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The study of how rivers cross obstructing mountains, once popular in the early twentieth century, has seen a dramatic resurgence in the last decade. Since Hutton's scholarly introduction to a possible cause for transverse drainage, however, no single study has compiled all of the various criteria that can be used to discriminate among the four possible mechanisms of antecedence, superimposition, overfl ow, and piracy. This paper presents the fi rst such compilation and related methodology to apply these criteria both in tabular and graphical formats, as well as an online interactive tool in the data repository. Combining nominal and ordinal data sources, this methodology generates an objective, reproducible assessment for the mechanism most likely to have established the transverse drainage at fi ve ordinal levels of confi dence. When applied to southwestern U.S. sites, randomly selected through an objective spatial procedure, four general observations emerged on the relationship between the development of transverse drainage and landscape evolution. (1) Streams persisting through lengthy periods of extension develop antecedent canyons. (2) In order to reestablish through-fl owing channels, streams overfl ow closed basins as active extension wanes.(3) Following a drop in base level related to the newly developed trunk channels, streams tributary to the trunk channels incise into basin-fi ll deposits-sometimes leading to the development of superimposed drainages.(4) Tributaries eroding headward, in response to the integration of two or more closed basins, can capture and redirect drainage; this permits transverse drainage through both piracy and superimposition upstream of the capture event. Because extant criteria use nominal and ordinal data almost entirely, considerable potential exists to refi ne this approach through future strategies that incorporate interval data. Future use of a criteria-based method has the potential to inform on prior geomorphic studies by providing a new perspective with which to study how basins evolve in active tectonic regions, the analysis of related basin sedimentation, the hydrological and biological aspects of drainage evolution, and transverse drainage found in Martian crater fi elds.

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 AI-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.

A 450-year correction is required to make Anodonta14C dates comparable to 14C dates on other materials in the Mojave River basin. The internal stratigraphic consistency of 34 conventional 14C dates on Anodonta in this drainage basin indicates that such dates are usually reliable. The validity of most conventional 14C dates in the Mojave River basin may be a product of the basin's crystalline bedrock in a region usually typified by thick Paleozoic carbonate sections.

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