SECT. 2J MICROTOPOGKAPHY 439 



2. Classification of Microtopographic Features 



Three types of feature can be studied from random photographs of the sea 

 floor and all pictures are analyzed under these three headings. 



A. Surface geology. The nature of the bottom is listed under one or more of 

 the classes ; bedrock, boulders, cobbles, pebbles and granules, sand and mud, 

 according to Table I, and rock types are identified where possible. 



Table I 



Classification of the Nature of the Bottom according to Size. (After 



Wentworth, 1922.) 



B. Indications of current activity. Ripple marks, scour pits, sediment cover 

 and the orientation of fauna are related, if possible, to current strength and 

 direction. 



C. Benthic fauna. The fauna is identified and its ecology studied. Organic 

 detrital material can sometimes be identified. Qualitative and quantitative 

 studies are made of the disturbances of the sediment surface by benthic 

 organisms. 



A. Surface Geology 



a. Bedrock outcrops 



Whereas fractured rocks in a dredge may suggest the existence of bedrock, 

 it can only be proved by direct observation or photographs. In searching for 

 suitable outcrops to sample, experience of the correlation of photographs of 

 outcrops with echo-sounding characteristics is invaluable. Most outcrops have 

 been observed, as one would expect, on steep slopes or cliffs (Fig. 2), but 

 comparatively recent submarine lava flows may solidify on a nearly level 

 surface (Fig. 3). The majority of outcrops have proved to be basaltic and 

 associated with seamounts of volcanic origin, although recent dredgings have 

 indicated that some seamounts are capped by limestone (Fig. 4). 



Bedrock not associated with topographic highs has been found in the 

 Romanche Trench (Fig. 5) at 7500 m where erosion and slumping have been 

 active, and on the cliffs of turbidity current cut channels in the abyssal plains 

 (Fig. 6) (Laughton, 1960). 



The identification of rock types from photographs is made difficult by a 

 coating of manganese dioxide that accumulates on the rock surface, and by 



