DROWNED ANCIENT ISLANDS — HESS 



285 



been noted between the depths of the flat upper surfaces and the depths 

 of the surrounding ocean floor, which normally ranges from 2,600 

 fathoms (15,600 feet) to 3,100 fathoms (18,600 feet). The observed 

 depths of the flat upper surfaces of typical guyots range from 520 

 fathoms (3,120 feet) to 960 fathoms (5,760 feet), with most values 

 concentrated near the center of this group (800 fathoms). Thus 

 the guyots rise from 10,000 to 15,000 feet above the ocean floor. The 

 flat tops of guyots in general do not exhibit accordance of summit 

 levels. It is quite common to find groups of guyots in a relatively 

 small area with flat tops varying several hundred fathoms from one 



I SLOPING I I SHELF I 



U- SHELF _^L, rL«t top ^U. iJ 



Figure 4. — This diagram was traced directly from figure 3 and adjustments for 

 steep slopes to the side made. The vertical and horizontal scales and numerical 

 values of the slopes in degree are given. 



to another among the group. Less commonly two or three guyots in a 

 group will have approximately the same depth. 



A few guyots were found to have upper surfaces which were gently 

 undulating rather than flat. These undulating or hummocky surfaces 

 have a maximum relief of about 40 fathoms. In most cases the flat 

 surface can be seen here and there in the profiles and it passes beneath 

 the hummocky material (fig. 5). Judging from the evidence, most 

 guyots have been swept clean of the fine sediments which must be 

 continually settling upon them. In the case of the rare, hummocky 

 ones it would appear that the fine precipitates had for some reason not 

 been completely swept off. It is rather surprising that the normal 

 guyots are swept clean since water currents at such depths as these 

 are thought to be slight. One must look to occasional bottom stir-up 

 by tsunami (Bucher, 1940), though possibly currents related to tides 

 might be strong enough. Once the sediment on these isolated, flat- 

 topped peaks is stirred up, very little of it would be expected to fall 

 back on top of the guyot. It would be dispersed over the surrounding 

 area. 



