The sea floor off southern California is unlike that adjoining most other coasts of the world in its great topographic irregularity. The area of about 30,000 square miles consists of a checkerboard-like pattern of basins and ranges which are believed to be grabens and horsts that were formed in late and post-Miocene time. A general southward decrease in elevation of the tops of the ranges and the bottoms and sills of basins suggests that regional tilting followed or accompanied the local faulting. Basins far from shore are only slightly filled with sediments, those near shore have been nearly completely filled, and the Los Angeles and Ventura basins have been filled to overflowing so that they now are land areas rather than part of the sea floor. Grain-size, calcium carbo ate, and organic content of the sediments present a smooth graduation from the Los Angeles basin through the nearshore basins to the offshore basins.

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