PRICE 21 



If one were to analyze the few contour maps we have of the submarine canyons, one would 

 find their upper parts cut into the shelf and then emerge as channels in deltas. The shaded 

 portion of the figure indicates the built-up section of the delta that becomes an apron, apparently 

 a very widely dispersed, widely extended, thin layer of depositional material extending beyond 

 the definite delta. The delta is at the foot of the submarine canyon, and the apron is where the 

 turbidity currents flow out on the abyssal plain. Maurice Ewing's work of last year in the At- 

 lantic shows that the turbidity flows which come down from the northeastern coast of North 

 America enter the great western Atlantic Canyon from Greenland and flow down a tremendous 

 distance to the abyssal plain and into the deep off Puerto Rico. Figure 11 shows, diagrammati- 

 cally, the same situation with the apron reaching a trough which might be receiving some of the 

 turbidity-current material. 



Ewing showed that along the great Atlantic canyon the landward side of the abyssal plain 

 was higher than the other side. Justification of similar conditions shown here is found in the 

 contouring of the bottom of the Gulf of Mexico. 



Computations by C. L. Bretschneider, V. J. Henry, my son William, and myself indicate 

 the astonishing volume of 19,000 cubic miles for the deep delta at the foot of the Mississippi 

 canyon. In computing the volume of the delta, I assumed that the continental shelf originally 

 had the steep slope which is shown at the north (Fig. 7) and at the west (Fig. 3) and that the 

 delta was built on such a shelf and with a nearly flat Gulf floor. If we assume major irregu- 

 larities of the floor and projections of the slope, we may subtract a third of the volume and 

 still have approximately 12,000 to 13,000 cubic miles of material. 



I did not know that this shoreline study was going to involve consideration of the bottom 

 of the Gulf, but from the shoreline to the abyss there is a continuity of events and processes. 

 This is itself a major scientific result of this research project. 



You will notice on Fig. 11 that the three different regimes each have a concave profile. 

 At the lower ends of the upper two profiles where the energy is about spent, the curves become 

 convex in the terminal dumps. This low-energy convexity is an overlap and an interfingering 

 with the next lower regime. 



A basic similarity in these three profiles is the drop in elevation (with the resulting 

 gravitational difference) between the ends of each profile to provide energy in a moving body 

 of water. Material is transported down the river. There must be transport of material across 

 the shelf, although the exact method is not known to me. Water coming in from the land must 

 get off the shelf. Water transported landward with the waves must also get off the shelf. There- 

 fore, there must be a movement of water across the continental shelf to the oceanic basin, and 

 this movement must effect the transportation of material. You find the finest material, clay, 

 commonly building up the terminal dump of the shelf in a zone of spent energy. The energy is 

 mostly confined to a thin zone at the surface. This fine land-derived clay has been transported 

 across the shelf in suspension in the water which we have deduced must somehow flow outward 

 off the shelf. Whatever movement there may be at the surface of the shelf in deep water re- 

 mains to be discovered. 



Figure 12 : Diagrammatic bottom profile of continental shelf illustrating some of my 

 terminology. I extend the shoreface out somewhat farther than some writers, and place the 

 seaward edge at the point of rapid change in rate of concavity, not merely at the edge of the 

 beach or intertidal zone. Where I have data, the shoreface of a sandy barrier island seems to 

 extend to the bottom of the sand structure. 



The ramp is a gently sloping surface, slightly concave, and asymptotic to the horizontal. 

 In this figure a broad irregular submerged deltaic mass is shown rising from the plane of the 

 ramp. It is being smoothed in its Gulfward parts and eroded and graded into the shoreface- 

 ramp profile near shore. Where the submerged deltaic deposits and other obstructions have 

 been removed, as they have in a few places, the ramp continues to join the terminal convexity, 

 shown by the extrapolated broken line. I have applied the word camber meaning convexity, to 

 the terminal convexity of the shelf. Some people call the zone where the curvature reverses 

 from concave to convex the "shelf break," others pick any point where the curvature is chang- 

 ing rapidly near the shelf edge; some consider a terrace or some kind of slope irregularity as 

 the "break." If we are to use such a term, it should be defined. 



