Rock varnish dating
Another first-order control on distribution rests in whether the varnish formed in a geochemical landscape (Perel'man 1961) favorable for stability; exposure to minimal acidity helps maintain the manganese and iron in their oxidized and immobile condition in varnish. Enumeration, isolation, and characterization of ultraviolet (UV-C) resistant bacteria from rock varnish in the Whipple Mountains, California, Icarus 174, 585-595.
A second-order issue turns on the nature of the erosional process. Rock varnish exists in geomorphic catenas (Haberland 1975; Palmer 2002), much in the way that soils vary along slopes (Jenny 1941).
Consider just Antarctic research, that has been consistently unable to distinguish between iron films and weathering rinds usually termed either desert varnish (Glasby et al.
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Note that rock varnish dominates the more stable hill crest; iron films increase in abundance towards the slope bottom, while rock surfaces mostly lacking either coating occurs in a similar abundance on all slope positions.
Palmer's (2002) general observation that different geomorphic positions influence rock varnish carries over to a single boulder.
Manganese enhancement, two orders of magnitude above crustal values, remains the geochemical anomaly of rock varnish and a key to understanding its genesis.
Field observations have resulted in a number of informal classifications.
Just considering varnish thickness, millimetre-scale topographic highs host thinner varnishes than "microbasins" or broad depressions a few millimetres across (Fig. Depressions more than a few millimetres deep favor retaining enough moisture to host varnish-destroying microcolonial fungi (Fig. Granodiorite boulders on a moraine in the Karakoram Mountains, for example, illustrate the importance of aspect where varnish grows on south-facing surfaces, while lichens and salt efflorescence occupy north-facing surfaces (Waragai 1998). 8.3 Originally named black globular units (Borns et al.