After understanding the weaknesses that relate to the overall setting of the rock type, the indexer then looks for processes that could lead to erosion of the panel.  Erosion is a two-step process of first detachment and then movement.  The idea is to predict the future, by identifying those factors that could lead to the detachment or separation of rock from rock. This section's purpose is to identify the potential loss of rock.

Fissuresol (future location of flaking or break-offs)

Fissuresols are miniature soil-type sequences found inside hairline cracks.  Cracks or edges on a rock surface behave as materials traps in which aeolian transported dust, weathered materials, and other atmospheric particulates land and collect.  Dust slips into these fractures.  Clays will expand and contract through wetting and drying cycles --- ultimately mechanically wedging cracks and fissures open. The carbonate is leached from the dust and precipitates on the walls of the fracture, and progressive growth of this calcium carbonate slowly widens the fracture.  The larger the crack, more the fissure opens. The surface being wedged will eventually fall off of the face of the panel.
 

Source: Villa, N., Dorn, R. I., & Clark, J. (1995). Fine material in rock fractures: aeolian dust or weathering? In V. Tchakerian (Ed.). Desert aeolian processes, pp. 219-231.

Villa et al. (1995) indicate that defined sorting or order occurs within the material fines in the crack, much like soils. The miniature soil horizons in these fissures can also be seen on the rock coatings on the walls of the fissure.  A fissuresol acts as a mechanical wedger, both from the perspective of the expansive clays and from the perspective of the growth of the rock coatings.
 
 

Basalt, Coso Range, California

Basalt, Coso Range, California

Roots

Root growth in rock is mechanical weathering agent where the root of the vascular plant forces or wedges itself into cracks present on the rock and forces the crack to open. In addition, roots secret acids that help to chemically weather the rock surface in immediate contact with the root. Rooted plants growing into the rock can be extremely destructive. However, their removal can be worse. Decisions regarding root growth should be made as early as possible in the growth of plants near panels.
 

Schist, northern Portugal

Granodiorite, central Arizona

Plant growth on or near the panel can impact the rock art surface in ways other than root growth.  The most dramatic effect would be a wildfire.  As shown in Tratebas et al. (2004) , burning plants near use prior weaknesses to generate immediate erosion, as well as additional spalling after the fire. Plants that overhang a panel will also drop plant detritus that enhances chemical weathering and promotes the growth of such lithobionts as fungi. Swaying branches near the panel may abrade rock coatings on the panel. Transpiration from an overhanging plant changes the microclimate of the area, increasing moisture, decreasing temperatures,  and promoting the growth of such lithobionts as lichens.

Silificied Dolorite, South Australia	Sandstone, central Utah	Basalt, Coso Range, California
Sandstone, Black Hills, Wyoming	Silificied Dolorite, South Australia	Basalt, Kaho'olawe Island, Hawai'i
Basalt, southeast Colorado	Sandstone, Black Hills, Wyoming	Sandstone, Chevelon Canyon, Arizona
Basalt, Coso Range, California	Sandstone, southeast, Colorado	Sandstone, Black Hills, Wyoming

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Preface: Spalling of rock pieces simply describes the breaking off of rock pieces.  The terminology used to describe the myriad of spalling forms can be confusing and contradictory.  No matter the term that oner indexer is used to, another indexer will use that word in a different way.  Thus, we ask the indexer patience in using our definitions.

RASI uses the term exfoliation only where the spalls first depends on rock formation,  second is parallel to the rock weakness, and third is parallel to the panel surface (e.g. follow bedding, follow foliations, pressure release shells). This use the term exfoliation follows Fitzner's classification system.

Where the spall does not depend on rock formation, the spall is independent of the rock structure and then we use the size of thickness of the spall to name it:

• Flaking - fingernail
• Scaling - up to the thickness of your fist
• If larger the thickness of the spall is larger than your fist, then move to the page for Loss of Stone by breaking-off chunks.

Sandstone, central Wyoming

Sandstone, southeast Colorado

Splintering looks like wood that is just starting to separate along the tree rings, where there exists a pattern of linear splitting apart of the rock.   Another way to describe splintering would be similar in appearance to a book that got soaked in water and then dried out. Splintering can sometimes follow bedding planes, and it can sometimes follow mineral contacts such as mica in schist.  But it does not have to, and most often splintering is not obviously connected to weaknesses in the stone structure.

RASI uses this term exclusively in circumstances where the splitting is not parallel to the panel face.  So splintering is different from flaking and scaling, because these spalls must be parallel to the panel surface.  When you see splintering that is preparing for detachment (i.e. not yet eroded), it will usually be right next to places that have eroded by splintering.  The RASI indexer would simply look to areas adjacent to splitnering erosion to notice the next spots on the panel to erode.

Sandstone, Black Hills, Wyoming	Sandstone, central Wyoming	Sandstone, central Wyoming
Granodiorite, southern Nevada	Sandstone, Black Hills, Wyoming	Sandstone, northern Arizona
Sandstone, Black Hills, Wyoming	Sandstone, northern Arizona	Sandstone, central Utah
Sandstone, central Wyoming	Silicified Dolomite, South Australia	Sandstone, central Wyoming

Undercutting

The progressive removal of the footing support of a rock panel is called undercutting. One type of undercutting is would be removing the base of a rock face by stream erosion.  Another type of undercutting might be spring sapping at the base of a cliff.  Other types of undercutting settings might be put into place by erosion of fine sediment at around a boulder, called pedastal erosion. Trail use erosion might also generate undercutting.
 

Sandstone, southeastern Colorado	Sandstone, central Wyoming	Sandstone, Black Hills, Wyoming
Sandstone, central Wyoming	Sandstone, central Wyoming	Sandstone, central Wyoming
Sandstone, central Utah	Sandstone, Chevelon Canyon, Arizona
Granodiorite, southern Arizona	Granodiorite, Southern Arizona

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The weathering rind encompasses the upper few millimeters of the rock surface that is exposed to air. Rinds contain rock coatings and weathered rock material. Rock coatings can form a barrier to other weathering agents and act as preservation tools for the deeper rock. The more developed the weathering rind, the more likely that the coatings are acting as a case hardening agent.  More information on nature of weathering rinds can be found in Gordon and Dorn (2004). The interaction of weathering rinds with rock art is detailed in  Tratebas et al. (2004).
 

Schist, northern Portugal	Sandstone, eastern Wyoming	Basalt, Coso Range, California
Sandstone, Black Hills, Wyoming	Sandstone, Black Hills, Wyoming	Sandstone, Black Hills, Wyoming
Sandstone, Black Hills, Wyoming	Sandstone, central Wyoming	Basalt, Mojave Desert, California
Sandstone, northern Arizona	Sandstone, Black Hills, Wyoming

The above categories do not represent all of the possible locations of future panel erosion.  Thus, if the indexer sees other processes preparing for future detachment operating at a panel, these should be scored here.
 

Dorn, R.I. and Cerveny, N.V. (2005) Atlas of Petroglyph Weathering Forms used in the Rock Art Stability Index (RASI). http://alliance.la.asu.edu/rockart/stabilityindex/RASIAtlas.html (originally posted April 1, 2005; last modified March 10, 2007).