144 C. A. COTTON 
submarine portions of deltas, which have been shown by Barrell’ to 
cover often a larger area than the subaérial portions and to form 
large seaward protuberances of the continental shelf, to the con- 
sideration of the shelf as a whole. 
Off all coasts deposition of a greater or less amount of sediment 
is always in progress, not only off the mouths of rivers, but along 
the whole length of the coast lines, and there can be a difference 
only in degree between such deposition and that by which the 
seaward, submarine portions of deltas are built forward. In the 
shallow water near shore conditions may be entirely different from 
those on the shallow-water portions of deltas; for here, as has 
already been shown, erosion may be going on, either deposition or 
erosion being in progress according to the state of balance between 
waste supply and transportation. Erosion near shore, however, 
will, generally speaking, merely have the effect of pushing the zone _ 
of deposition farther seaward. 
From observation of the form and structure of the small deltas 
laid bare and dissected owing to the lowering of the level of the 
water in lakes it is known that waste, after being discharged from 
a river, is transported outward from the shore owing to agitation 
and forward movement of the water, and traverses the upper, 
gently sloping surface of the mass of sediment already deposited 
(‘‘subaqueous plain,’’ Barrell) until, reaching the edge, it slips over 
into deeper and stiller water and comes to rest at the angle of 
repose on the more steeply sloping front of the mass (“‘foreset 
slope,” Barrell). Similarly, in the case of larger deltas of finer 
waste being built into the ocean, the waste is transported seaward 
across a subaqueous plain which is essentially a part of the con- 
tinental shelf, extending outward to the depth at which the bottom 
is no longer sensibly stirred by wave action (‘‘wave base,” Gulli- 
ver’). This depth in the open ocean is clearly indicated by the 
t Op. cit., Fig. 1, p. 388; ‘“‘The Strength of the Earth’s Crust,” Jour. Geol., XXII 
(1914), 39-42. 
2F. P. Gulliver, “Shoreline Topography,” Proc. Amer. Acad. Arts and Sci., 
XXXIV (1809), 176-77. As defined by Gulliver wave-base, the limiting plane 
toward which marine erosion will tend to lower the wave-cut platform, is ‘‘the depth 
to which the maximum wave action is possible.”” The term has been redefined by 
Fenneman as the depth ‘“‘at which wave action ceases to stir the sediments” (N. M. 
Fenneman, ‘‘Lakes of Southeastern Wisconsin,” Wis. Geol. and Nat. Hist. Surv., Bull. 
No. 8, 1902, p. 25). 
