/ 



470 LAUGHTON / [CHAP. 18 



/ 



areas of recent displacement of rocks on steep slopes resulting in screes where 

 there is no sediment to be seen. 



The central rift valley has been described by Heezen et al. (1959) and Hill 

 (1900). Photographs taken on the floor of the valley by Cousteau have shown 

 water-cooled lavas similar to pillow lavas. In the Romanche Trench, which 

 cuts through the Mid- Atlantic Ridge, exposed rocks have been seen at 7500 m 

 (Cousteau, 1958). 



e. Seamounts (Figs. 2, 3, 4, 7, 8, 15, 16, 23, 24, 25, 26, 27, 28, 29, 30) 



The features of microtopography on seamounts are very varied over short 

 distances. Successive photographs in a camera station may show bedrock 

 strewn with boulders followed by a current-rippled sand bed. There is a high 

 proportion of visible rock and much coarse clastic material. Judging by the 

 effect of disturbances of the sediment by animals and by currents, the finer 

 sediments contain a high pro])ortion of sand and are frequently found with 

 ri]iple marks. It is probable that the clay portion is winnowed away by the 

 currents leaving a comparatively well sorted sand. 



Photographs of seamounts in the Atlantic have been jiublished by Elmendorf 

 and Heezen (1957), Heezen, Tharp and Ewing (1959), Laughton (1957, 1959), 

 and Laughton, Hill and Allan (1960), and in the Pacific, apart from those in 

 the seas off Southern California, by Menard (1952) and Carsola and Dietz 

 (1952) and Emery, Tracy and Ladd (1954). 



Acknowledgements 



The following underwater photographs are published by permission of: 



N.I.O. National Institute of Oceanography, Wormley, Surrey, England. 



Figs. 2, 4, 6, 8, 9, 10, 14, 18, 19, 20, 25, 26, 28, 33, 36, 37. 

 L.G.O. Lamont Geological Observatory, Palisades, New York, U.S.A. 



Figs. 3, 12, 15, 16, 17, 21, 22, 23, 24, 27, 29, 30, 31, 32, 35. 

 N.E.L. U.S. Navy Electronics Laboratory, San Diego, California, U.S.A. 



Figs. 7, 11, 13, 34. 

 N.G.S. National Geographical Society, Washington, D.C., U.S.A. 



Fig. 5. 



References 



Anderson, A. G., 1953. The characteristics of sediment waves formed by flow in open chan- 

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Bagnold, R. A., 1946. Motion of waves in shallow \\ater. Interaction between waves and 

 sand bottoms. Proc. Boy. Soc. London, A187, 1-18. 



Bagnold, R. A., 1947. Sand movement by waves : some small-scale experiments with sand 

 of very low density. J. In.st. Civ. Eng., 27, 447-469. 



Bagnold, R. A., 1955. Sonic flinii(> exjioriments on large grains but little denser than the 

 transporting fluid and their implications. Proc. Inst. Cir. Eng., Pt. Ill, 174-205. 



Bagnold, R. A., 1956. The flow of cohesionless grains in fluids. Phil. Trans. Roy. Soc. 

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