Prevailing concepts regarding the geomorphic modification of the continental margin rely on the theory of wave base and its corollaries, namely, a wave-built terrace, topset shelf beds, and steep foreset slope beds. These views are rejected because they are theoretically unsound and they do not adequately explain the form of the terrace as revealed by modern echo-sounding profiles. Instead, the continental margin is considered to be modified by various types of currents that act at all depths in the ocean and by surf action. Turbidity currents are considered to be especially important in the erosion of steep slopes and in the transportation of sediment. Consequently, clastic sediments are thought not to accumulate on the shelf and slope in any large amount except in local depressions. Instead, most sedimentation is considered to take place largely either at the base of the continental slope, forming an apron of low gradient, or inside the surf zone, forming near sea-level "hinter-surf" beds, that is, littoral, estuarine, lagoonal, and deltaic deposits.

Much of the variation in present form of continental terraces can be accounted for by assuming that these processes have acted for different periods of time--or, in other words, by the Davisian concept of youth, maturity, and old age. Initially, the continental slope is considered to be structural. In youth, surf action cuts the shelf and an apron of sediment forms at the base of the slope. In maturity, this apron becomes so large that its weight depresses the underlying crust and warps down the adjacent shelf. This permits preservation of any hinter-surf deposits which have prograded across the shelf. In old age, the apron covers the entire initial continental slope so that it finally becomes a depositional rather than an erosional environment. Partial rejuvenation by epeirogenic upl ft of the continental margin or complete rejuvenation by orogeny may occur.

Based on an examination of terrace profiles obtained from fathograms, the terrace around much of the Pacific Ocean is believed to be in youth; the terrace of the eastern United States, in maturity; and the terrace off the Mackenzie Sea sector of Antarctica, in old age. The Gulf Coast terrace off Texas may have been rejuvenated from late maturity.

If the proposed geomorphic evolution is correct, some sedimentational consequences, which can be tested in the field, are: (1) the depth of breaker formation, topography, and sea-level should dominantly control continental terrace sedimentation; wave base is not important; (2) except in topographic lows, the shelf and slope should be zones of non-accumulation of large amounts of clastic sediment; (3) the important zones of sedimentation should be inside the surf zone and the deep sea floor along the base of the slope; (4) wave-built terraces should not be formed in the ocean; (5) sediments should show no marked tendency to decrease in grain size seaward across the shelf; (6) turbidity currents should play an important role in erosion and in transportation; (7) the sedimentary strata c pping some continental terraces (for example, the eastern United States) should be found to be dominantly hinter-surf beds; (8) preservation of hinter-surf deposits should result from depression of the shelf due to overloading the crust at the foot of the slope; (9) these beds should be found to be neither truncated by a fault at the shelf margin nor should they become foreset beds; instead they should end as littoral facies deposits. In places where algal-coral reefs grow, much limestone deposition, of course, takes place on the shelf. Such reefs also commonly provide topographic traps in which clastic sediments collect.