Saturday, January 25, 2014


Winnemuca Lake petroglyphs. Photograph: Larry Benson.

A recent scientific investigation of petroglyph boulders on the west side of the Winnemucca Lake basin in Nevada has yielded hard dates on the age of the petroglyphs.

Paleoclimatologist Larry Benson (an emeritus scientist with the U.S. Geological Survey who does research for the University of Colorado and its Museum of Natural History) had noticed that the symbols are much whiter than the gray rock they're carved into.

Winnemuca Lake petroglyphs. Photograph: Larry Benson.

Benson needed permission from the Pyramid Lake Paiute Tribe to sample the rock coating. He did finally get permission to sample the coating on rocks near the petroglyphs although he has not yet been allowed to sample any of the ancient rock art.  The whitish coating proved to be Calcium carbonate (CaCO3) and had been deposited when rising lake water lapped over the lower portions of the petroglyph boulder.

Winnemuca Lake petroglyphs. Photograph: Larry Benson.

His paper in the Journal of Archaeological Science, 40 (2013), 4466-76, Dating North America’s oldest petroglyphs, Winnemucca Lake Sub-basin, Nevada, by L.V. Benson, E.M. Hattori, J. Southon, and B. Aleck, outlined the reasoning involved. Benson was aware that Carbonate crust could not have been deposited in the petroglyphs from the lake water unless they were actually underwater at some period. This period was determined using two methods. Laboratory analysis "determined the amount of Calcium carbonate in layers of lake sediment over time. When the amount was close to zero, the lake covered the lower part of the mound below 1206m and the petroglyphs below this level. When the value was relatively large, the lake had fallen below the mound and the petroglyphs and made them accessible for carving." (Benson)

Additionally, they detected a fresh-water plankton (Stephanodiscus hantzschii) found today in lakes in British Columbia in those layers proving that a large quantity of fresh water was injected into the lake water. During those periods water level would have been high and the rocks partly covered, thus the carbonate. They also assumed that the petroglyphs were carved during a period when water was low and people could walk to the site. This is, of course, a very simplified explanation and readers are encouraged to refer to the original paper for the scientific details. The second method of age determination was to 14C date the carbonate layer itself. Fifteen carbonate samples were taken near the petroglyphs and  were 14C dated at the University of California-Irvine W.M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory.

Winnemuca Lake petroglyphs. Photograph: Larry Benson.

The results in Benson’s words were “to provide a minimum age for carving of the low-elevation (1202-1206 m) petroglyphs, we dated the carbonate crust that coats the petroglyphs. The six carbonate-crust samples from the petroglyph site (WDL12) exhibited an age range of 10.23-9.77 ka with one outlier at 8.69 ka. As the sample abrasion process did not always reach the inner (oldest) part of the carbonate crust, we conclude that initial deposition of the carbonate crust occurred at 10.2 ka and continued until 9.8 ka, a conjecture consistent with the TIC data discussed in Section 3.5, which indicates that lake level was constrained by overflow at 1207 m until 9.3±0.1 ka. (Benson 2113:4473)

Additionally, the time frames indicated by the sediment coring supported that by indicating “the TIC records resulting from the two age models indicate that the base of the petroglyph site was subaerially exposed between 15.0 and 13.2 ka and was subject to the carving of petroglyphs. However, the TIC records resulting from the two age models indicate different times of possible subaerial exposure after 13.2 ka. One age model (Fig. 5A) indicates that the base of site WDL12 was subaerially exposed between 11.3 and 10.5 ka and the other age model (Fig. 5B) indicates that the base of site WDL12 was subaerially exposed between 11.5 and 11.1 ka.” (Benson 2013:4473) Applying another age model gave Benson an age range of 11.3 – 10.5 ka. (Benson 2013:4476)

By combining the date ranges from sediment coring and 14C testing on the carbonate layer Benson could state “We, therefore, conclude that the petroglyphs were carved sometime between 14.8 and 10.2 ka.” (Benson 2113:4473)

I asked Benson some questions based upon my own observations (and lack of detailed knowledge). First, I could imagine Calcium carbonate molecules floating around in the lake for hundreds or thousands of years until rising water brought them to a position to be deposited upon the petroglyph rocks, “wouldn’t that give an excessively ancient date?” Benson explained that the inrush of fresh water that raised the lake level to cover the base of the petroglyphs also flushed the bulk of the preexisting carbonates out of the lake (remember the fresh water plankton indicating that the brackish water had been greatly diluted and/or displaced. (personal communication). I also asked about the appearance of sharp edges on the lines of some of the carvings. “Did they exhibit any evidence of more recent additions or modifications?” Benson answered that some of the sharp-edged lines actually showed carbonate deposition on their surfaces proving that they had not changed since that event (personal communication). This is seemingly iron-clad, with the results of more than one type of test providing results that agree like this. Indeed, this is significant enough that Archaeology magazine named it one of the Top Ten Discoveries of 2013. (Powell 2014:28)

So, thank you Larry Benson. Bringing the knowledge of different disciplines to work on rock art questions can provide surprising benefits. And while I am at it, thank you Archaeology for including a rock art analysis in your Top Ten list for 2013.

NOTE: I am grateful to Larry Benson for taking the time and effort to correspond with me about this, and for providing a copy of their paper and photographs for my use, and to illustrate this. If any of the technical details above are incorrect it is entirely due to misunderstanding on my part, not any lack of consideration and generosity on Larry's part.


Benson, L. V.
2013    Dating North America’s oldest petroglyphs, Winnemucca Lake, Subbasin, Nevada, Journal of Archaeological Science, 40 (2013) 40 (2013) 4466-76.

Powell, Eric A.
2014    North America’s Oldest Petroglyphs: Winnemucca Lake, Nevada, from Top Ten Discoveries of 2013, Archaeology, January/February 2014, Vol. 67, No. 1, p.28.

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