RAIN SHADOW FORESTS OF THE ANCESTRAL ROCKIES; THE IN SITU BURIAL OF CALAMITES GROVES, PENNSYLVANIAN HERMOSA GROUP, SOUTHWESTERN COLORADO
Within the study area there are at least twenty five stem casts, rhizomes and impressions of Calamites. In situ stems range from 6–80cm tall with diameters ranging from 5-20cm. Rhizomes are preserved below 8 of these stems, and there are several examples of multiple stems arising from a common rhizome. Prostrate impressions of Calamites also occur on bedding planes. Internodal distances on the stems range from 3-20 cm. Inconsistent internodal length may suggest variable duration of growth seasons and moisture availability (Taylor et al., 2008), or timing between inundation events. No fruiting structures, lateral branches, or signs of paleosol development were observed.
This locality was very close to the Grenadier Highlands (Baars, 1966) of the Ancestral Rockies uplift and on the margin of the Paradox basin in western Pangea. Our data suggest that even in the rain shadow of the Ancestral Rockies, on the downwind side of Pangea, there was sufficient moisture to support dense growth of Calamites forests in the floodplains of braided rivers. In contrast with the coals of the underlying Atokan Pinkerton Trail Formation, this lower Desmoinesian paleoflora and the associated sediments suggest a well drained environment, with seasonal rainfall on these braid plains in the shadow of the Ancestral Rockies.