The majority of these petroglyphs were made by the ancestors of modern Pueblo people. The Ancestral Puebloans made petroglyphs by carefully removing the desert varnish with hand-held stone tools to expose the lighter color of the basalt's interior. After centuries of weathering, older petroglyphs begin to oxidize and this darkening confirms their authenticity and great age. Archeologists refer to these images as being made in the Rio Grande style. This style developed rather suddenly around the year 1300, coinciding with a dramatic increase in the local population and the construction of many pueblo villages along the Rio Grande. Elements frequently seen in Rio Grande style images include human figures, animals, and geometric designs.