 

 and on the rate of sedimentation in raising the surface of their habitation. 



E. Description of Microtopograx)hy by Physiographic Regions 



a. Continental shelves (Figs. 13, 20, 22, 34) 



According to the classification of Heezen, Tharj) and Ewing (1959) the 

 continental margin can be divided into three categories, the first category 

 consisting of the continental shelf, the second the epicontinental marginal seas 

 and the last the marginal plateaus. These regions have been more closely 

 studied i^hotographically than any of the deeper ones since the bottom is more 

 accessible. Early work on underwater photography by Ewing, Vine and 



