The central Aden-Afton field includes Aden, Afton, the Gardner cones, and the Kilbourne Hole and Hunt's Hole maars. The two maars erupted through portions of pre-existing Afton series basalt flows. The Afton flows may have erupted through a fissure upon which the Gardner cones were emplaced. Aden is a well-preserved shield volcano that at one time had a lava lake, which later solidified and partially collapsed to the west. Its lavas have been dated as between 15 and 19 thousand years old.