150 C. A. COTTON 
dunes. ‘The structure of the resulting shelf will be similar to that 
with stationary shore line with the exception that each (theoretical) 
foreset bed will have a subaérial as well as a subaqueous portion. 
In an actual case the stratification will be very irregular in the 
shoreward portion of the shelf, as there will be much cross-bedding 
in both the shallow-water and the subaérial deposits. 
A very similar result is produced where the shore Jine advances 
seaward owing to the growth of an alluvial plain formed by con- 
fluent deltas, as, for example, along the coast of the Canterbury 
Plains, New Zealand. The structure of the whole mass of deposits, 
Fic. 4.—Ideal sections across the continental shelf; a, the shore line fixed in 
position; }, the shore line advancing; c, the shore line retreating. 
including the alluvium of which the plain is built and the shelf 
which fringes it, will be similar to that of the delta of a single river 
as described by Barrell. 
With the shore line retreating.—In Fig. 4c the commoner case is 
represented in which the waves reaching the shore have power to 
erode and where, therefore, retrogradation of the shore takes place 
(see also Fig. 1). The shelf will consist of two portions, a cut 
platform and a built platform, and it is obvious that the ratio of 
the width of the cut to that of the built platform may vary widely. 
As has been pointed out on an earlier page, it is theoretically possible 
that, in exceptional cases, there may be no built platform, the 
material broken by waves together with that coming from the land 
being swept away by along-shore currents and the shelf being 
entirely or almost entirely a cut platform such as that of the 
