Rapid formation of rock varnish and other rock coatings on slag deposits near Fontana, California

Manganiferous rock varnish, silica glaze and iron skins have formed on 20‐ to 40‐year‐old slag piles near Fontana, southern California. Rapid rock‐varnish formation is associated with an unidentified cocci bacterium that grows rapidly in culturing experiments, combined with the likelihood that Mn‐rich solutions flow over slag surfaces. A new model is proposed for the formation of silica glaze, involving soluble Al‐Si complexes and the nature of wetting films. Iron skins occur where runoff is concentrated, and where iron bacteria grow. Observations indicate: (i) substrate can be important in supplying Mn, as well as maintaining a stable surface and channelling runoff to microspots where varnish grows; (ii) ignorance of boundary layer pH/Eh conditions makes chemical‐physical models of varnish formation speculative; (iii) the balance between cementation of recently nucleated varnishes and erosional shear stresses is poorly understood, but is probably an important limiting factor on varnish growth rates; and (iv) there must be at least two steps in biotic varnishing‐accretion of Mn‐Fe in casts, followed by cementation of clays by either in situ cast residue or Mn‐Fe remobilized from casts. Varnish accretion may be limited by the rate at which both steps occur. Rapid development of readily distinguishable manganiferous rock varnishes at this post‐1952 site highlights the danger of speculation on the ages of landforms based solely on the appearance of rock‐varnish coatings.