DEPOSITION ON CONTINENTAL SHELF AND SLOPE Tse 
Norwegian coast as interpreted by Nansen. On the other hand, 
owing to nearness of a large source of supply of waste from the land, 
the shelf may differ but little from that shown in Fig. 4a, there 
being but little retrogradation of the shore and the shelf consisting 
almost entirely of a built platform. 
Davis, in considering the development of the graded shore 
profile at Cape Cod,’ comes to the conclusion that ‘‘the critical 
point, where marine action changes from degrading the near-shore 
bottom to aggrading the off-shore bottom, migrates seaward.” 
Clearly, however, in the general case, after a continental shelf has 
been developed, the direction of migration of this critical point will 
depend upon the relative rates of seaward growth of the shelf and 
landward retreat of the shore line. When the rate of growth of 
the built platform is rapid compared with the rate of retreat of the 
shore line the critical point in the profile will generally migrate 
landward. In this case no part of the built platform will be sub- 
sequently eroded and each topset bed will overlap the preceding 
one, lying upon and protecting from further erosion a strip of the 
cut platform. If, on the other hand, owing to rapid shore recession, 
the critical point migrates seaward, some of the earlier-formed 
topset beds will be obliquely truncated by the cut platform. While 
the former case is perhaps the commoner in nature, it is the 
latter which lends itself to diagrammatic representation, owing to 
the mechanical difficulty of drawing a broad shelf on a narrow 
page. 
With the shore line alternately retreating and advancing.—The 
shore-line features of parts of the coast of New Zealand, notably 
in eastern Marlborough and western Wellington, indicate that, 
owing to some disturbance of the balance between load and trans- 
porting power of waves and along-shore currents, retrogradation 
and progradation have occurred alternately. Where such alter- 
nation is taking place, somewhat complex structures in the topset 
beds will result. During each period of shore retreat the previously 
formed topset beds near shore will be eroded, and during each 
advance fresh topset beds will be laid down unconformably on an 
eroded surface. 
tW. M. Davis, Geographical Essays (Boston, 1909), pp. 702-3. 
