The upper 22 inches of the sediment is medium to coarse carbonate 

 sand containing pelecypod, gastropod, and coral fragments and 

 represents material deposited on the algal plain. Beneath this, a 

 drastic change in grain size and composition occurs. Approximately 

 one-third of the clasts in the lower part are well-rounded, non- 

 carbonate cobbles with a maximum diameter of 6 inches. The other 

 two-thirds are rounded clasts of the corals Montastrea annularis , 

 Siderastrea siderea , Acropora cervicornis and Porites porites plus 

 smaller fragments of corals, pelecypods, and gastropods. The corals 

 making up the larger clasts are species that do not live on the 

 algal plain at present. This fact, plus their roundness and the 

 presence of rounded bedrock clasts, suggests deposition under higher 

 energy conditions than those now present at these depths on St. John. 

 The deposits are readily explained, however, by the postulated 

 stillstand of the sea at approximately 43 feet below the present mean 

 sea level. 



An erosional platform more than 1000 feet long and varying in width 

 from 30 feet to approximately 200 feet is cut into the bedrock at 

 depths of 45 to 55 feet below the present mean sea level along the 

 southwest end of Cabritte Horn Point. Landward of the eroded bedrock 

 platform is a bedrock cliff or large blocks of bedrock rubble. The 

 cliff and the rubble slope rise to a depth of 30 to 35 feet where 

 another platform is partially cut into the bedrock. 



Additional evidence for a stillstand at 43 + 3 feet is at the east 

 end of St. John, where an erosional platform is cut into the bedrock 

 on the east side of Ram Head at a depth of 44 to 50 feet , and broad 

 constructional platforms exist at depths of 40 to 50 feet at Eagle 

 Shoal and at LeDuck Island. The beachrock at 43 feet may represent 

 part of the beach of Vaughn's (1916) 36-60 foot erosional platform. 



RELICT SEA LEVEL AT 24 +2 FEET 



The five other extensive areas of beachrock conglomerate (figure 1) 

 are widely scattered, but the base at their seaward edge lies 

 consistently beneath 22 to 26 feet of water. The depths of the 

 landward margin of the deposits are more variable, ranging from 

 15 1/2 to 8 1/2 feet below mean sea level. The deposits extend 

 parallel to the present shoreline for distances of 27 feet to more 

 than 700 feet, and their width varies from 9 feet to 108 feet. In 

 cross-section the conglomerate deposits are wedge shaped, ranging 

 from 2 to 7 feet (averaging 5 feet) in thickness seaward, thinning 

 to 1 1/2 to 2 feet landward. The margins of the s\ibmerged deposits 

 are vertical and usually are undercut as much as 15 feet on the 

 lower beachrock-bedrock contact. Landward the beachrock rests on 

 bedrock; seaward it may rest partly on bedrock or on carbonate sand 

 or on coral rubble. 



Profiles across the beachrock deposits (figure 3) show them in 

 relation to the bottom slope and coral reef development. Seaward 

 of the beachrock, the bottom slope is gentle and coral reefs are 

 fully developed. Landward of the beachrock, the bottom slope 



VI-75 



