A patterned surface on the underside of a siltstone slab from Wyoming Triassic red beds bears a striking resemblance to Dendrophycus triassicus described by Newberry in 1888 from Triassic red beds of the Connecticut Valley. D. triassicus has been considered a rill mark by some and a plant imprint by others. The Wyoming specimen was found in the Red Peak Member of the Chugwater Formation. The pattern represents a system of branching ridges which filled grooves on a continuous underlying surface, rather than a series of tubes or imprints of independent branches. Distinctive features shared by the Chugwater pattern and D. triassicus specimens from Connecticut include development of the pattern on a broad surface with significant relief, straightness of major ridges, close spacing and sharp definition of long, narrow, straight-sided ridges, dichotomous aspect of branching of these ridges, lack of anastomosis of closely spaced ridges, and lack of undissected areas between adjacent minor branches. Laboratory experiments have failed to duplicate the pattern in full detail, but suggest that it is inorganic and formed at the edge of a body of water which repeatedly transgressed and regressed over a sloping surface of unconsolidated silt.