282 GuiLCHER [chap. 13 



But some shelves are not tied to continents, or to large islands, although 

 they do not lie very far from them and cannot be considered as true oceanic 

 features. The Bahama Banks fall into this category. They are only 4 to 6 fm 

 deep and are surrounded by steep escai^pments and deep bodies of water 

 (Newell and Rigl)y. 1957). Other shallow, flat banks not connected with the 

 mainland are found in the Caribbean Sea between Honduras and Jamaica. 

 Rockall Bank, Faroe Bank, Bill T^aileys Bank. Lousy Bank off Ireland and 

 Scotland (Robinson, 1952 ; Hill, 1956) and the Flemish Cap near Newfoundland 

 may also be quoted. They may be referred to as confincnfal horderJands, a term 

 introduced by She])ard and Macdonald in 1938 for irregular shelves, such as those 

 off Southern California, which include deep basins. But the distinction from 

 oceanic banks is not always easy. The structure, either basaltic or consisting of 

 continental rocks, cannot be used as a general criterion, since the banks off 

 North-west Europe are probably made of basalt, which outcrops widely in 

 Northern Ireland : and Ireland has obviously a continental structure. 



Another difficulty is the definition of the foot of the continental slope, which 

 in many areas gradually merges into the deep-sea floor. The range in depth, 

 between 750 and more than 2000 fm in the North Atlantic, is considerably 

 larger than at the edge of the shelf. Before the abyssal plains are reached, a 

 gently sloping continental rise is often encountered (Heezen, Tharp and 

 Ewing, 1959). 



2. Topography of the Continental Shelf 



Minor ridges related to recent marine sedimentation need not be described 

 here, since they are considered in Chapter 12 of this volume. A]iart from 

 these, other irregularities are of common occurrence, so that perfectl}' flat 

 plains are fairly rare features on the shelf. Most of the irregularities belong to 

 tjrpes well represented in the continental topography, especially in areas 

 adjacent to the shelf. 



Submarine vaUeys are probably the most interesting element. Although 

 they have been obscured by sedimentation in many areas, they are clearly seen 

 in others, for example off the coast of Brittany (Fig. 1), where strong tidal 

 scour contributes to keeping them o])en : but the tidal currents have not been 

 able to carve them into the solid rock, since the latter consists of granite or of 

 hard Palaeozoic sediments. Beautiful ])atterns of branching valleys have been 

 found by Molengraaf on the Sunda Shelf between Borneo, Sumatra and Java 

 (see Kuenen, 1950, fig. 203), and others have been discovered more recently by 

 French hydrographic engineers off Guinea, where the narrow African shelf 

 widens considerably. The submarine Hudson channel (not canyon) on the east 

 American continental shelf falls into the same category (Veatch and Smith, 

 1939). In all the areas mentioned above, the submarine valleys have evident 

 connections with the subaerial valleys inland. They can be followed down to 

 dei)ths of 25 to 50 fm. 



In other parts of the shelves, discontinuous elements of submarine valleys 



