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<arttitle> How fire erodes rock engravings
<aug> Alice M. Tratebas*, Niccole Cerveny & Ronald I. Dorn
<aff> *Alice M. Tratebas, Bureau of Land Management, 1101 Washington Blvd.,
Newcastle, WY 82701, Alice_Tratebas@blm.gov; Niccole Cerveny & Ronald I. Dorn,
Department of Geography, Arizona State University, Tempe AZ 85287-0104, USA
<abs> The first analyses of prehistoric rock engravings collected before and after a
wildfire reveal new insights to how petroglyphs erode. Engravings spall in rock
coatings 1 and in weathering rinds that are zones of enhanced mineral porosity mm
to cm beneath the surface 1. Given the prevalance of wildfire in drylands that host
most of the world's outdoor rock art sites 2, new insights on processes wildfireinduced erosion can aid future research on how to preserve priceless
archaeological remains.
<p> A 2002 wildfire (fig. a) at Whoop-up Canyon, Wyoming, provided the
opportunity to compare 60 1991 pre-fire samples with 51 post-fire 2003 samples.
Optical microscopy of ultra thin-sections, backscattered electron microscopy (figs. b, c,
d), X-ray (energy dispersive and wavelength dispersive), and high resolution
transmission electron microscopy (fig. e) reveal that wildfire uses pre-existing
weathering weaknesses in petroglyph erosion. Weathering rinds1 about 1-10 mm
beneath Whoop-up sandstone surfaces are zones of enhanced porosity (fig. b). Erosion
occurs in part because of reduced cohesion and in part because of enhanced capillary
water retention. While some fire spalling takes place in relatively unweathered quartz,
weathering rinds detach in weathered minerals such as mica (fig. c) and most commonly
weathered quartz (figs. d). Erosion also occurs within porous zones within rock
varnish1 (fig. e) that forms on top of the petroglyphs. While weathering-rinds naturally
spall without fire, these zones were targets of enhanced erosion during the
conflagration. Site managers can now focus preservation efforts in areas where
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weathering rinds undelay engravings and their panels, in efforts to clear fuel and in
efforts to relocate water runoff away from these weaknesses.
<received> Style tag for received and accepted dates
<bibcit>
1.
Dorn, R. I. Rock Coatings (Elsevier, Amsterdam, 1998).
2.
Whitley, D. S. in Handbook of rock art research (ed. Whitley, D. S.) 7-51
(Altamira Press, Walnut Creek, 2000).
Correspondence and requests for materials should be addressed to
Alice_Tratebas@blm.gov
<LEGEND> (a) Whoop Canyon, Wyoming, 2001 fire impacted engraving panels
on sandstone spall faces. (b) Engravings spalled in bedrock weakened by
chemical weathering long before the fire. Detachment took place along
weathered mineral grains such as mica (c) and most commonly quartz (d).
Some panel erosion took place within rock coatings (e). Scale bars 200 µm, 10
µm, 30 µm for backscattered electron micrographs b,c,d, and and 400 nm for
transmission electron micrograph image e.
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