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Case Hardening Vignettes from the Western USA: Convergence of Form as a Result of Divergent Hardening Processes

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The rock weathering literature contains the hypothesis that case hardening exemplifies equifinality, where the same end state can be reached by many potential processes in an open system. We present analytical data from six different sites in the western USA to assess the hypothesis of equifinality. Case hardening can be produced on: (1) sandstone in Petrified Forest National Park, Arizona, from the addition of silica glaze, rock varnish and heavy-metal skins; (2) sandstone in Whoopup Canyon, Wyoming, from silica glaze that formed originally inside subsurface joints combined with externally applied iron film, silica glaze, and rock varnish; (3) welded tuff in Death Valley, California, from the accumulation of rock varnish and heavy metal skins of Mn and Fe; (4) sandstone in Sedona, Arizona, from the protective effects of rock varnish accretion and heavy metal skins of Mn and Fe; (5) basalt on the Big Island, Hawai'i, from the accumulation of silica glaze inside vesicles; and (6) sandstone at Point Reyes, California, from a lithobiont mat of fungi and lichen. Each developed the general form of a case-hardened shell, protecting the surface from erosion. In accordance with the hypothesis of equifinality, the processes that led to similar appearance differ.
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53
Case Hardening Vignettes
from the Western USA:
Convergence of Form as a Result of
Divergent Hardening Processes
Ronald I. Dorn, Jacob Dorn, Emma Harrison, Eyssa Gutbrod,
Stephen Gibson, and Philip Larson
Arizona State University
Niccole Cerveny and Nicholas Lopat
Mesa Community College
Kaelin M. Groom and Casey D. Allen
University of Colorado Denver
ABSTRACT
e rock weathering literature contains the hypothesis that case hardening
exemplies equinality, where the same end state can be reached by many
potential processes in an open system. We present analytical data from six
dierent sites in the western USA to assess the hypothesis of equinality.
Case hardening can be produced on: (1) sandstone in Petried Forest
National Park, Arizona, from the addition of silica glaze, rock varnish and
heavy-metal skins; (2) sandstone in Whoopup Canyon, Wyoming, from
silica glaze that formed originally inside subsurface joints combined with
externally applied iron lm, silica glaze, and rock varnish; (3) welded tu
in Death Valley, California, from the accumulation of rock varnish and
heavy metal skins of Mn and Fe; (4) sandstone in Sedona, Arizona, from
the protective eects of rock varnish accretion and heavy metal skins of
Mn and Fe; (5) basalt on the Big Island, Hawai‘i, from the accumulation
of silica glaze inside vesicles; and (6) sandstone at Point Reyes, California,
from a lithobiont mat of fungi and lichen. Each developed the general
form of a case-hardened shell, protecting the surface from erosion. In
accordance with the hypothesis of equinality, the processes that led to
similar appearance dier.
54 "1$(:&"3#00,t7PMVNFt
Introduction
Differential weathering of rocks leads to varying degrees of resistivity
of dierent rock types to erosion. Such dierences in weathering provide
a fundamental control on Earths topography (Ollier 1984, Pain and Ollier
1995), as exemplied by dierent classic landforms settings. For example,
dierential weathering has led to dierences in erosion of the sedimentary
layers of the Colorado Plateau, resulting in certain resistant sedimentary
layers serving as caprocks in a staircase topography (Dutton 1882). e
talus atirons that occur in front of such cuestas develop because the talus
rock falls from the caprocks, in turn protecting the weaker rock beneath
(Gerson 1982). Of particular importance is the concept of topographic
inversion, where the most erosion-resistant materials become topographi-
cally prominent features within the landscape, despite a depositional origin
within a topographic low (Pain and Ollier, 1995). Examples include basalt
ows where lava owed down valleys, eventually becoming the high point
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stream-channel deposits are now preserved as relatively high topography
(Maizels 1990); and even duricrusts like calcrete (Reeves 1993) and silcrete
(Summereld 1983), at a much larger scale.
We examine the role of dierential weathering on the scale of individual
boulders or rock outcrops. Case hardening exemplies the importance of dif-
ferential weathering inuencing the dierential erosion of rock surfaces—in
our study, at the scale of meters to millimeters. Oen darkened by coatings
such as rock varnish, case hardening creates a dierential resistance to
detachment where indurated surfaces erode more slowly than unprotected
rock. Hardening the outer shell of a rock surface can also promote the
formation of visually interesting forms such as honeycombed weathering
features and rock shelters used by prehistoric people.
Although there exists a long-held belief that weathered solutions rise
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Walther 1891, Peel 1960, Longwell et al. 1950, Holmes 1965), very little
evidence exists for an internal origin of indurating agents. Instead, avail-
able evidence indicates that case hardening occurs when abiotic or biotic
materials, applied externally, increase the resistance to detachment in the
outer few millimeters of a rock surface (Dorn 2004, 1998).
Case hardening is important in physical geography and stone conserva-
tion. Geomorphologists connect case hardening to such issues as tafoni
GPSNBUJPO.FMMPSFUBMDMJČSFUSFBU&NFSZBOE'PTUFSBOE
55
Dorn et al: Case Hardening Vignettes from the Western USA
pedestal rocks (Crickmay 1935). Case hardening processes may play a role
JOUIF QSFTFSWBUJPO BOEEFDBZ PG IJTUPSJDCVJMEJOHT 'JU[OFSFU BM 
contemporary buildings (McAlister et al. 2003), and rock art (Tratebas et
al. 2004, Cerveny et al. 2006, Cerveny 2005).
Figure 1.—Newspaper Rock member sandstone within Petried Forest National
Park hosts many examples of case-hardened sandstone surfaces. Each image
in this gure displays petroglyphs originally engraved into the case hardening
on a planar joint surface darkened by rock varnish. Many of the engravings
in the park range from 600 to 1100 calendar years BP (Dorn 2006). Decayed
engraved surfaces are progressively undermined by erosion of the underlying
rock. With the assumption that the engravings were manufactured when the
case hardened surface was fully intact, the rate of erosion of many panel faces
can be roughly estimated as ve to ten percent of a panel surface per century.
56 "1$(:&"3#00,t7PMVNFt
Weathering forms such as case hardening support the geomorphic
concept of equinality (Goudie and Viles 1999, Turkington and Paradise
0MMJFS*OFRVJĕOBMJUZUIFTBNFFOETUBUFDBOCFSFBDIFECZNBOZ
potential means in an open system. We present analytical data from dierent
sites in the western United States to assess the hypothesis of equinality for
case-hardened surfaces.
Field Sites
Sampling sites analyzed for this study were not selected randomly. We
compiled analyses collected for other projects that have been repurposed
to understand case hardening in selected western United States sites. For
example, the granodiorite of South Mountain, Arizona, was sampled to
explore the eect of paintball on stone surfaces. Welded tu in Death Valley
was collected to understand rock varnish processes, and the basalt on the
Big Island of Hawai‘i was collected as part of a petroglyph dating project.
us, the electron-microscope imagery gathered in previous studies can be
used in our attempt to assess equinality of case-hardened surfaces.
Four sampled sites in three very dierent environments involve sand-
stone. Samples from the high desert of Petried Forest National Park (PEFO)
on the Colorado Plateau were collected to understand rock art chronology
(Dorn 2006). e pine forests of the Black Hills provide the environmental
backdrop for samples collected to understand how re impacts rock art pan-
els (Tratebas, Cerveny, and Dorn 2004). Marine terrace-derived sandstone
from Point Reyes was sampled to understand the nature of rock coatings in
this maritime climatological setting, as were Sedona’s semi-arid cli faces.
In addition to samples collected for electron microscopy, Petried Forest
National Park provided the setting for a systematic study of petroglyph panel
stability. Students from multiple institutions collected data on over 2,500
panels distributed across the park.
Methods
Dierent microscopic techniques generated imagery and chemistry to un-
derstand the nature of case hardening at dierent sites. Light microscopy
provides information on the scale of microns to millimeters. Scanning
electron microscopy (SEM) generates a topographic perspective using sec-
ondary electrons (SE). SEM with a back-scattered electron detector (BSE)
generates an image of average atomic number (Z) of a at surface that, here,
is typically a cross-section from the surface of a sample down into the rock.

Dorn et al: Case Hardening Vignettes from the Western USA
Energy dispersive X-ray (EDS) and wavelength dispersive (WDS) analyses
measures the elemental composition of specic areas—typically micron-
sized spot sizes.
In addition to electron microscopic techniques, case hardening was
studied in the context of a rock art project carried out at PEFO. e Rock
Art Stability Index (RASI) is a triage technique for condition assessment
of rocks containing Native American rock art (Dorn et al. 2008, Cerveny
2005). RASI is used extensively throughout PEFO, and in this study col-
lege students and K–12 teachers gathered data relating case-hardening
with adjacent aking and scaling erosion. e methodology requires eld
researchers, typically docents or introductory science students in college,
to utilize basic training to gather observation eld-data for approximately
three dozen weathering characteristics. Scores are compiled in the laboratory
and a value of stability assigned on a panel-by-panel basis. Generalizations
or regional causation for specic weathering patterns can then be analyzed
and communicated, giving sound data for site managers to make informed
conservation and preservation decisions. In the PEFO research, over 2,500
panels were measured using RASI, resulting in data being gathered on case
hardening for an unprecedented number of sites.
Results
Petried Forest National Park
Case hardening at Petried Forest National Park (Figure 2A) results from a
mixture of dierent added constituents. e process starts when silica glaze is
added to the walls of buried joint faces. Water moving through joints carried
dissolved silica that reprecipitated in pores (Figure 2C). en, upon exposure
of a joint at the surface, rock varnish formed. Although portions of varnish
are geochemically stable, much of the varnish can dissolve. Remobilized
iron and manganese has then moved into the pores in the sandstone and
added to the case hardening (Figure 2B).
Reports were collected on the weathering condition of over 2,500 rock
art panels in Petried Forest National Park. e RASI scoring system is an
index amounting to a score of 0 if there is no weathering and over 60 if the
panel is falling apart from rock decay. Because case hardening provides
a stabilizing inuence, researchers scored case hardening with negative
numbers in an ordinal scale of not present (0), present (-1), obvious (-2),
or dominant (-3). e overall pattern (Figure 3) reveals that case harden-
58 "1$(:&"3#00,t7PMVNFt
ing is present in about forty-two percent of the scored panels, and that case
hardening dominates only ve percent of the rock art panels.
e spatial distribution reects the presence of the Newspaper Rock
member sandstone, the locations of petroglyph manufacturing, as well
as a clustering of dominant case hardening whose cause has not yet been
determined.
Figure 2.—Case hardening of sandstone at Petried Forest National Park. (A) A case-
hardened joint face forms a stable surface used for carving of a bird motif and an earlier
anthropomorphic image. Note, however, that the case hardening has been breached—
with wristwatch for scale. (B) Back-scattered electron-microscope image of a cross-section
collected at location c” in image A. The bright material between the grains is a heavy metal
skin of iron and manganese constituents, likely dissolved from rock varnish and mobilized
into the sandstone to stabilize the upper millimeter of a surface. Note also the abundant
porosity underneath the case hardened heavy metal skin. Once the case hardening is
breached, this porosity makes erosion rapid. (C) Another type of case hardening exists
at the very surface of this panel. A mixture of silica glaze and rock varnish can oer
additional stability in the upper few microns, where this secondary electron image was
from a sample also collected from the letter C above the watch in image A.
59
Dorn et al: Case Hardening Vignettes from the Western USA
Whoopup Canyon, Black Hills, Wyoming
Whoopup Canyon in the Black Hills, Wyoming, exhibits sandstone joint
faces that were originally case hardened by silica glaze before the joints
became exposed. Iron lm then remobilized under acidic conditions and
further impregnated the sandstone. Later rock varnish formation led to
remobilization of the varnish constituents that also moved downward into
the rock to mix with silica glaze. us, the case hardening is a complex mix-
ture of dierent materials. e porosity from enhanced weathering in the
underlying sandstone allows detachment of rock fragments as rock aking.
Figure 3. —Histogram and mapping of observations of case hardening at rock
art panels in Petried Forest National Park. “Dominant” observations of case
hardening are the smallest symbol on the map; they represent the smallest
number of observations and hence are visible because they are portrayed on top.
60 "1$(:&"3#00,t7PMVNFt
Death Valley, California
Studies of rock varnish in Death Valley have yielded insight into the impor-
tance of lichens, even in hyperarid settings. Lichens colonizing the surfaces
of rock varnish secrete acids.
Figure 4.—Sandstone
surface at Whoopup
Canyon, Black Hills,
Wyoming (width of upper
image about 30 cm) has
case hardened due to
the accumulation of
three types of materials
as revealed in the lower
back-scattered electron-
microscope image. First,
silica glaze impregnated
a joint surface, while it
was still unexposed in the
subsurface. Second, iron
seeped into the sandstone
and lled in pore spaces
to cement sand grains
together. Third, rock
varnish formed on
the surface and also
remobilized to mix
with silica glaze.
Enhanced dissolution
of quartz underneath
the case hardening led
to detachment of the
millimeter-scale ake.
61
Dorn et al: Case Hardening Vignettes from the Western USA
e acidity reduces Mn (IV) and Fe (III), which causes the varnish to
develop nanoscale pore spaces (Figure 5C) as the Mn (II) and Fe (II) migrate
into the underlying rock.
Figure 5.—Death Valley rock varnish undergoing dissolution from the acid-
producing activity of lichens. (A) Secondary electron (SE) image that shows the
topography of a cross-section. (B) Back-scattered electron (BSE) image that shows
the atomic number (Z) of the material in this cross-section. Notice that most of
the lichen turns black in the BSE image, because of its lower atomic number.
The bright specks in the lichen are oxalate minerals that often precipitate inside
lichens. (C) Close-up BSE image of the pocket of porous material underneath
the lichens.
62 "1$(:&"3#00,t7PMVNFt
e process of varnish remobilization was studied at the nanoscale
level through energy-dispersive X-ray analyses. Prior electron-microscope
elemental data on rock varnish chemistry generated only data from microm-
eter-scale spot analyses that are typically two microns in diameter. rough
the use of higher-resolution electron microscopy and higher energy levels,
spot sizes were reduced to about 1.3 nanometers. e descriptive statistics
of these data reveal tremendous variability that could be from the ongoing
dissolution of the varnish. Whereas larger spot sizes would “average” these
tiny pores, the tremendous nanoscale chemical variability suggests that
varnish undergoes uneven dissolution on a submicron scale.
Figure 6.—(A) Welded tu, Death Valley, forms a rock shelter, because of a
case-hardened outer shell. Image A is 2 meters wide. (B) In this back-scattered
electron image, the brighter material accumulating in the fractures is manganese
(Mn)-Iron (Fe)-clay rock varnish. C) In addition, some of the Mn and Fe have
reprecipitated in micron-sized pore spaces. The lichens (li) appear in images A
and B because of its lower atomic number.
63
Dorn et al: Case Hardening Vignettes from the Western USA
Table 1. Descriptive statistics of 128 nanoscale measurements of rock varnish
undergoing remobilization from lichens.
Element Average Median Standard
Deviation
Minimum
Value
Maximum
Value Range
C 0.14 0.00 1.20 0.00 11.21 11.21
F 3.32 1.45 3.71 0.00 13.18 13.18
Mg 1.46 1.31 1.42 0.00 18.34 18.34
Al 17.89 18.11 1.24 10.72 19.90 9.18
Si 11.43 10.96 2.78 1.58 21.56 19.98
P 1.17 1.22 0.24 0.37 1.69 1.32
Cl 0.00 0.00 0.03 0.00 0.20 0.20
K 0.48 0.44 0.21 0.23 1.56 1.33
Ca 0.69 0.58 1.41 0.26 17.78 17.52
Ti 0.73 0.00 2.86 0.00 15.50 15.50
Mn 14.90 15.48 4.08 3.96 24.21 20.25
Fe 6.57 6.85 2.02 0.77 12.04 11.27
Cu 0.95 0.92 0.18 0.50 1.76 1.26
Zn 0.00 0.00 0.06 0.00 0.68 0.68
Ba 0.94 1.24 0.84 0.00 2.97 2.97
Ce 0.28 0.00 0.38 0.00 1.32 1.32
Re 0.01 0.00 0.09 0.00 0.80 0.80
O 39.06 39.06 2.96 33.10 55.23 22.13
e instability of varnish very much relates to case hardening. e
indurated rock has formed a rock shelter where rock varnish and heavy
metals have been dissolved by lichens. Aer uneven dissolution of nanoscale
pockets, some of the varnish moves into fractures in the rock and reprecipi-
tates. Occasionally, just the iron and manganese ll in pore spaces as heavy
metals without the clays. us, even in a hyperarid environment, the acid-
ity produced by lithobionts can be important in freeing up case-hardening
agents to penetrate into the rock.
Sedona, Arizona
e case hardening of sandstone by the accumulation of manganese in the
pore spaces does not necessarily have to do with the dissolution of varnish
constituents, as exemplied in the previous results sections. e dark streaks
BU4FEPOBTPNFUJNFTSFTVMUGSPNUIFQSFDJQJUBUJPOPGNBOHBOFTF'JHVSF
64 "1$(:&"3#00,t7PMVNFt
e manganese moves into the pore spaces between the sand grains
and sometimes pushes the grains apart. e material composing these dark
streaks is oen a mix of true clay-Mn-Fe rock varnish and heavy metal skin
dominated by manganese. e case hardening can extend into the rock up
to a millimeter.
Hawai‘i
Basalt surfaces on the rainshadow portions of the island of Hawai‘i oen
accumulate silica glaze. e silica glaze that coats ow surfaces dissolves
and reprecipitates in vesicles inside the basalt. One result is a color change
from the dark black of a basalt ow to the light brown of a silica-glaze coated
surface. A second result is the case hardening of the basalt surface.
Figure 7.—Waterow streaks at Sedona can form through the accumulation of
fungi, lichens, heavy metal skins, and rock varnish. In this case, the sampled streak
is a mixture of rock varnish (at the surface) and heavy metal skins (remobilized
magniferous varnish) that move downward through pore spaces to impregnate
the sandstone of Sedona, at Schnebly Hill Road, Arizona.
65
Dorn et al: Case Hardening Vignettes from the Western USA
Point Reyes, California
Sandstone weathering at Point Reyes includes a variety of forms, such as
tafoni, alveoli, gnamma pits, and case hardening. A sample of case-hardened
sandstone reveals the presence of a lithobiontic crust composed of lichen
mixed with fungi. e biolm also contains calcium oxalate minerals, per-
haps secreted by the lichens. e biolm was typically less than 0.1 mm thick.
However, the laments are clearly able to bind the underlying sandstone in
a way that indurates the surface.
Figure 8.—Hawai‘ian petroglyph panels and petroglyphs are often case hardened
by silica glaze that originally accretes on surfaces, and then remobilizes into the
rock and inlls vesicles. The back-scattered electron-microscope image on the
left shows dark-colored silica glaze, where about 30% of a vesicle has been lled
by silica glaze. In addition to silica glaze, other precipitates (e.g. iron skins, rock
varnish, carbonate) formed inside the vesicle. The image on the right from the
Ki’I site on the Big island of Hawai‘i shows a panel composed of basalt, but the
lighter color of the engraved surface in the foreground derives from silica glaze.
66 "1$(:&"3#00,t7PMVNFt
South Mountain, Arizona
We think it is important to mention anthropogenic agents in this discussion
of case hardening, since humans continue to inuence natural surfaces. One
way that natural rock surfaces are impacted rests in grati and activities like
paintball gaming. e study of modern paints on rock surfaces has heretofore
been limited to conservation eorts designed to remove or hide these scars.
In a pilot study to examine the inuence of paintballs, we examined
samples of natural rock surfaces hit by this recreational activity using
back-scattered electron microscopy. Preliminary results indicate that the
paint has begun the process of physically separating from the underlying
rock coating. ere are still abundant attachment points. However, in the
unknown amount of time between the paintball attachment and sampling,
substantial physical separation has occurred. us, paintballing may not
become a case-hardening agent and will eventually detach itself from rock
surfaces, eliminating paintballs as agents of rock case hardening.
Figure 9.—Point Reyes case hardening in Image A, where the ice plant in the lower
right provides scale. The indurated sandstone erodes because of undermining
as the unhardened sandstone detaches. A secondary electron image of the case
hardened shell reveals that it is composed of a ~40 µm thick layer of a mat of
lichen mixed with fungi. The angular materials underneath the lamentous mat
are minerals in the underlying sandstone.

Dorn et al: Case Hardening Vignettes from the Western USA
Figure 10.Anthropogenic paint balls applied to granodiorite rock surfaces
at South Mountain Park, Arizona. BSE imagery reveals that the anthropogenic
paint (p) separates from the underlying rock surface after application of epoxy
(ep) application of the vacuum used to carbon coat the samples. The BSE
imagery and EDS analyses also reveals two case-hardening agents are present
on the rock surface: rock varnish (rv) and iron lms (if). The iron lms tend to
incorporate silt-sized fragments as preprecipitating containing iron envelope
dust particles that are ubiquitous on rock surfaces. Note: normally, BSE imagery
shows epoxy as black. However, the contrast and brightness were adjusted in
a way to minimize contrast and reveal the epoxy and the paint.
68 "1$(:&"3#00,t7PMVNFt
Discussion
Our ndings indicate that equinality does appear to be valid for the wide
variety of study sites and environments in the western United States that
were studied using electron microscopy. Case hardening can occur through
lithobiontic crusts composed of fungi and lichen, by lichens dissolving heavy
metals that are then reprecipitated in the outer shell of a rock, by silica glaze
infusing into vesicles in basalt ows, and by mixtures of silica glaze, rock
varnish, and heavy metals that hold sandstone grains together.
e persistence of equinality can be understood further by examin-
ing environmental change over time for the studied sandstone surfaces of
Petried Forest National Park and Whoopup Canyon.
The first step in-
volved the formation of
silica glaze inside un-
opened joint faces that
started the process of
case hardening. Second,
erosion of soil and more
weathered rock exposed
silica-glaze impregnated
joint faces to the subaer-
ial environment. ird,
subaerial rock varnish
formed on the surface.
Fourth, leaching of the
iron and manganese out
of the rock coating and
into the underlying rock
added to the initial case
hardening started by
the silica glaze. Thus,
the equinality of case
hardening forms that
started with silica glaze
inside a subsurface joint
Figure 11.Weathering profiles in many different
rock types develop in a fashion generalized in this
diagram. Then, enhanced erosion can strip the overlying
weathering zone. At Petried Forest National Park, for
example, the soil and highly weathered rock materials
that may have been present during the late Pleistocene
have eroded away. Moderately weathered and slightly
weathered rocks are now exposed at the surface. The
silica glaze that formed inside joint faces represents the
rst step in case hardening. This diagram is modied
from (Ehlen 2005).
69
Dorn et al: Case Hardening Vignettes from the Western USA
was enhanced by completely dierent processes operating in a surcial
environment.
Another perspective on equinality can be understood by examin-
ing a time sequence of case hardening on a single rock face (Turkington
and Paradise 2005), where case hardening stabilizes a surface for a time
as the underlying rock continues to weather. When the hardened surface
is breached, this leads to rapid erosion of the underlying heavily decayed
weathering rind. Aer the weathered material erodes away, the surface can
restabilize and begin to case harden.
is generalized sequence can be seen in operation at the Petried For-
est site. Case hardening has stabilized the surface, but the back-scattered
electron-microscope image reveals that weathering has continued under-
neath as indicated by the abundant porosity in this weathered rind. e
breach keeps growing until the weathered rind is eroded down to fresher
rock, at which time a new round of case hardening takes place. e cycle
continues when this new case hardening is breached. With forty-two percent
of panels hosting case hardening, and a process that reforms induration aer
Figure 12.From Turkington and Paradise (2005), a sandstone surface can
undergo a series of changes. In this idealized diagram, subsurface dissolution
leads to the creation of a weathering rind (A), followed by the surcial
accumulation of minerals and lithobions (B) to case harden surfaces (C). Then, the
case-hardened surface is breached (D), leading to rapid erosion of the decayed
weathering rind underneath (E and F). Eventually, the new surface stabilizes (G).
 "1$(:&"3#00,t7PMVNFt
Figure 13.A petroglyph panel of sandstone at Petried Forest National Park
supports the general model of sandstone face erosion (Turkington and Paradise
2005), where the outermost case-hardened panel was breached, leading to
the enlargement of the cavity. Then, after the weathered rind eroded away,
the surface restabilized with the formation of rock varnish and some case
hardening. Then this restabilized surface was breached in turn. The lower,
back-scattered electron-microscope image (collected from SEM sample arrow)
shows how the rock underneath the varnish-stabilized surface continues to
decay. Much of the quartz has dissolved, and the porosity of the weathering
rind has increased over time.

Dorn et al: Case Hardening Vignettes from the Western USA
breaching of the initial surface, case hardening appears to be a persistently
developing land surface within the Park.
Conclusion
e general eld of rock decay concerns itself with processes and forms
produced by biogeochemical and biophysical mechanisms. e end result
of rock decay is typically thought of as unidirectional, where processes of
rock decay promote further rock decay. e output” of decay is typically
thought of as products transported by erosional processes, where sucient
particle-size dimunition eventually generates transportable material.
rough investigation of rock data from six dierent sites around the
western United States, we explored whether the concept of equinality ap-
plies to case hardening. In each of the sites from which data were collected,
case hardening had occurred in a way that was both unique and consistent
with the environment that housed the rock. Stone surfaces collected from all
locales demonstrate evidence of case hardening due to a variety of external
factors. Although silica glaze is present in half the sites we considered, the
conditions under which that glaze formed were dierent at each location.
Furthermore, case hardening caused by rock varnish and heavy metal skins
of Mn and Fe was observed at two sites in Arizona, and we determined
that case hardening was caused by an accumulation of fungi and lichen
in California. us, we concluded that despite dierent processes coming
into eect at each of the sites, a protective covering or shell that performs
the same ultimate function—case hardening—is present in all cases. ese
ndings support our hypothesis of equinality.
Case hardening stands as a testament to the existence of negative feed-
backs in this progression of rock decay, where the products of rock decay
act to inhibit erosion. In doing so, case-hardened rock surfaces stand out as
similar forms in a host of dierent rock types and dierent environmental
settings. ere is an outer shell, typically only millimeters thick, that stands
out in relief through the more rapid erosion of unprotected rock.
Negative feedbacks, where the products of rock decay promote surface
stability, appear to be a universal phenomenon where many dierent types
of processes can produce the form of case-hardened surfaces (Turkington
BOE1BSBEJTF1IJMMJQT(SBCFUBMćFSFTFBSDIQSFTFOUFE
here is consistent with and fully supportive of the hypothesis of equinality.
In dierent open systems involving the decay and reprecipitation of rock
materials, many dierent processes have produced case hardening.
 "1$(:&"3#00,t7PMVNFt
Our conclusion is that with equinality being such a persistent negative
feedback to surface denudation in a host of dierent contemporary bare-
rock geomorphic settings, case hardening may have been a very important
QSPDFTTJO&BSUITFBSMZIJTUPSZ1SJPSUPUIFBEWFOUPGMBOEQMBOUTNJMMJPO
years ago and land fungi 1.3 billion years ago (Heckman et al. 2001), Earths
surface could have been protected from erosion by the action of case hard-
ening. To the best of our knowledge, the possible role of case hardening in
protecting the early Earth is a hypothesis that has not yet been considered
in the scholarly literature.
Acknowledgments
Research was supported by National Science Foundation awardsDUE-
 BOE UISPVHI DPPQFSBUJWF FČPSUT XJUI 1FUSJĕFE'PSFTUڀ /BUJPOBM
Park. We thank the many Mesa Community College, Universityof Colorado
Denver, and Arizona State University students who compileddata on case
hardening at Petried Forest. We also thank Death ValleyNational Park
and Point Reyes National Seashore for permission to takesmall samples
for analyses.
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In order to identify those petroglyph and pictograph panels most susceptible to damage, we propose a field-friendly index that incorporates elements of existing strategies to characterize the stability of stone. The Rock Art Stability Index (RASI) has six general categories: Site Setting (geological factors); Weakness of the Rock Art Panel; Evidence of Large Erosion Events On and Below the Panel; Evidence of Small Erosion Events on the Panel; Rock Coatings on the Panel; and Highlighting Vandalism. Initial testing reveals that training of individuals with no prior background in rock decay can be conducted within a two-day period and yield reproducible results. RASI’s use as a tool to promote cultural resource sustainability includes the use of a Geographic Information System to store, display and analyze rock art.
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This paper presents results of the first study of pre-fire and post-fire samples collected from rock engravings and adjacent sandstone joint faces. A 2001 wildfire at Whoopup Canyon, Wyoming, stimulated a comparison of 1991 and 2003 samples. Opti- cal microscopy of ultra-thin sections, backscattered electron microscopy, x-ray (energy dispersive and wavelength dispersive) analysis of cross sections, and high-resolution trans- mission electron microscopy reveal that fires create some thermal fractures that enhance panel erosion, but most of the fire-induced erosion occurs along weathering rinds that form long before petroglyph manufacturing. In addition, rock varnish on top of petro- glyphs experiences spalling, and fire ash with a clear potassium spike strongly adheres to rock varnish on petroglyphs and spalled sandstone. In the past, site managers assumed minimal damage away from massive spalls and other macrodamage on fire affected petro- glyphs, an assumption no longer tenable. Since it is difficult to protect rock art after a fire starts, mitigation efforts can include identification of areas of intense weathering-rind development as locales most susceptible to erosion, and clearing trees and shrubs near rock art by hand. (Key words: geomorphology, fire, rock art, sandstone, weathering.)
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Sandstones of the Paleozoic Beacon Supergroup in the Dry Valley region of Antarctica (also known as the Ross Desert), undergo at least three chemical weathering processes. (1) Exudation of oxalate ± other chelators by endolithic microorganisms causes translocation of elements, producing distinctive Fe pigment patterns and accelerating mechanical weathering (exfoliation) in rocks colonized by endolithic organisms. (2) Formation of thin siliceous crusts (<0.1 mm) stabilizes rock surfaces. The siliceous crusts form by accumulation and in situ alteration of airborne dust composed of quartz, clays, and Fe oxy hydroxide s. Crust textures include petrographically amorphous lenses of silica, stained and laminated birefringent coatings, and chalcedonic protrusions in surface pits. (3) Silicification of porous quartz sandstones by growth of quartz in optical continuity with host grains produces impermeable rinds up to several cm thick. The rinds form by transport of silica toward subaerially-exposed surfaces due to wet/dry and warm/cold cycling of microenvironments. The resulting interstitial precipitates commonly trap pre-existing grain coatings in pores, thereby preserving biogenic pigment patterns in colonized rocks as trace fossils of endolithic microorganisms.
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Pedestal rocks of granite, commonly associated with residual boulders, occur at many places in the southern Appalachian Piedmont. The rocks are essentially homogeneous in composition but exhibit marked contrasts in weathering of the cap and the shaft. They have previously been thought to have originated from joint blocks through frost action, rain wash, and decomposition. It is here shown that granular disintegration initiated by expansion through hydration is the dominant process involved. Hydra-tion is at a maximum on the lower slopes where there is some protection from the direct rays of the sun and where evaporation is at a minimum. The pedestal rocks are restricted to peneplains of Tertiary? age.
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The recent discovery of two calcium carbonate-cemented drainage channels in the Pliocene Ogallala section of the Southern High Plains, Texas and New Mexico, indicates that Ogallala drainage channels were commonly cemented with calcium carbonate, due to leaching of calcrete from adjacent Ogallala sands. Parallel drainage lines near Lovington, New Mexico, originally thought to represent calcrete incisement along ancient dune swales, are now interpreted as an example of recent drainage between a suspenparallel drainage pattern in bas-relief.
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Silcrete has been widely cited as an indicator of arid or semi-arid environments, such a palaeoclimatic inference deriving from a number of specific studies of Cenozoic silcretes in southern Africa, Australia and elsewhere. Recent investigations of Cenozoic silcretes in southern Africa reveal, however, the existence of two petrographically and geochemically distinct silcrete types, one of which has apparently formed under humid conditions.Silcretes in the Kalahari Basin (Botswana) have developed through silicification of a variety of host materials, especially playa sediments and calcrete. Petrographic characteristics (including length-slow chalcedony vugh-fills), lack of associated deep westhering profiles and limited independent palaeoclimatic evidence, suggest silcrete genesis in an alkaline, arid/semi-arid environment. In contrast, silcretes in the Cape coastal zone (South Africa) are almost invariably associated with deeply weathered bedrock, and are TiO2-rich in comparison with Kalahari silcretes. Local mobilisation and co-precipitation of silica and titanium is indicated by the presence of authigenic anatase in colloform features and glaebules in silcrete. This implies a low pH (< 4) during silcrete formation by silica replacement of weathering profile clays. Such a highly acidic weathering environment suggests abundant vegetation and a humid tropical or subtropical climate.Other recent reports of occurrences in Australia and France support the view that TiO2-rich weathering profile silcretes form under a humid climate. A semi-arid/arid palaeoclimatic interpretation remains valid for non-weathering profile silcretes, and for silcretes occurring within, but not genetically related to, deep weathering profiles.
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Rock varnish, erosional grooves, and well-developed cavernous weathering phenomena occur in close association on a small biotite-monzogranite nunatak in the Northern Foothills region, Northern Victoria Land, Antarctica. The grooves, similar in appearance to the 'rinnenkarren' described in the karst literature, are developed on steeply inclined (>35degrees) bedrock surfaces while the rock varnish occurs on adjacent, more gently sloping (<15degrees) bedrock surfaces. The varnish forms a resistant carapace through which small weathering pits have developed and below which are large cavernously weathered hollows (taffoni). We argue that the intimate association between the grooves and the rock varnish indicate the nunatak has been exposed to a long period of subaerial weathering. The preservation of both phenomena supports (a) the idea that landscape modification in this exceptionally cold and and region of Antarctica is very slow and (b) the long-term stability of the Antarctic ice sheet. Copyright (C) 2002 John Wiley Sons, Ltd.