The geological history of the front range of the Rocky Mountains is complex, shaped by geological forces far to the west. About 70 million years ago, subduction of the Farallon tectonic plate under the North American plate gave rise to the Rockies. Over millions of years, the crust pushed upward and older rock slid west over younger rock in an overthrust fault that pushed up, deformed, and folded the rocks as it moved. Later, Pleistocene ice sheets carved the rocks into their scrambled appearance. The gray rocks are Madison Limestone. About 350 million years ago, much of Montana was submerged under a shallow sea where billions of tiny marine creatures lived, and when they died their bodies settled into muck on the sea bed. After hundreds of millions of years of accumulation and many more millions of years, this material metamorphosed into the pale gray rocks of the front range. Because Madison Limestone resists weathering and erosion better than most other rocks, it formed the spectacular cliffs and dramatic ridges to the west. The front range has also been a paradise for wildlife for at least 70 million years, from duck-billed hadrosaurs and triceratops along the shores of an ancient sea to grizzlies, elk, mountain lions, deer, wolves, moose, and rattlesnakes today.