The Buesaco-Aranda Faults, which are parallel, extend in a north-northeast to northeast direction from near the Galeras Volcano in southwestern Colombia. The Buesaco Fault is located to the west of the Aranda Fault. On the eastern block of the Buesaco Fault, the basement rock consists of basic volcanic, andesite and dark sedimentary rocks which probably developed in a marginal basin environment during Early Cretaceous time. On the western block of the fault are a group of low-grade metamorphic rocks which consist of greenschist, amphibolite, quartzite and black schist, all of Paleozoic age. The area is mostly covered by a Pliocene blanket of pyroclastic rocks and calc-alkaline lavas, Quaternary lahar deposits and fluvio-glacial deposits.