An extensive volcanic field of basaltic cinder cones and barren lava flows near Lake Güija, which straddles the border between El Salvador and Guatemala, is named after its largest feature, Volcán de San Diego. A large basaltic lava flow from the San Diego cinder cone dammed the drainage and was responsible for the formation of Lake Güija. Volcanism is concentrated in two areas near Lake Güija: volcanic cones including San Diego E of the lake, and small cones N of the lake near the Guatemalan border, N of the Río Ostua. None of the eruptions have been dated, but Williams and McBirney (1955) estimated that the latest eruption occurred within the past few thousand years. Quaternary volcanic rocks are also located across Lake Güija in Guatemala.