The beaches and shallow offshore areas of Santa Cruz and Santa Rosa, the two major northern channel islands off the California coast, are the sole source areas for sediment that ultimately is deposited in the Santa Cruz basin. The intermediate position in this three-stage sediment-transfer system is occupied by the Santa Cruz submarine canyon. Through a network of tributaries at its head the canyon receives material from the islands. The material is moved down the canyon axis and dispersed across the basin as a submarine fan. A main channel, with natural levees, persists across the fan and a series of distributary channels develop from it.

Cores recovered from the canyon axis and fan contained graded layers of sand and gravel, including one such unit 116 centimeters long. The gradation in this last unit is somewhat unusual, in that it differs greatly from the "fine-at-the-top, coarse-at-the-bottom" conceptual model of graded bedding. Instead, there is a constant compositional population with a modal class at +2^phgr, which decreases in over-all percentage as the bottom of the unit is approached. A second, coarser-grained population appears midway in the unit, with a modal class at +0.75^phgr. This second population increases in over-all percentage and shifts its modal class gradually to -4.25^phgr toward the bottom of the unit.

The coarsest particles in all the graded beds are disseminated in a narrow zone above the base of the bed. Thus the bottom of each graded bed is a zone of reverse graded bedding.

All graded sand and gravel layers are extremely clean, usually containing less than 1 per cent silt and clay. Contacts with overlying mud are sharp and distinct. Lower contacts, where visible, commonly are gradational. Detailed pipette analyses show that overlying "pelagic" mud in many places is graded (in the usual sense). Electron micrographs show a decrease of organic remains (diatoms, coccoliths, etc.) toward the base of the graded beds, with a marked, increase at the base. Maximum organic remains are in mud surrounding the sand layers.

The writer postulates the existence of an originally clean sand and gravel body that was able to move down the canyon or across the fan, maintaining its internal integrity. Such a body would exhibit properties described above. It may act as a triggering mechanism; itself setting off "true" turbidity currents.

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