SECT, 2] MICROTOPOGRAPHY 463 



is that they are sometimes grouped in regvilar patterns such as straight hnes or 

 circles, six or eight holes comprising the group (Figs. 31 and 32). 



Among the most spectacular features in the sediment of the sea floor are the 

 tracks made by mobile fauna. The largest track so far observed is 15 cm wide 

 showing a central ridge, parallel lines of small depressions on either side, and 

 transverse markings on the ridge (Fig. 33). It seems likely that these tracks 

 were made by a mud feeder such as a holothurian, the transverse marks being 

 made by scooping the mud into suspension, and the depressions by its feet. 



« •"<• 



Fig. 33. 41° 18']Sr, 14° 23..5'W. Depth 5341 m. Iberia abyssal plain. Area of picture 1 by 1 m. 

 (Photo by N.I.O.) 



Broad tracks (15 cm wide) in Globigerina ooze made by a mud-feeding animal on 

 the surface. Note the regular indentations (5 cm apart) either side of the tracks and 

 the transverse markings (top left) suggesting a mud-scooping action resulting in a 

 central ridge. 



Smaller but more numerous tracks have been identified as having been made 

 by smaller holothurians and sea-urchins (Fig. 34). Unidentified animals with 

 long thin conical shells (resembling scaphopods), with two extended arms, have 

 often been photographed at the end of thin and rather angular tracks (Fig. 35). 

 Occasionally fish feeding near the bottom leave marks where the lower tip of the 

 tail touches the sediment surface. 



The greatest contribution to the disturbance of the sediments is due, how- 

 ever, to the galleries and dwelling holes of burrowing worms and other infauna 

 which are visible to the camera only where they come to the surface. However, 

 they are shown up well in cross-sections of cores in the region where the sedi- 

 ment colour changes. Galleries bored in the lower material become filled 

 subsequently with the upper material and this constitutes a mixing process. 

 The only evidence of these galleries in underwater photographs is the sudden 



