Silcrete

Silcrete is an indurated (resists crumbling or powdering) soil duricrust formed when surface soil, sand, and gravel are cemented by dissolved silica. The formation of silcrete is similar to that of calcrete, formed by calcium carbonate, and ferricrete, formed by iron oxide. It is a hard and resistant material, and though different in origin and nature, appears similar to quartzite. As a duricrust, there is potential for preservation of root structures as trace fossils.

Silcrete (siliceous paleosol) in the Waddens Cove Formation (formed during the Pennsylvanian), Sydney Basin, Nova Scotia

Silcrete is common in the arid regions of Australia and Africa often forming the resistant cap rock on features such as the breakaways of the Stuart Range of South Australia. Silcrete can be found at a lesser extent throughout the world especially England (e.g. Hertfordshire puddingstone and sarsen stone), and France.[1] In the Great Plains of the United States, polished silcrete cobbles are locally common on the surface and in river gravels east of the outcrops of the Ogallala Formation.[2][3]

Human use

In Australia, silcrete was widely used by Aboriginal people for stone tool manufacture, and as such, it was a tradeable commodity, and silcrete tools can be found in areas that have no silcrete groundmass at all, similar to the European use of flint.

Tools made out of silcrete which has not been heat treated are difficult to make with flintknapping techniques. It is widely believed by stone tool experts that the technology to treat silcrete by burying under a hot fire was known 25,000 years ago in Europe. Heating changes the stone structure making it more easily flaked.[4] This process may have been the first use of so-called pyrotechnology by early mankind.[5][6]

Bifacial silcrete point from Blombos Cave, South Africa, Middle Stone Age
(71,000 BCE) (scale bar = 5cm)

In South Africa at Pinnacle Point researchers have determined that two types of silcrete tools were developed between 60,000 and 80,000 years ago and used the heat treatment technique. There is evidence to suggest the technique may have been known as early as 164,000 years ago.[4][7]

The peoples of the African Middle Stone Age (MSA) showed a preference for silcrete tools, sourcing the material from up to 200 km to use in place of more accessible quartz and quartzite. MSA quarries have recently been found in Botswana south of the Okavango Delta. Evidence was found that raw silcrete blanks and blocks were transported prior to heat treating during the MSA. The geochemical signatures of the fragments can be used to identify where many of the individual pieces were quarried.[8]

The builders of Stonehenge in southern England used this stone for the Heel Stone and sarsen circle uprights. Avebury and many other megalithic monuments in southern England are also built with sarsen stones.

In the Great Plains of the United States, silcrete cobbles and boulders up to 16 kilograms (35 lb) of Neogene/early-Quaternary age are found on uplands bordering the Ogallala outcrop and were used as chipped tool stone as early as the Early Ceramic (ca. 400–1100 CE) Keith phase of the Woodland culture.[3][2]

References
  1. Ullyott, J.; Nash, D.; Whiteman, C.; Mortimore, R. (2004). "Distribution, Peterology, and Mode of Development of Silcretes (Sarsens and Puddingstones) on the Eastern South Downs, UK". Earth Surface Processes and Landforms. 29 (12): 1509–1539. Bibcode:2004ESPL...29.1509U. doi:10.1002/esp.1136. S2CID 128835601.
  2. McCoy, Zaneta (2011). The Distribution and Origin of Silcrete in the Ogallala Formation, Garza County, Texas (PDF) (MSc). Texas Tech University. Retrieved January 24, 2021.
  3. Robert J. Hoard, Kansas Historical Society, John R. Bozell, Nebraska State Historical Society, Gina S. Powell, Kansas Historical Society (2017). THE KRAUS 1 SITE, 14EL313 A Keith Phase Component in West Central Kansas. Cultural Resources Division, Kansas Historical Society, Topeka. Retrieved 2021-01-24. The samples included fossil traces and fragments of plant materials—wood or roots—that in one instance is replaced by calcite which in turn is being replaced by silica. This evidence suggests that the material is not metamorphic quartzite but more likely is sedimentary silcrete that is being formed in an ancient paleosol in the Ogallala formation.{{cite book}}: CS1 maint: multiple names: authors list (link)
  4. Brown KS, Marean CW, Herries AI, Jacobs Z, Tribolo C, Braun D, Roberts DL, Meyer MC, Bernatchez J (2009). "Fire as an engineering tool of early modern humans". Science. 325 (5942): 859–62. Bibcode:2009Sci...325..859B. doi:10.1126/science.1175028. PMID 19679810. S2CID 43916405.
  5. Borrell, Brendan (13 August 2009), "Cooked Results: Modern Toolmaker Uses Fire to Solve 72,000-Year-Old Mystery", Scientific American, retrieved 4 April 2013
  6. "Early modern humans use fire to engineer tools from stone", Phys.org, Arizona State University, 13 August 2009, retrieved 4 April 2013
  7. Jones, Cheryl (30 October 2008), "Technological innovation may have driven first human migration", Nature, doi:10.1038/news.2008.1196, retrieved 4 April 2013
  8. Nash, D.; Coulson, S.; Staurset, S.; Ullyott, J.S.; Babutsi, M.; Hopkinson, L.; Smith, M. (2013). "Provenancing of silcrete raw materials indicates long-distance transport to Tsodilo Hills, Botswana, during the Middle Stone Age". Journal of Human Evolution. 64 (4): 280–288. doi:10.1016/j.jhevol.2013.01.010. PMID 23453438.
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