Tuffaceous ephemeral lake deposits on an alluvial plain, Middle Tertiary of central California

The Oligocene and Miocene Valley Springs Formation represents a large fluvial depositional system that extended westward from sediment-filled palaeovalleys in the high Sierra Nevada to a piedmont alluvial plain under the present Central Valley. The Valley Springs Formation consists largely of tuffaceous mudrocks, tuffaceous sandstone, polymict conglomerate and rhyodacitic tuff. The tuffaceous mudrock lithofacies probably represents a complex of ephemeral lake and marsh environments on a low gradient alluvial plain. The inferred abundance of shallow lakes, ponds and marshes implies a climate that was wetter than the semi-arid climate of the region today. -from Author