DEPOSITION ON CONTINENTAL SHELF AND SLOPE 149 
shelf bordering a recently uplifted coast which has been cut back 
to a line of cliffs by energetic wave action must, at its landward 
edge, be a wave-cut platform; but even in this case farther seaward 
the waste derived from this retrogradation of the shore together 
with that brought down by rivers while cliff recession was In progress 
must have been deposited, forming a seaward extension of the shelf 
in the manner described on an earlier page. Off uplifted coasts the 
shore lines of which have not retreated an appreciable distance from 
the initial position, the shelf must be almost entirely constructional, 
and the same is true in the case of depressed coasts the initial 
embayed outline of which has not yet been much modified by 
marine erosion. 
THE STRUCTURE OF THE BUILT SHELF 
The principle is now well understood that, just as in a river the 
ratio of load to transporting power determines whether degradation 
or aggradation shall take place, so along a shore line retrogradation 
or progradation of the shore occurs according as the waves breaking 
on the shore are underloaded and hungry or are overloaded with 
waste from another source. 
With the shore line fixed in position.—It will simplify the dis- 
cussion of the structure of the built shelf if an ideal case is considered 
first, in which the load and the transporting power of waves and 
along-shore currents remain exactly balanced during the building 
of the shelf, so that the shore line neither retreats nor advances. 
Fig. 4a is an ideal section of a shelf built under such conditions. 
The front of the shelf being situated at the level of wave-base 
during all the stages of growth, it is clear that for each foreset bed 
there will be a relatively very thin topset bed, the two being differ- 
ent facies of the same stratum, that the topset beds will thicken 
seaward, and that successive topset beds will approach more and 
more nearly to perfect horizontality. 
With the shore line advancing.—Fig. 4b represents the case in 
which the waves as they reach the shore are overloaded throughout 
the whole period during which the shelf is being built. The coast 
will be continuously prograded, and upon the growing strip of 
strand plain additional sand from the beach will be piled to form 
