The Valley by William Meikle

In 1863, a group of mercenaries are hired to help out a mining town in Montana. They arrive to find the town empty and some of the buildings wrecked and strewn over a wide area. A new cave leads them to a land that time forgot, a high valley full of animals that are extinct elsewhere, but have thrived in the remote environment. But something else is loose in the land, something even older that has emerged from the cave system. Soon the remaining men are fighting for their lives, and the lives of everything in the valley.