DEPOSITION ON CONTINENTAL SHELF AND SLOPE 155 
If there is still a small advance there will be a terrigenous foreset 
bed, but it will be relatively thin (Fig. 50). It is only a step from 
this case to one in which there will be no terrigenous foreset bed, 
the waste being just sufficient or even insufficient to build a topset 
bed for the full width of the built shelf, the edge of the shelf in the 
latter case retreating landward. In no case, however, will the con- 
tinuity of a stratum be broken, for, if the foreset bed fails, the 
pelagic deposits will extend up the continental slope to join the 
topset bed (Fig. 5c). 
During long-continued subsidence, the duration of which may 
be comparable with the length of a geological period, the movement 
Pelagic 
Pelagic 
LE 
Fic. 5.—a, Variation in thickness of a stratum deposited during stillstand; 
b, a stratum deposited during subsidence with abundant supply of waste; c, a statum 
deposited during subsidence with restricted supply of waste. 
is probably never absolutely uniform and continuous. The geo- 
logical record affords evidence to the contrary, and what is known 
from physiographic evidence of the more recent movements points 
in the same direction. Oscillations being left out of account, con- 
sideration may be given to the consequences of fluctuations in the 
rate of subsidence. Clearly, with a constant rate of supply of 
waste, alternating periods of extremely slow and relatively rapid 
subsidence may determine alternating advances and retreats of the 
edge of the shelf. It is obvious also that, with slow subsidence at a 
uniform rate in progress, fluctuations in the supply of waste may 
produce similar results. Increase in the supply of waste may result 
from differential elevation of a portion of the neighboring land 
mass, after which a falling off will take place as a result of regional 
subsidence or peneplanation of the land, to be followed by a further 
increase when differential elevation is renewed, and so on. 
