The Diligencia Formation, exposed in southern California’s Orocopia Mountains, preserves a valuable record of inland environments and mammal life during the transition from the Oligocene to the Miocene. Defined in 1975, this non-marine formation reflects landscapes of rivers, alluvial deposits, and lakes. Its rocks include conglomerates, sandstones, siltstones, limestones, and volcanic layers of basalt and andesite, dated to about 24 to 21 million years ago, along with andesitic sills and dikes. Although its fossil record is limited, it includes mammals such as the oreodont Merychyus cf. minimus, making the formation an important source for understanding southern California’s ancient terrestrial ecosystems.