is strong current activity on the outer shelf in 

 this region (see Fig. 8). The siting of the Wil- 

 mington Canyon is such that sediment being 

 transported westward across the outer shelf 

 (in accord with the direction of non-tidal bot- 

 tom drift noted by Bumpus, 1965, Fig. 5) would 

 be intercepted by the canyon head. The tongues 

 of gravel, coarse sand and shell debris which 

 extend down the eastern flanks of the canyon 

 head (Fig. 7) evidently result from this process 

 of entrapment, since materials of comparable 

 grade are lacking on the sheltered western 

 walls. The first prerequisite for active canyon 

 sedimentation — a continuing supply of sedi- 

 ment to the head — is thus fulfilled. 



(2) A narrow tongue of silty sand extends 

 down the canyon axis to a depth of about 500 

 fms suggesting down-slope movement of coar- 

 ser detritus, at least to this depth, at the pre- 

 sent time. 



(3) While clay muds cover the continental 

 slope and upper continental rise in areas re- 

 mote from the canyon, the distribution of silt 

 follows the trend of the major depressions as- 

 sociated with Wilmington canyon (compare 

 Figs. 1 and 7). The pattern of silt distribution 

 closely resembles that discovered by Stanley 

 (1967, Figs. 7 and 8) in The Gully Canyon. 



(4) Patches of gravel and sand presently 

 exposed on the upper rise may be relict depos- 

 its swept clear of recent mud by the deep-sea 

 currents detected in their vicinity. On the 

 other hand, they may indicate active, if peri- 

 odic, supply of coarse detritus to the upper rise 

 via the Wilmington Canyon complex. Ample 

 evidence of such periodic influxes of debris 

 within the recent past is provided by the pres- 

 ence of sand and shell debris in cores (Stanley 

 and Kelling, 1968b, Fig. 8). 



One point is clear : the supply of coarse shelf 

 sediment moving toward the west and, thus, 

 toward the head of the canyon is such that 

 most of the material must be transferred 

 quickly to the deep sea. Were it not, the head 

 would soon be filled. 



The fine-grained veneer that floors much of 

 the area of the slope and rise is being contrib- 

 uted, in part, by a rain of pelagic material set- 

 tling to the bottom. The origin of this material 

 is unknown at present. Muds cou i originate 



from deposition of suspended matter concen- 

 trated in the water column by deep turbulence. 

 This nepheloid layer on the slope and rise 

 north of the Wilmington Canyon has been de- 

 scribed by Ewing and Thorndike (1965). Geos- 

 trophic contour bottom currents of the type 

 recognized at the base of the Atlantic margin 

 by Heezen et al. (1966) and Schneider et al. 

 (1967) may also play an important role in the 

 transport and deposition of sediments. 



Evidence from bottom photographs of south- 

 west and south-southwest flowing currents on 

 the outer sector of the upper rise are compati- 

 ble with the geostrophic contour patterns re- 

 corded by these other workers. However, in 

 this study area, flow also occurs at a level on 

 the upper continental rise, which Schneider et 

 al (1967, p. 358) regard as a tranquil, current- 

 free region. Strong, northwest-flowing cur- 

 rents that are capable of moulding sand and 

 gravel at station 43 (Plate 28) may form part 

 of an eddy system developing counter to the 

 main Western Boundary Under Current. The 

 location of this eddy is probably controlled, in 

 part, by the presence of the southeast-north- 

 west trending Nyckel Ridge. Observation of 

 this ridge shows that bottom currents are ca- 

 pable of moving material of fine to coarse sand 

 grade and that they may well play an impor- 

 tant role in modifying the dispersal pattern of 

 sediment emanating from the Wilmington Can- 

 yon on the lower slope and rise. 



At present, the Nyckel Ridge bounding Wil- 

 mington Canyon on its south side forms an im- 

 portant locus of current activity and of prob- 

 able slope instability, i.e. slumping and gravita- 

 tional gliding, perhaps of the type described by 

 Rona and Clay (1967) and Uchupi (1968). The 

 occurrence of large angular slabs of rock with 

 talus at Station 43 suggest either active ero- 

 sion of outcropping rock as described by 

 Schneider et al. (1967, p. 358) or slumping and 

 sliding of large rockmasses off the adjacent 

 Nyckel Ridge onto a sand-silt bottom. Evidence 

 for displacement of these blocks is provided by 

 a series of linear fractures in the rock ledge 

 proper (Plate 27). The general trend of main 

 fractures affecting these rocks is N50°E, i.e. 

 parallel with local isobaths. The breaks are not 

 unlike crevasses in glaciers or in areas of in- 

 cipient avalanches such as steep snow-covered 



89 



