Rock Varnish Dating, Surface Features and Archaeological Controversies in the North American Desert West

Archaeological surface features on desert pavements, including geoglyphs, are notoriously difficult to assess. Absent temporally diagnostic artifacts they may be impossible to place chronologically, limiting their inferential utility. Whether they are cultural or natural in origin itself may sometimes be uncertain, not only again crippling their interpretative value but potentially creating problems for heritage management. Not surprisingly, a series of controversies have developed in the North American desert west over certain of these features. We describe methods for chronometrically constraining the ages of desert pavement features using three approaches to rock varnish dating: varnish lamination (VML) and lead-profile dating, as well as cation-ratio (CR) as an additional tool. Each of these techniques may be applied to rock varnished cobbles that have been upthrust into areas that have been previously cleared of the original pavement through cultural or natural processes. We apply these methods to three unresolved archaeological issues: the age of the intaglios (geoglyphs) along the lower Colorado River corridor; whether the Topock (or ‘Mystic’) Maze is the product of Precontact Indigenous or late nineteenth century railroad construction; and if commonly occurring cleared circles are natural features likely associated with now-missing vegetation and/or bioturbation, or were cultural products.