Unit is named the Mokelumne River Formation. Formerly called the Meganos-Martinez Formation; more recently referred to as the informally named Mokelumne formation (not to be confused with the formally named Mokelumne Formation of Taliaferro (1951), of Permian age, in El Dorado Co., CA). Consists of white to gray-green, fine- to medium-grained, friable, quartzose sandstone and gray to brown carbonaceous shale interbedded with thin layers of lignite. Overlies the H & T shale; unconformably underlies the Eocene Capay Formation. Gas is produced from three sandstone zones in the Mokelumne River Formation; the Middle zone is informally named the IOC sand. Thickness is 1,542 feet. Age is Late Cretaceous (B1565).

Source: GNU records (USGS DDS-6; Menlo GNULEX).