According to classical theory, wave-cut platforms are developed by deep submarine abrasion while sea level is stationary. Modern writers, however, believe that wave erosion takes place only in shallow water. Fragile, subaerially etched pyroxene sand grains are mechanically unstable at depths of less than about 30 feet in the Santa Cruz area of California but are mechanically stable at greater depths, supporting the conclusion that significant wave-produced submarine abrasion is restricted to the surf zone with a maximum depth of about 30 feet. Most sediment in transit beyond the surf zone is too fine-grained to act as an abrasive. If sea level is stationary, marine planation may produce a wave-cut platform up to about one-third of a mile wide; a wider platform can be cut only during slow submergence. The modern and former platforms in the Santa Cruz area were carved while sea level was rising.