| A cat track--probably genus Pseudaleurus--from the middle Miocene Barstow Formation, preserved on the bedding plane of volcanic tuff radiometrically dated at 14.8 million years old--hardened material deposited into clear lake waters whose normal freshwater alkalinity was interrupted on occasion by evaporative periods and enormous quantities of volcanic ash ejecta which, eventually reworked by erosion and redeposited as firm mudflats, provided a perfect medium (analogous to wet concrete) to preserve the trackways of horses, camels, pronghorns, gomphothere proboscideans, and bear-dogs. The trackways are now found along the tilted and folded bedding planes of those lithified volcanic tuffs. Photograph courtesy Raymond M. Alf. |