Surficial geology in the vicinity of Washington Ranch
— David Love and Lewis Land

Piedmont slopes and river terraces near Washington Ranch reflect late Neogene geologic history, but are affected by both local and regional solution subsidence of the underlying evaporitic Castile Formation. The Black River currently is a discontinuous stream with local solution sinks and springs. Holocene and late Pleistocene alluvium, marsh, and tufa deposits mark the current river level, whereas two types of older terraces are preserved in the vicinity of Washington Ranch-- fluvial gravel terraces, and a fine-grained gypsiferous “terrace” (?) deposit that indicates an episode of no coarse clastic transport along the river. Seven levels of clastic piedmont deposits with preserved surfaces ranging from highest remnants to modern tributary channels reflect changing base levels from the Guadalupe Escarpment toward the Black River and beyond. Older high piedmont levels west and northwest of the Black River cap outcrops of Castile Formation, whereas younger piedmont levels are graded close to the level modern tributaries and the Black River. The underlying Castile Formation is thinned markedly by solution and up to 100 m of coarse alluvium underlie the younger piedmont deposits, reflecting long-term subsidence of the distal parts of the piedmont and Black River. Ovoid karst features cut the piedmont deposits at all levels. Differences in spring and sinkhole water chemistries reflect different aquifers including the piedmont gravel aquifer of Rattlesnake Spring and the gypsic aquifer of Bottomless Lakes.