Small-scale sedimentary features in medium- to coarse-grained siltstones (loessites) of the Maroon Formation (Middle Pennsylvanian to Early Permian) of northwestern Colorado are interpreted to be of rain-impact origin. These features, which have been experimentally produced by simulated oblique rain impact, are herein termed rain-impact microtopography, or RIM. Experiments indicate that RIM forms only in sediments ranging from fine silts to medium very fine sands (7 phi to 3.5 phi ). Probable fossil (ancient) examples of RIM superficially resemble numerous other sedimentary and biogenic structures, but a variety of observations and experimental procedures demonstrate their uniqueness. Recognition of RIM can be valuable as an indicator of subaerial exposure and paleowind direction.

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