Holocene evidence includes petroglyphs and trail segments that are probably related to outcrops of local high-quality siliceous rock (primarily chalcedony in freshwater limestone).

In the Manix Basin (Lower Mojave Valley) of San Bernardino County, California a fortuitous ensemble of environmental factors relating to mountain building, climatically controlled conditions for lake formation, alluviation, and erosion, faulting and folding and significant erosion of ancient lacustrine plain sediments by the modern drainage have rendered relatively accessible for archaeological investigation a series of deposits that represent more than 350,000 years of Quaternary history.

This lake drained, probably catastrophically, approximately 18,100 years ago, probably as a result of a major increase in river inflow or tectonic movement on the Manix fault.

Haynes (1973) postulated that rock fracturing by tectonic stresses, weather, rock-on-rock percussion in streams and mudflows, pressure retouch of buried cobbles, and successive generations of flake removal and separation from cores through cycles of erosion and redeposition could have occurred during deposition of the alluvial deposits at Calico and produced specimens indistinguishable from artifacts.

work is being done on compiling and publishing the most recent finds at the site to be submitted to the local tribes, community and journals for peer review.

He tested a method developed by Barnes (1939) who had compared frequency of obtuse angles on eoliths, natural fractures, and artifacts.

Duvall and Venner (1979:462) examined a sample of Calico artifacts and concluded they were form-selected examples of naturally flaked rocks.

This consensus developed based on a number of factors, including: In 1959 Louis Leakey, while at the British Museum of Natural History in London, received a visit from Ruth DeEtte Simpson, an archaeologist from California.

Excavations in an area stratigraphically separate from a verified 10,000-year-old Paleoindian site were carried out by Leakey and Simpson, who believed that they had located stone artifacts that were dated 100,000 years or older, suggesting a human presence in North America much earlier than estimated.

[9] The archaeologist Jeffrey Goodman who worked at the site with Leakey had also claimed the stone artifacts to be human made.

"[11] The geologist Vance Haynes had made three visits to the site in 1973 and had claimed that the artifacts of Leakey were naturally formed geofacts.