The distinctive sedimentary environment of Cook Inlet, south-central Alaska, may be attributable in part to a unique combination of the gross morphologic and climatologic characteristics of the area. The sediments consist predominantly of cobbles, pebbles, and sand with minor admixtures of silt- and clay-size material. Three sedimentary facies are suggested on the basis of the detailed grain-size distributions. The boundaries of each facies are very well defined.

During the summer months, large quantities of glacially derived sediment are added to the upper reaches of the inlet. Strong currents prevent early deposition of most of the silt and clay which are transported seaward toward the Forelands morphologic constriction. In this area intense tidal flushing removes almost all material of less than gravel size. The rate of sediment supply to the inlet is minimal during the extended winter season, and sediments are reworked predominantly by ice rafting. Much of the material probably is deposited initially in the area adjacent to the Forelands.