The unit is of middle to upper Eocene age, being deposited in a shallow marine environment, during a time in which the sea was receding and becoming more shallow.Minor et al., map legend Because the process was not uniform, with periods of shallow-water deposition alternating with deeper water conditions, the predominantly sandstone unit is interbedded with siltstones and shales, with the siltstone and shale making up around 20% of the total composition. Late in the period of deposition, large oyster beds appear in the stratigraphic record, indicating a brackish-water nearshore environment. The source rock for the sediment making up the Coldwater was mostly granitic, and pebbles or cobbles of granite and quartzite appear in some of the nearshore sandstones near the top of the unit. At its top the unit grades into the redbeds and conglomerates of the Sespe Formation, which was deposited in a floodplain, riverine, and estuarine environment onshore.
