terraces. Barrier reefs at the outer edges of the two deeper 

 terraces presxjmably were formed during submergence of the 

 St. John platform. Meyerhoff (1926) postulates an older geologic 

 age for the formation of the Virgin Island platform. The plat- 

 form initially underwent deposition in the Oligocene. Emergence 

 and fluvial dissection of the platform followed in the late 

 Tertiary. Two stages of recent submergence drowned the platform 

 and established a stillstand 27 feet below the present sea level 

 (Meyerhoff, 1926, p. 119) . The sea then advanced to the present 

 level. A fossil coral deposit discovered during this investiga- 

 tion on White Point at an elevation of 65 feet above the present 

 sea level may represent another strand deposit. 



This study investigates the most recent part of the complex sea 

 level history, that preserved beneath the sea in depths of less 

 than 70 feet. 



BEACHROCK 



Beachrock consists of sand or coarser clastic beach material 

 cemented by calcium carbonate into an indurated deposit. The 

 cementation takes place in the intertidal zone. The mechanism of 

 beachrock cementation, though not fully understood, is discussed 

 by Ginsburg (1953), Emery and Cox (1956), Kaye (1959), Russell 

 (1960, 1962), Deboo (1962), Stoddart and Cann (1965), and others. 

 Beachrock deposits lying above or below the present day intertidal 

 range therefore indicate a former position of sea level. The 

 submerged conglomerates on the south side of St. John are inter- 

 preted herein as initially forming at lower sea levels . The 

 depths of the submerged beachrock represents the approximate 

 depth of a temporary stillstand below the present mean sea level. 



The distribution of both the contemporary and the submerged 

 beachrock along a part of southeast St. John is discontinuous and 

 patchy (figure 1) . Along the present shoreline, beachrock exists 

 only on the cobble-boulder beaches at the head of the bays or on 

 the margins of the bays where the cobble-boulder beaches are 

 developed. The submerged beachrock deposits are extensive, long 

 and narrow, and trend parallel to the present shoreline. The 

 deposits are best developed on the west side of the bays. Of the 

 six occurrences of stibmerged beachrock, five are at depths of 22 

 to 26 feet on the seaward border, extending to 8 1/2 feet on the 

 landward border below the present mean sea level. The sixth 

 deposit off Cabritte Horn Point lies between 40 and 46 feet below 

 mean sea level. The submerged deposits stand above adjacent 

 coral reefs and bedrock outcrops. 



Beachrock along the present shore and that which is submerged 

 consists of poorly sorted, well-rounded carbonate and noncarbonate 

 pebbles, cobbles, and sand cemented by calcium carbonate. The 

 carbonate clasts consist of coral, hydrozoan, and algae fragments. 

 The noncarbonate clasts are composed of volcanic material derived 

 from the erosion of the bedrock on St. John. Carbonate fragments 



VI-71 



