Back in 2004 the BLM (it think) was looking at the area for public use. They stated that Gypsum was mined in Hidden Valley from 1915 to 1965 by U.S. Gypsum Corporation and was hauled on a three-foot gauge rail line to the plaster mill. During the mining process miners found several mammal fossils and bison bones.
Anyway....The mill operated just north of Highway 34 from 1887 until 1965 when a flood on Buckhorn Creek destroyed it. It sounds likt the old quarry was used to dump debris from the Big Thompson flood in 1976 (?)
Now, pictures....does anyone have pictures?????
Jerry