SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 241 



a number of ways. For example, the numeroiis banks in the North Sea are 

 elongate, parallel to the streams ; while the depth and position of certain 

 channels are subject to such important changes in position that resurveys have 

 to be made each year or so. In the same region, the streams, reaching about a 

 knot or more at the surface, have brought a large area of sand into ridges 

 normal to their path. Off the Dutch coast, the grade of the sand decreases in 

 their inferred direction of advance. Similar relief is found even at 90 fm near 

 the edge of the continental shelf at the western approaches to the English 

 Channel" (Stride, 1959). Sand ridges on the continental shelf were first recog- 

 nized on records made by normal echo-sounders. But a recently developed, 

 horizontally beamed echo-sounder has provided a much better method of 

 studying small-scale linearities on the shelf. In addition to studying in more 

 detail the previously recognized sand ridges, Stride (1960) has found that in 

 some areas, notably in the area south of the Dorset coast, the curving linear 

 patterns represent the outcrop of sedimentary beds. In areas totally devoid of 

 recent sediment, small cuestas a few feet high mark the outcrop pattern of 

 beds of varying resistance. In this manner, he has traced anticlines, synclines 

 and fault lines from near the shore out to a depth of 40 or 50 fm. There is a 

 misleading tendency to assume that sediment covers the sea floor everywhere. 

 Now that we realize that strong currents have swept various areas of the con- 

 tinental shelf clean of sediments, it is not at all surprising that the topographic 

 expression of outcrop patterns long recognized in the emerged coastal plain are 

 also seen in the submerged coastal plain. Thus, the topography of the con- 

 tinental shelf differs only slightly from that which can be observed on the 

 emerged adjacent coastal plains. 



Structural benches, the topographic expression of outcropping beds, have 

 been identified on the continental slope (Heezen et al., 1959). Near Cape 

 Hatteras, Virginia, structural benches have been dated by extrapolating data 

 obtained in several test borings near the coastline (Fig. 10). Benches on Georges 

 Bank have been dated by dredging, and the dips of beds have been deduced by 

 correlation of structural benches along the walls of submarine canyons. It 

 appears that, through the action of slumps, bottom currents and turbidity 

 currents, sediments are continually being removed from the continental slope. 

 There is little cover of recent sediments to obscure the outcrops of the ancient 

 formations. 



c. Shelf break 



An abrupt change in slope known as the shelf break marks the boundary 

 between the continental shelf and continental slope (Fig. 11). The slope change 

 has been variously attributed to : (1) sedimentation at wave base and in equi- 

 librium with present sea-level ; (2) sedimentation at wave base but related to 

 some past lower sea-level ; (3) wave abrasion in equilibrium with present sea- 

 level ; and (4) wave abrasion related to some past lowered sea-level. 



Modern knowledge of the activity of waves on the continental shelves has 

 resulted in the conclusion that the break in slope is caused by wave abrasion 



