The environment of deposition at San Miguel Lagoon is characterized by wide ranges in the measured factors. When the lagoon is isolated from the ocean, large fluctuations in bottom-water temperature with time mask any variations from place to place. Chlorinity, on the other hand, changes so slowly with time that areal variations are meaningful. Dissolved oxygen concentration and pH vary diurnally and areally, and these variations are used to delineate five aqueous environments within the lagoon.

Three different types of sediment within the lagoon are related to the following processes of transportation and deposition: wave processes, river processes, and slow deposition from quiet water. Analyses of sediments from other west coast lagoons show that these same processes may be inferred. Distribution of Foraminifera reflects the influence of the barrier in restricting water exchange with the ocean. Calcium carbonate values in the sediments are low (0.2-2.3 per cent), and the erratic distribution pattern results from variations in the sources, rate of dilution, and physico-chemical factors. In a lagoon where large amounts of organic material are contributed, low values for the organic carbon content of the sediments (0.36-5.60 mg./g.) are attributed to a high rate of decompositi n. Higher organic carbon values are found in the finer-grained sediments.

The sediments reflect the overlying water in only those areas where they are genetically related to that water. In these areas lower calcium carbonate content was found in sediments underlying water with lower pH. Finer grain sizes are accompanied by less dissolved oxygen in the overlying water, and higher organic carbon content occurs with less dissolved oxygen and lower pH in the overlying water. Inferences are made from this study about the nature of ancient coastal lagoon deposits.