Much of the watershed is dominated by Oligocene to Quaternary age volcanic rocks formed by basalt, basaltic andesite, andesite and dacite. In the central and eastern parts there are also sedimentary formations of Pleistocene to Holocene, including alluvial and lacustrine deposits; these sedimentary formations are concentrated in the central parts of the catchment high Isluga volcano is still active, with eruptions in 1900 and 1963; other important mountains are the high Cabaray and the high Cerro El Fraile.