204 W. O. CROSBY 
series of Cretaceous and later sediments. This general crustal 
movement—depression seaward and elevation landward—has 
continued until the originally nearly level bed-rock surface (the 
Cretaceous peneplain) now slopes seaward beneath the coastal 
plain 75 to 100 feet per mile. Upon this sloping peneplain the 
coastal plain, which is continued seaward in the continental plat- 
form, holds the relation of a built terrace—thinning landward and 
thickening seaward. 
The oscillatory character of the movement has determined 
repeated seaward and landward migrations of the shore, with the 
result that within what may be called the axial zone of the sea- 
board or landward margin of the coastal plain, deposition and ero- 
sion alternated and stratigraphic conformity has been interrupted 
by unconformity; while seaward throughout a large part of the 
coastal plain and the whole of the continental platform, deposition 
has been relatively uninterrupted and conformable; and land- 
ward beyond the limits of the coastal plain, erosion has similarly 
prevailed. 
Obviously these conditions are likely to continue until the dis- 
tant future time when the increasing burden of unconsolidated 
sediments, accompanied by a gradual rise of the isogeotherms, 
has induced softening of the sub-crust or true bed-rock, and plica- 
tion, with consequent crustal thickening and uplift, ensues. Thus 
when the revolution is complete and the land is again base-leveled 
by erosion, the way will have been prepared for another wide- 
spread or universal unconformity and the beginning of another 
grand cycle of deposition, leading to the development of a new 
continental platform and coastal plain. 
UNCONFORMITY IN THE COASTAL PROVINCE 
No attempt at a complete enumeration of the unconformities 
of the coastal plain (the accessible part of the coastal province) 
above the Cretaceous peneplain—the universal and absolute uncon- 
formity which forms its floor—is likely to be successful, except 
locally, for the simple reason that the crustal oscillations in which 
the unconformities have their origin have not affected uniformly 
all parts of the seaboard. The oscillations are known to have 
