VI. Conclusions 



The development of the continuously recording echo sounder makes 

 possible much more detailed studies of sea-floor geomorphology than 

 was possible in the past. However, echo-sounder records still have many 

 limitations such as generalizing the bottom because of the poor directivity 

 of sound so that fathograms must be interpreted with caution. It is 

 also unfortunate that standard echo sounders have a tape recording limit 

 of 2,000 fathoms as nearly all of the ocean floor is located at depths in 

 excess of this. 



In studying sea-floor features from fathograms one is generally 

 assisted by a simplicity of form resulting from the absence of competent 

 erosion processes like those which add greatly to the complexity of 

 subaerial features. It follows from the lack of erosion that some sea-floor 

 features are extremely old, perhaps as old as the ocean basins themselves. 

 Such physiographic " museum pieces " are in contrast to subaerial 

 geombrphic features which are generally considered to be entirely late 

 Tertiary or younger. 



References 



(1) Hess, H. H. (1946) : Drowned Ancient Islands of the Pacific Basin. Am. 



Jour. Sci., Vol. 244, pp. 772-791. 



(2) Shepard, F. p., and Emery, K. O. (1941) : Submarine Topography off the 



California Coast. Geol. Soc. Am. Special Papers No. 31, pp. 35-41. 

 (3) (1948) : Submarine Geology. Harper and Bros., New York, pp. 193-194. 



THE GEOLOGY OF BIKINI, MARSHALL ISLANDS 



By H. S. Ladd and J. I. Tracey, U.S. Geological Survey 



(Read by Dr. K. O. Emery) 



[Ahstracf] 



Soundings have been carried out on the sloping shelf around Bikini, 

 and the slope indicated was relatively steep. A seamount was also 

 indicated lying to the north-west. The projections of the reef are reflected 

 in projections at depth. Soundings around the Rongelap Atoll also 

 indicated steep shelves and an associated seamount at Eniwetok Atoll. 

 A deep terrace at 750 fathoms lay to the north-west and a similar one to 

 the south-east. North of Bikini is a shallow seamount which was illu- 

 strated in some detail. Deep-sea sediment distribution was illustrated 

 on plan. Deeper sediments are red clay, but at 2,300 fathoms to 2,000 

 fathoms it is replaced by globergina ooze, and between this zone 

 and the atolls there is a variety of sedimentary types. Sonic soundings 

 around Bikini indicate a slope of a maximum of 45°, and close soundings 

 were carried out adjacent to the atoll with leadline. Cross-sections of 

 atolls and seamounts were illustrated. The former indicated steeper 

 slopes at the atoll, but the latter had a somewhat different profile which 

 was comparable up to 600 to 700 fathoms, and above that a decrease in 

 slope or a rounding off. Sounding within the lagoon itself indicated a 

 terrace around the perimeter, widest at the outward projections and 

 narrowest at the inward projections. The subsidence theory may hold 

 ground for atolls built up from the seamounts, but this would require 

 a deep drill-hole to prove. At Bikini there had been planing at lower 

 sea-levels during glaciations. Growth of the rate of coral is estimated at 

 1 mm. per year from the reef surface. There was a marked concentric 

 distribution of sediments within the lagoon itself. 



344 



