DEPOSITION ON CONTINENTAL SHELF AND SLOPE 141 
while, as the shore line recedes, erosion of the sea bottom must go 
on near shore, as the shelf front advances the graded profile is 
maintained farther seaward by deposition. 
The stream of waste in transit seaward will generally, on account 
of its slow movement, form a thick layer on the outer part of the 
shelf precluding the possibility of erosion of the deeper layers of 
strong currents along the shelf, so that the built portion of the 
shelf may be narrow compared with the cut portion. In sucha case 
erosion must extend into relatively deep water, parts of the eroded 
sediment or of the rock floor. It is, however, conceivable that in 
exceptional cases much or all of the waste may be swept away by 
Fic. 1.—Diagram to illustrate the formation of a continental shelf by marine 
erosion and deposition. 
floor of bedrock being occasionally swept clean. In this way are 
perhaps to be explained some recorded occurrences of hard bottom 
far out on the continental shelf. 
The modification of this hypothesis which requires subsidence 
following marine planation has nothing to recommend it. Fig. 2a 
is a copy of the figure used by Gardiner in illustration of the 
hypothesis, modified by the addition of a thick layer of sediment 
(coarsely stippled) on the top of the cut platform, which must be 
added to restore a normal profile. The profile of a later stage of a 
shelf developed by cutting and building on this initial form is also 
added (lightly stippled). 
The hypothesis that subaérial planation followed by submergence 
explains the continental shelf —In considering this possible mode of 
shelf formation care must be taken to exclude cases of temporary 
emergence of a shelf already formed. References to portions of the 
shelf thus temporarily uplifted, partially dissected, and again 
submerged have been already quoted; and oscillations of the 
