The types of sediment found on the narrow shelves around Catalina show distinct relation to the nature of the environment as do also the topographic characters of the shelves. Thus the flat platform off the protected area around Avalon is covered largely with organic sediments presumably because of the absence of detrital material washed from the shore. On the other hand, the shelf on the exposed side slopes outward continuously and is covered with fine sediments which are interpreted as coming principally from the present day erosion of the shore cliffs. The zone northwest of Avalon, which lacks a notable shelf, suggests the absence of wave erosion because of its protected position. Extensive tracts of rocky bottom near the northwest end of the island are explained by the currents which impinge against this end and prevent deposition of fine debris. Currents are also given as part explanation of the coarse sediment, both detrital and organic, which was discovered in many places along the outer margin of the narrow shelves.