The Hidalgo Formation is a latest Cretaceous geologic unit in southwestern New Mexico that records both volcanism and Laramide mountain-building in a changing regional landscape. Composed mainly of basalt and andesite lava flows with breccia, pyroclastic rock, and locally thick limestone and shale, it varies greatly in thickness and lies between older formations below and the Rubio Peak Formation above. Geologists interpret it as the product of a volcanic center inland from the southwest coast of the Ringbone depositional basin, erupting as tectonic forces partitioned that basin. First named by Samuel G. Lasky in 1978, it remains important for reconstructing the volcanic and tectonic history of the New Mexico-Arizona borderlands.