An abrupt change in slope marks the boundary between the continental shelf and the continental slope. The slope change previously has been attributed to (1) sedimentation at wave base and in equilibrium with present sea-level, (2) sedimentation at wave base but related to some past lowered sea-level, (3) wave abrasion in equilibrium with present sea-level, and (4) wave abrasion related to some past lowered sea-level. To test these various possible origins, the concept of wave base is examined theoretically. It is found that wave base appears unimportant in the regimen of the sea, and it does not account for the observed "shelf-break." The break appears to be related to the depth at which breakers form which, excluding rare cases, is at a maximum of about 5 fathoms. The wr ters term this the depth of vigorous abrasion. On theoretical grounds it appears, therefore, that the break-of-slope is caused by wave abrasion related to a sea-level lowered to within about 5 fathoms of the break. This theory is tested with several field facts and these appear to confirm it. For example, the depth of the shelf-break is approximately the same on the windward and the leeward side of islands which is in agreement with the concept of the break being formed at the depth of vigorous abrasion, but is in disagreement with the concept of wave base.

Some geological implications of this theory are briefly discussed. These are: (1) sea-level has recently (geologically speaking) risen eustatically, (2) this recent rise has taken place more rapidly than changes of land-level effected by erosion, sedimentation, or diastrophism.