The western West Potrillo field includes the Western Potrillo Mountains, a cavalcade of hundreds of coalescing cones and flows formed upon older, thick platform that is possibly fissure-fed stacked flows. Also within the western area are the Riley, Malpais, and Potrillo maars. Potrillo maar is included with the western alignment due to its position west of the Robledo fault. The West Potrillo field is the oldest of the Potrillo volcanic fields, with lavas ranging in age from 262 to 916 thousand years.