Cleared Circles: Anthropogenic or Biogenic? Use of Non‐Invasive Geophysical Techniques to Determine Origin

PreviousNext You have accessSymposium on the Application of Geophysics to Engineering and Environmental Problems 2011Cleared Circles: Anthropogenic or Biogenic? Use of Non‐Invasive Geophysical Techniques to Determine OriginAuthors: Todd CaldwellEric McDonaldSteven BaconRina SchumerThomas BullardTodd CaldwellSearch for more papers by this author, Eric McDonaldSearch for more papers by this author, Steven BaconSearch for more papers by this author, Rina SchumerSearch for more papers by this author, and Thomas BullardSearch for more papers by this authorhttps://doi.org/10.4133/1.3614205 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Cleared circles (sleeping circles) are a dominant surface feature across much of the desert southwest. Their origin has been attributed to both anthropogenic and biogenic sources. Cleared circles consist of 1–3m diameter areas of exposed smooth soil, partially surrounded by a low relief berm, with deeply varnished desert pavement and underlying vesicular (Av) soil horizon extending beyond the berm. Their cultural significance has been associated with prehistoric aboriginals scraping back pavement stones to create a more comfortable sleeping area. Contrarily, biologic and pedogenic characteristics associated with circular plant scars and mounds, have been identified beneath cleared circles. Terrestrial laser scanning (TLS) and ground penetrating radar (GPR) surveys were used to evaluate non‐intrusive methods of differentiation. The 250 MHz GPR grids were collected at 0.5m spacing over areas encompassing 29 circles. Local background subtraction (1m) was used to remove planar soil features prior to rendering a 3D volume isosurfaces. Morphological features of the berm and associated subsoil were then characterized by trenching across cleared circles. Results suggest that many cleared circles are byproducts of long‐term faunal and floral bioturbation created beneath desert shrubs. Burrows, rooting structures, and soil chemistry indicate higher levels of biotic activity beneath cleared circles relative to soil under the adjacent undisturbed pavement. TLS clearly indicate that the circular area is a depression roughly 2–5cm lower with an elevated berm on all down gradient edges. Volume renderings of GPR data clearly show substantial velocity anomalies at 20 of the 29 circles with areas extending well beyond the surficial cleared area. The observed subsurface anomalies were not distinctly traceable during targeted excavation. The presence of pedogenic carbonate nodules and gravel lenses is the most likely cause. These results suggest that cleared circles were likely much larger and quite possibly occupied by a shrub complex. Non‐invasive techniques proved valuable, however, these results do not preclude the use of these circles by aboriginals, but they do strongly suggest a natural origin.Permalink: https://doi.org/10.4133/1.3614205FiguresReferencesRelatedDetails Symposium on the Application of Geophysics to Engineering and Environmental Problems 2011ISSN (online):1554-8015Copyright: 2011 Pages: 642 publication data© 2011 Copyright © 2011 The Environmental and Engineering Geophysical SocietyPublisher:Environmental & Engineering Geophysical Society HistoryPublished Online: 06 Jul 2011 CITATION INFORMATION Todd Caldwell, Eric McDonald, Steven Bacon, Rina Schumer, and Thomas Bullard, (2011), "Cleared Circles: Anthropogenic or Biogenic? Use of Non‐Invasive Geophysical Techniques to Determine Origin," Symposium on the Application of Geophysics to Engineering and Environmental Problems Proceedings : 581-581. https://doi.org/10.4133/1.3614205 Plain-Language Summary PDF DownloadLoading ...