The artifacts and the fossils were lying in or under clays left by a lake that has now disappeared. If Lake Cochise was, like Lake Bonneville, one of the bodies of water created while the last great ice sheets were growing, then the Cochise culture must date from the last wet period of the final glaciation—perhaps as much as 35,000 years ago. Antevs thinks, however, that the water which laid down the clays belonged to a number of ponds, not to a single large lake, and that the ponds could have formed and disappeared, formed again and disappeared again, just before postglacial times brought a much drier climate. Any single pond may have appeared as late as 10,000 years ago to provide the Cochise clay. Radiocarbon dates for Cochise all seem too recent to fit the geology and paleontology of the site. Dates for the earliest stage, the Sulphur Spring, range from 6,210 to 7,000 years ago; the Chiricahua from 2,850 to 7,000; the last, the San Pedro, from 1,762 to 2,463.[53] Antevs, on the other hand, believes that the Sulphur Spring stage at Double Adobe—twenty-four miles east of the Lehner mammoth site where Clovis points were found—dates back more than 12,500 years.[54] Finds of milling stones have also been made at Signal Butte, Nebraska, by W. D. Strong; at Pinto Basin by the Campbells[55]—the first site probably about as early as Gypsum, the other 10,000 years old—and under hardpan at that much disputed site near Frederick, Oklahoma, where some claim that artifacts and fossils of extinct animals appear together in an interglacial formation. Milling stones have also been found at sites of later cultures such as the Edwards Plateau in Texas and Santa Barbara in California. There are indications of an ancient horizon of milling stones, as well as Pinto and older points and scrapers, in Sonora and Lower California. The fact that they were found with the bones of elk and bison and along the shores of dry lakes in Mexico argues that this culture is as old as Cochise, perhaps older. Carl Sauer, who found the materials, believes that only during the last glaciation could the climate of these two desert areas have been moist enough to produce lakes and support so much animal life.[56] At Borax Lake in California, in several years of work following 1938, Harrington found more milling stones. There were also quite a variety of points—fluted, Gypsum, Pinto, Mohave, and Silver Lake—scattered through an alluvial fan of dirt carried down by some early stream that flowed when the country was well watered and verdant, instead of arid like so much of California today.[57]