26 NV. M. FENNEMAN 
currents may occasion this condition ; (3) the excessive power 
of the water on the shallow bottom may be employed in the 
transporting of a greater load or even in erosion. This is quite 
generally true; (4) the material may be heterogeneous, the 
larger stones coming to rest in the shallower water because of 
their ability to withstand the greater agitation at a higher level. 
Of all these reasons, it will be seen that only the first can pro- 
vide fora permanent slope: the others depend upon a continual 
supply of fresh drift. 
Necessity of a continuous supply of load.icSuppose now that a 
short section of coast line be enclosed between perfectly resist- 
ant walls or piers perpendicular to the shore line, and extending 
out to deep water. The transportation of material alongshore 
will thus be prevented. If the shore also be supposed to be 
perfectly resistant, so that no new drift can be furnished to the 
waves, then the profile of equilibrium, toward which the bottom 
will tend, is a steep descent from the water line to the depth at 
which undertow becomes ineffective, and then a low slope out- 
ward, following the base of effective undertow. This base is 
necessarily on a slope because of the increasing volume of under- 
tow with distance from shore. 
Effect of a supply of drift—Ilf now, drift be supplied at the 
shore line at a given rate, filling will occur at the foot of the 
steep descent leading down from the water line, until the bottom 
has risen to a level at which the power of the water is sufficient 
to transport the material at the rate at which it is furnished, and 
this filling will advance off-shore, ending in a convex front as 
shown above. 
At the shoreward boundary of this filling area is an angle 
made by the plane of deposition, with the steeper descent lead- 
ing down from the water’s edge to the line at which deposition 
becomes possible. In an actual case, where the material of the 
shore yields to erosion, the water’s edge is carried landward, 
and the first descent is not only far from vertical, but in weak 
material, is very gentle ; probably always steeper, however, than 
the slope made by deposition farther out. This may be observed 
