Several different effects were observed during the course of the 

 experiment. Very commonly the pebbles would be lightly to heavily 

 covered with a dusting of sand thrown into the water by organisms 

 digging near by. In all plots the pebbles, particularly the 

 flatter ones tended to sink a centimeter or two into the sand. In 

 the channel mouth plot, many of the pebbles covered small burrows, 

 most of which seemed to be dug by small fish, mainly gobies. Such 

 pebbles rather rapidly sank into depressions in the sand. In every 

 plot pebbles were displaced by the passage of organisms across the 

 sand; one of the largest bedrock cobbles was moved several centi- 

 meters and some of the smaller pebbles were displaced even farther. 

 A mediimi flat coral pebble was taken from the channels mouth plot 

 by an octopus, who used it as a door to his den about 5 meters 

 away. 



After 32 days, two parameters were measured for each pebble re- 

 maining in the plots. The first of these was the distance from 

 the top of the pebble to the surface of the saiad above or below, 

 which indicates the degree of burial of the pebble. The second 

 was the distance from the base of the pebble to the sand surface 

 above, which indicates the depth to which the pebble sand and, 

 unlike the first parameter, is independent of the size (thickness) 

 of the clast. 



By averaging the data for each type of clast from all four plots, 

 the depth of burial and depth of sinking can be related to clast 

 size, shape, and composition. The averages indicate that depth 

 of sinking, approximately 2 cm for all types, is unrelated to the 

 dimension or the density of the clast. The depth of burial is 

 related to size and shape but not to composition of the clast. 

 Such a relation to size and shape is predictable; for pebbles and 

 cobbles which sink more or less the same distance into the sand, 

 the smaller and flatter are likelier to become buried. 



The experiment demonstrated that the depth of sinking and burial 

 is related to the style of bioturbation. In the channel mouth 

 plot, where undermining was active, the top of the pebbles lay 

 on the average nearly even with the sand surface; in the other 

 three plots, the tops of the pebbles averaged 2.5 to 3.5 cm above 

 the sand surface. The average depth of sinking of the pebbles in 

 the channel mouth plot was nearly 4 cm, whereas that in the other 

 plots was only about 1-2 cm. Of the clasts in the channel mouth 

 plot, the most deeply buried were a large flat bedrock cobble that 

 sank to a depth of 8 cm and lay (from its top) 3.5 cm beneath the 

 surface and a small flat coral pebble that sank to a depth of 

 5 cm and lay (from its top) 4 cm beneath the surface. The plots 

 in the sand strip and the patch reef sites contained 2 or 3 pebbles 



VI-29 



