Geoscience Reference
In-Depth Information
Stage 1
Stage 2
Stage 3
Etampes
limestone
marl
silcrete
5 metres
unsaturated
Fontainebleau
sand
saturated
Fontainebleau
sand
groundwater
flow direction
500 metres
Figure 8.15
Model of groundwater silcrete development in the Paris basin (after Thiry, Ayrault and Grisoni, 1988) showing the
development of superposed silcrete lenses within bedrock adjacent to a progressively incising valley system.
8.7.2
Rock varnish
in subsoil environments or fissures and are 'exhumed'
during soil erosion or rock spalling; these coatings are
not considered here. During the initial stages of devel-
opment, faster-growing coatings will tend to dominate
over slower-growing types (Dorn, 2009). In many environ-
ments, this means that lithobiontic coatings (e.g. those as-
sociated with lichen and fungal growth; see Souza-Egipsy
et al.
, 2004) will out-compete the formation of most natu-
ral inorganic coatings. Many fast-growing lithobionts are
capable of weathering rock surfaces (Viles, 1995), includ-
ing pre-existing rock coatings. Assuming that a stable rock
surface is available, a mechanism is required to transport
the rock coating constituent minerals to the rock face.
In many cases, constituent availability may directly de-
termine the type of coating that develops (Dorn, 1998).
For example, Fe- and Mn-rich water flowing over a rock
face may result in the formation of a heavy metal skin
rather than rock varnish (Dorn, 2009), since a supply of
clay minerals (often absent from such waters) is essen-
tial to the development of a varnish (Potter and Rossman,
1977).
Rock varnish (often called 'desert varnish') ranges in
thickness from between 2.0 µm and 1.0 mm (Krumbein
and Jens, 1981; Dorn and Oberlander, 1982; Dorn, 1991).
It is usually orange, grey, brown or black in colour (Fig-
ure 8.16) and has a lustrous appearance. Most varnishes
comprise alternating layers of light and dark material
ranging in thickness from nanometres up to 20 µm, the
darker laminae containing a higher proportion of MnO
2
(Perry and Adams, 1978; Dorn, 1984; Dragovich, 1988;
Krinsley, Dorn and Tovey, 1995). The composition of
individual layers is linked to environmental conditions,
since Mn fixation is dependent on pH; alkaline condi-
tions restrict fixation and orange-coloured varnish devel-
ops (Dorn and Oberlander, 1982). Varnishes exhibit other
microtextures including dendritic growth patterns (Billy
and Cailleux, 1968) and lamellate and botryoidal struc-
tures (Dorn and Oberlander, 1982). A variety of factors
can affect the textures present, including the rate of dust
supply during formation, the clay mineralogy, the relative
