Unit is named the Frenchman Flat Sandstone Member of the Ridge Route Formation of the Ridge Basin Group. Exposed in the central Ridge basin, Los Angeles Co., CA. Is a clastic tongue of the Ridge Route and consists of white, medium-grained, arkosic sandstone interbedded with mudstone and rare conglomerate. Contains freshwater mollusks, plant remains, ostracodes, and stromatolites. Extends laterally into the basin for 4620 m. Interfingers with the Ridge Route undivided to the northeast and with the Violin Breccia to the southwest. Conformably overlies the Osito Canyon Shale Member (new) and conformably underlies the Cereza Peak Shale Member (new), both of the Peace Valley Formation (revised). Thickness is 172 m at the type section and ranges from 50 m to more that 400 m. Age is late Miocene based on its stratigraphic position below the fossiliferous late Miocene Cereza Peak Shale Member. (B1565)

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