Rock Bottom 



63 



(Fowler, 1958). Through these detailed 

 studies the number of rock samples avail- 

 able for identification has been considerably 

 increased. About 450 samples contain rock 

 other than phosphorite and are judged to be 

 in place according to the following criteria: 



1. Fresh fractures. 



2. Large size of individual rocks. 



3. Abundant rocks of similar lithology. 



4. General angularity. 



5. Fragile or poorly consolidated rock. 



6. Catching of the dredge on rock firm 

 enough to stop the ship's progress abruptly. 



7. Strong pull on the cable as measured 

 by an accumulator spring or other device. 

 In addition, about 250 other samples con- 

 tain rocks that had probably been trans- 

 ported, as suggested by 



1 . Varied lithology. 



2. General rounded character. 



3. Small size of individual rocks. 



Rock bottom has also been observed locally 

 by divers (Shepard, 1949; Menard, et al., 

 1954), and more than 200 photographs of 

 rock bottom (Fig. 59) have been made by 

 remote-control cameras (Shepard and 

 Emery, 1946; Emery, 1952<3). Irregular 

 topography shown by fathograms (Figs. 33 

 and 55) reveals the larger features of rocky 

 bottom as peaks of uneven height and slopes 

 too variable in direction and steepness to be 

 depositional surfaces. In short, each of 

 several methods of investigation covers a 

 different portion of the size spectrum of 

 bottom features, generalized approximately 

 as follows: echo sounder, more than 100 

 feet; camera and diving, 100 to 1 foot; and 

 dredging, 1 foot and less. Still other indi- 

 cators of rock bottom are the presence of 

 kelp and nearness to rocky shores. 



When plotted in map form (Fig. 60), rock 

 bottom based on samples alone is present in 

 a variety of environments but nowhere is 

 the sample density very great. Use of the 



Figure 59. Top of Osborn Bank (lat. 33°22.0', long. 119°'02.9', 190 feet). Irregular masses of rock, covered byhydroids 

 and encrusting organisms, project through a blanket of coarse foraminiferal glauconitic sand having abundant ripple 

 marks. Bottom edge is about 10 feet wide. November 19, 1951. (AHF 2086.) 



