Introductory Note: different types of rock coatings (click here for an introduction) are frequently misidentified in the field. Visual inspection is often insufficient to distinguish different types.

For the purposes of this field-based system, it is not necessary to be definitive.   One key distinction that you need to learn is whether the deposit strengthens or weakens the surface.

Almost all rock coatings strengthen rock surfaces (case hardening effect), with the exception of:

• anthropogenic activities (illustrated below)
• efflorescence or subflorescence (illustrated below)
• fissuresols that open up cracks and promote the break-off of blocks
• lithobionts that pit surfaces and promote decay under the biological agents (Note: biological agents that promote loss of stone material are illustrated elsewhere).

Thus, the first sets of images help train your eye in distinguishing rock coatings from case hardening from these weakening agents.

Anthropogenic (chalking, graffiti, other)

Human erosion of rock art would not fall into this category, but in either incremental (small) bits of erosion or removal of panel material in larger pieces or chunks of rock.

This category is reserved for the activities of applying different types of anthropogenic coatings, such as chalking or painting over the rock art.
 

Sandstone, Black Hills, Wyoming	Sandstone, eastern Wyoming	Sandstone, central Wyoming
Sandstone, Black Hills,Wyoming	Sandstone, southeast Colorado	Sandstone, southeast Colorado
Sandstone, Black Hills, Wyoming	Sandstone, southeast Colorado

Case Hardening

Case hardening is the addition of cementing agent to rock matrix material; the agent may be manganese, sulfate, carbonate, silica, iron, oxalate, organisms, or anthropogenic. Whereas rock coatings are paper thin, case hardening of the rock material can be millimeters thick.
 

Basalt, Coso Range, California	Sandstone, Black Hills, Wyoming	Sandstone, central Wyoming
Basalt, Kaho'olawe Island, Hawai'i	Sandstone, central Utah	Sandstone, southeast Colorado
Sandstone, central Wyoming	Silicified Dolomite, South Australia	Sandstone, Black Hills, Wyoming
Sandstone, central Wyoming	Sandstone, northern Arizona	Sandstone, central Utah
Case Hardening, Black Hills, Wyoming	Case Hardening, central Wyoming	Sandstone, Black Hills, Wyoming
Sandstone, Black Hills, Wyoming	Sandstone, central Wyoming	Sandstone, central Wyoming

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Efflorescence

Sandstone, western Oklahoma	Sandstone, central Utah
Sandstone, Black Hills, Wyoming	Granodiorite, southern Arizona

Subflorescence

Sandstone, central Wyoming

Granodiorite, southern Arizona

Lithobiontic coatings are organic remains form the rock coating, for example lichens, moss, fungi, cyanobacteria, algae. These biological agents are usually destructive.  However, the indexer should be aware that the removal of lithobionts can cause more erosion than simply leaving them alone.
 

Sandstone, eastern Wyoming	Silicified Dolomite, South Australia
Silicifed Dolomite, South Australia	Sandstone, eastern Wyoming

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The rock coatings below are shown for purposes of illustration.  You do not need to know how to distinguish these to complete the rock stability analysis.

Combinations

The indexer will often find a host of different types of rock coatings on the same panel.  It can be extremely difficult to distinguish these different types of accretions in the field, and even experts in rock coatings require advanced instrumention to tell the difference between different coating types.  Please do not think that the indexer will need to identify the type of rock coating.  That is not necessary.  Completing the index simply requires that the indexer characterizes the extent of all different types of rock coatings taken together.

These images show examples of different types of combinations that might be seen.
 

Sandstone, eastern Wyomin	Sandstone, Black Hills, Wyoming	Sandstone, central Wyoming
Sandstone, Black Hills, Wyoming	Sandstone, eastern Wyoming	Sandstone, southeast Colordao
Sandstone, Black Hills, Wyoming	Sandstone, central Wyoming
Sandstone, Black Hills, Wyoming	Sandstone, eastern Wyoming

Iron Film

Iron films are composed primarily of iron oxides or oxyhydroxides; unlike orange rock varnish because it does not have clay as a major constituent.
 

Sandstone, eastern Wyoming	Sandstone, Black Hills, Wyoming	Sandstone, Black Hills, Wyoming
Sandstone, eastern Wyoming	Sandstone, central Wyoming
Sandstone, central Wyoming

Oxalate

Oxalate crusts are typically composed of mostly calcium oxalate and silica with variable concentrations of magnesium, aluminum, potassium, phosphorus, sulfur, barium, and manganese. Oxalate crusts are often found forming near or with lichens, are typically dark in color, but they can be as light as ivory
 

Sandstone, Black Hills, Wyoming	Sandstone, Black Hills, Wyoming	Sandstone, eastern Wyoming
Sandstone, central Utah	Sandstone, southeast Colorado	Sandstone, Black Hills, Wyoming

Rock Varnish

Rock Varnish  is the most studied rock coating. Although it is found in all terrestrial settings, it is most easily recognized on the plentiful bare rocks seen in deserts (hence another common name is desert varnish). Also, the physical and chemical stability of rock surfaces in deserts allows sufficient time for this slow-growing accretion to form.  It is structured much like a brick wall, but its thickness (ranging from <5µm to almost 500µm) is typically less than 30µm  The clay minerals that comprise the bulk of the structure (~50-70%) are cemented to the rock by hydroxides of manganese and iron (15-50%).  The enigma of varnish is its great enrichment in manganese, typically over 50 times compared with the adjacent environment (soils, underlying rock, dust).
 
 

Sandstone, southeast Colorado	Sandstone, central Wyoming	Basalt, Coso Range, California
Sandstone, central Wyoming	Sandstone, northern Arizona	Sandstone, Black Hills, Wyoming
Sandstone, central Wyoming	Sandstone, eastern Wyoming	Silicified Dolomite, South Australia
Sandstone, Black Hills, Wyoming	Sandstone, central Utah	Sandstone, central Wyoming

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Silica Glaze  is a broad category of rock coatings that are dominated by amorphous silica with variable amounts of aluminum and iron.  They are usually less than 200µm thick, with a clear white to orange shiny luster, but they can be darker in appearance.  Silica glazes have been noted in warm deserts, cold deserts like Antarctica, on dry tropical islands, along tropical rivers, mid-latitude humid temperate settings, and various archaeological contexts. Silica glazes probably precipitate from soluble Al-Si complexes [Al(OSi(OH)3)2+] that are released from the weathering of phyllosilicate minerals.
 

Sandstone, Black Hills, Wyoming	Basalt, Kaho'olawe Island, Hawai'i	Sandstone, Black Hills, Wyoming
Sandstone, southeast Colorado	Sandstone, central Wyoming

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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).