steepens and is irregular. Corals grow only as isolated colonies 

 on the beachrock itself and landward on the bedrock and bedrock 

 nibble surfaces. The geometry of the submerged beachrock deposits 

 resembles closely that of present day beachrock deposits on the 

 east side of Salt Pond Bay. 



At Yawzi Point cemented conglomerates at depths of 15 to 23 feet 

 bridge the bedrock walls across four channels. The bottom of the 

 beachrock bridges lies 3 to 6 feet above the floor of the channels. 

 Coarse clasts, such as cobbles and boulders, are not found on the 

 surrounding bedrock surfaces nor in the channels, indicatir^g that 

 higher energy conditions than present during this investigation have 

 removed uncemented material from beneath the beachrocks . The higher 

 energy conditions may have occurred during a lower sea level or 

 during recent storms. Beachrock exists in Salt Pond Bay at a depth 

 of 6 feet and in Kittle Bay at a depth of 10 feet. Both deposits 

 are at depths below beachrock formation on the present shoreline, 

 indicating possible foirmation during a rising sea level. 



Other evidence supports a stillstand of the sea at approximately 

 24 feet below its present level. Uncemented concentrations of non- 

 carbonate pebbles are at depths of 20 to 25 feet on carbonate sand 

 flats on the east side of Salt Pond Bay and Grootpan Bay. Such 

 concentrations may have formed along a previous shoreline. Erosional 

 platforms cut into bedrock of 18 to 30 feet are located on the south- 

 west end of Salt Pond Bay, on the west side of Greater Lameshur Bay, 

 and on the southwest side of Europa Bay. A steep bedrock rubble 

 slope marks the landward sides of the platforms. The presence of 

 beachrock at five widely scattered locations at a consistent lower 

 depth of 24 + 2 feet indicates that sea level remained near that depth 

 for some period of time at least. It is unlikely that localized 

 siibmarine cementation could occur at the same depth over so wide an 

 area. Moreover, the concentrations of noncarbonate clastic material 

 and eroded bedrock platforms at the same depth support a stillstand 

 of the sea at an approximate depth of 24 feet below the present mean 

 sea level. 



The stillstand of the sea at 24 + 2 feet below mean sea level concurs 

 with Meyerhoff's (1926, p. 119) contention that a stillstand occurred 

 at a depth of 27 feet below the present mean sea level. This 

 hypothesis is based on the numerous terraces or "wave-beaten gaps" 

 at the 27 foot depth throughout the Virgin Island chain. Kaye (1959) 

 suggests that a stillstand occurred at 25 feet below the present mean 

 sea level off the Puerto Rico coast. 



CEMENT 



X-ray diffraction using nickel-filtered copper Ka radiation, and 

 chemical staining methods (Friedman 1959, Warne 1962, Davies and 

 Till 1968) established the initial mineralogic and chemical composi- 

 tion of the beachrock cement. The method of Chave (1952) and Goldsmith 

 and Graf (1958) is used to determine the mol percent of magnesium 



VI-77 



