slopes. The presence of straight and sHghtly convex slopes suggests 

 faulting and folding. The 30° . southern flank of the northernmost 

 mountain is so steep that it is probably a fault-scarp. Here again there 

 is no good evidence of summit truncation by wave erosion. 



IV. Escarpments 



A remarkable escarpment (Fig. 3a) was discovered in the Antarctic 

 Ocean about 80 miles north of the Easter Island Swell, to which it may 

 be genetically related. This feature, which has the appearance of a north- 

 facing, tilted fault-block, rises from a depth of about 3,000 fathoms 

 to a sharp peak at 1,180 fathoms, making a total relief of about 2 miles. 

 Eye-ear soundings obtained prior to crossing the escarpment reveal 

 a foredeep at its base. The fault-face has a straight profile and a 

 declivity of 63°, which is a minimum because, in calculating the slope, 

 it was assumed that the ship traversed the escarpment at right angles. 

 The gentler south slope is straight and has a slope angle of 11°. The 

 steepness of the escarpment, the asymmetry of the feature, and the 

 presence of a foredeep clearly indicate that this feature was produced 

 by high-angle faulting of large magnitude. The absence of effective 

 erosive processes deep beneath the ocean has permitted the preservation 

 of this fault block with " text-book-like " simplicity. The precipitous- 

 ness of the escarpment probably exceeds that of any continental 

 escarpment of comparable relief. For example, the angle of slope of 

 the eroded east face of the Sierra Nevada fault-block averages about 18°. 



A second escarpment of considerable magnitude was located during" 

 two crossings of a submarine " spur " 300 miles in length extending 

 out from the Antarctic Continent adjacent to Princess Ragnhild Coast. 

 Figure 3b shows a profile of this escarpment obtained during the west- 

 to-east crossing. This profile indicates the probable faulted nature of 

 the eastward side of the " spur." The escarpment drops with a 44° 

 declivity from 1,100 fathoms to at least 2,700 fathoms, and eye-ear 

 soundings suggest the presence of a foredeep. This escarpment is possibly 

 the result of high-angle faulting or, more likely, by horizontal movement 

 along a fault which has displaced a portion of the Antarctic Continent 

 and formed the submarine " spur." This feature reminds one of the 

 Gordo escarpment off northern California, which, according to Shepard 

 and Emery (2), may have been formed by horizontal displacement of the 

 continental shelf along an extension of the San Andreas fault. 



V. Continental Slopes 



During her passage around the Indian Ocean sector of the Antarctic 

 Continent the " Henderson " remained, for the most part, in the deep 

 water of the basins surrounding the continent. These basins are extremely 

 level suggesting a thick fiU of sediments. No seamounts were found. 

 At times the vessel ran along the continental slope at depths shoal enough 

 to be recorded on the echo-sounder tape. Irregular fathograms were 

 obtained, but the presence of submarine canyons or other indentations 

 into the continental slope cannot be definitely determined because of 

 constant manoeuvring around the ice. There is no detailed record of the 

 changes in course during this manoeuvring so ' that many traces which 

 appear to show indentation in the slope may have been produced by 

 changes in course along a smooth slope.' 



340 



