This post will deal with the stone tools found at Calico site, in the U.S. and the possibility that they were made by Homo erectus.
Calico is located in the Mojave Desert of California, near the town of Barstow. The Calico site dates back to 1942, when some amateur archaeologists found some very primitive stone tools in the sediments of what once was Lake Manix.
This lake is now a dry basin in the desert, but it held considerable water during the Pleistocene Period, and there are records of “highstands” in its water levels during the following periods: 
- 28 to 38 ka (thousand years ago)
- approx. 89 ka
- approx. 244 to 199 ka
- approx. 279 ka
- approx. 412 ka
- approx. 505 ka
- 1.0 to 1.2 Million years ago, wet period in the Mojave
It is very likely that hominids (either modern humans or, as the Calico findings suggest, Homo erectus) would have lived, hunted, gathered food close to this lake during the “wet” periods shown above.
The formal excavations began in 1964 with Ruth DeEtte Simpson and later, Louis B Leaky, a famous archaeologist who made some amazing discoveries at Olduvai Gorge in Tanzania, Africa.
The controversial stone tools
The stone tools (or, “Calico Lithic Industry”) found at the site have been dated to about 200,000 years ago. Which is long before the first modern humans H. sapiens appeared, and obviously long before they even ventured out of Africa and walked all the way to America.
This dating matter has made “orthodox” archaeology dismiss the findings and class them as “geofacts” that is, naturally shaped stones instead of “artifacts” or man made tools.
Calico’s controversial lithic industry has sparked many debates and the crude stone tools do indeed look antique and lack the beauty of the artfully crafted stone arrowheads, scrapers and tools made by Indians or Paleo Indians in more recent times (since 15,000 years ago).
However, they are very similar to the stone tools made by H. erectus whose stone tools are classified as the “Acheulian lithic tradition” which first appeared in Eastern Africa about 1.6 to 1.4 Million years ago and expanded to Asia where it persisted until about 125 ka.
The H. erecuts chose specific raw materials from which they would make their tools, they then prepared these cores and knocking them against an “anvil” stone to obtain bifacial pieces with sharp cutting edges. The flakes produced in the process were also used as cutters or scrapers.
The image below shows a hand axe from Calico (left) , and one from Africa that was made by H. erectus 1.2 Million years ago . They are very similar, hard to tell apart.
But not all believe that the stones are man-made ( and  uphold that they are artifacts made by humans). Others that they are natural . The natural origin theory says that stones originally broke off from the Calico Mountains due to weathering and flowing down their slopes, banged against others, chipped and flaked. Then subjected to the rolling of waves (Lake Manix), pressure against other stones, removal and redeposition, etc, got shaped even further and resemble man-made tools.
Man made tools display delicate working and flaking while natural sources wear down the stone and round off sharp edges. This can be easily seen while walking by a stream: one does This means that geofacts are not so common. Why would there be so many of them at Calico?
My personal opinion is that the stone tools they are very (see the photographs at the Calico site's official website) similar to those made by men in other parts of the world, and are definitively human, man made. Their age is also a clear indication that they were crafted by H. erectus.
Nevertheless, and despite my own amateurish opinion, several scientific papers have been written on this subject and look into many factors such as the angle of the fractures in the stones, to decide if they are or not man made. The conclusions are mixed some opt for a natural origin others for artificial one.
Further research my clarify the point. Digging continues at the site.
 Calico Early Man Site. The Calico Lithic Industry. Online.
 Christopher Hardaker, (2009). Calico Redux: Artifacts or Geofacts?. Earthmeasure Research SCA Proceedings, Volume 22, p. 18.
 Leland Patterson, Louis Hoffman, Rose Marie Higginbotham and Ruth Simpson, (1987). Analysis of Lithic Flakes at the Calico Site, California. Journal of Field Archaeology Vol. 14, No. 1 (Spring, 1987), pp. 91-106
 Vance, Haynes, (1973). The Calico Site: Artifacts or Geofacts?Science 27 July 1973: Vol. 181 no. 4097 pp. 305-310.
 USGS, (2009). Changing Climates and Ancient Lakes. Online
 Image source. Anthro Tools: acheulian.
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