earth, it is obvious that the whole problem needs still far more data in 

 order to find a reliable explanation of the origin of the seamou'nts and 

 their age. In the meantime the interpretation of the chart could at 

 least suggest that these drowned islands show some features which go 

 back to the oldest history of our globe which we might still be able to 

 decipher in form of geologic time-marks. 



References 



Hess, H. H. : Drowned Ancient Islands of the Pacific Basin. Amer. Journ. Sci., 



244, 1946, pp. 772-791. 

 SoNDER, R. A. : Shear Patterns of the Earth's Crust, &c. Transact. A»ter. Geoph. 



Union, 28, 1947, p. 939. 

 Drowned Islands and the Origin of the Oceans. ' ISth Int. Geol. Congress, 



London. 1948. 

 Wells, J. W., and others : Drilling on Bikini Atoll. 18th Int. Geol. Congress, 



London, 1948. 



SOME PACIFIC AND ANTARCTIC SEA-FLOOR FEATURES 



DISCOVERED DURING THE U.S. NAVY ANTARCTIC 



EXPEDITION, 1946-1947 



By Robert S. Dietz, U.S. Navy Electronics Laboratory, San Diego 52, 



California 



[Ahstracf] 



During the U.S. Navy Antarctic Expedition, 1946-1947 (Operation 

 HIGHJUMP), the recording echo sounder of the U.S.S. " Henderson " 

 indicated the presence of a number of previously undiscovered sea-floor 

 features. Several large, isolated, non-flat-topped seamounts were found, 

 the shoalest of which rises to a depth of 540 fathoms. A volcanic origin 

 is presumed for most of these seamounts as they are symmetrical in form 

 and have slightly concave flanks varj'ing in slope from 10 to 21°. 



A spectacular, asymmetrical, fault-block escarpment with a foredeep 

 was discovered north of the Easter Island Swell. This two-mile-high 

 ridge has a slope of 11° on one side and a calculated slope of at least 

 63° on the other. Thus, this feature is more precipitous than any sub- 

 aerial fault-scarp of comparable magnitude on the face of the earth. 

 Another large escarpment was discovered extending out from Antarctica. 



Profiles of the continental slope of Antarctica south of the Indian Ocean 

 show it to be smooth and gentle, suggesting extensive deposition. No 

 submarine canyons were definitely identified. The break-in slope between 

 the shelf and slope occurs at depths from 230 to 280 fathoms. This is 

 much deeper than the usual break-in slope which occurs at about 75 

 fathoms in most other parts of the world. It may be due to deep erosion 

 by icebergs and glaciers, but certain other considerations suggest that it 

 may result from the isostatic depression of Antarctica because of ice 

 loading. 



Most of the Pacific traversed appeared to have irregular topography, 

 but the basins around Antarctica are smooth and comparatively 

 featureless. 



I. Introduction 



During operation HIGHJUMP the U.S.S. " Henderson " (DD-785), 

 under the command of Commander C. F. Bailey, made continuous 

 soundings along her track with a model NMC-2 (RCA) echo sounder 

 using 17 kilocycle sound pulses. Soundings of less than 2,000 fathoms 



335 



