With the exception of the doubtful terrace at 800 fathoms 

 on the seamount in figure 29C, there is no good evidence of 

 terracing or summit truncation of a type that would have 

 resulted from wave planation if any of the seamounts had been 

 near sea level at some time in its past history. This is in 

 contrast to the discovery made by Hess ' that most seamounts 

 in the west central Pacific show summit truncation. 



Easter Island Swell . Existing charts, such as H.O. 2562, 

 show that the Easter Island Swell extends between Easter 

 Island and the Antarctic Continent for a distance of several 

 thousand miles. This swell, which is obviously a system of 

 submarine mountain ranges comparable to some of the large 

 Cordilleras on continents must be the combined results of 

 folding, faulting, and volcanism. Aural soundings that were 

 obtained prior to passing over the swell revealed a flat bottom, 

 with no indication whatsoever of a foredeep. The profile of the 

 northernmost portion of the swell (fig. 30B), shows a general 

 absence of concave slopes. The presence of straight and 

 slightly convex slopes suggests both faulting and folding. 

 The southern flank of the northernmost mountain is so steep 

 (30 degrees) that it is probably a fault scarp. There is no good 

 evidence of summit truncation by wave erosion. 



Escarpments. A most remarkable escarpment (fig. 31A) 

 was discovered in the Antarctic Ocean about 80 miles north 

 of the Easter Island Swell. Aural soundings that were obtained 

 prior to crossing the escarpment revealed a foredeep at its 

 base. This north-facing escarpment has the appearance of a 

 tilted fault block. It rises from a depth of about 3000 fathoms 

 to a sharp peak at 1180 fathoms, thus making a total relief of 

 about two miles. The fault face has a straight profile and a 

 declivity of 63 degrees. This is a minimum since, in calcu- 

 lating the slope, it was assumed that the ship traversed the 

 escarpment at right angles. The dip slope is straight and has 

 a slope of 1 1 degrees. 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 preserva- 

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

 precipitousness of the escarpment probably exceeds that of 

 any continental escarpment of comparable relief. For ex- 

 ample, the angle of slope of the east face of the Sierra Nevada 

 fault block averages only about 18 degrees. 



59 



