The Rocky Mountains are relatively young when compared to other major mountain ranges; they only began rising 75 million years ago. But to appreciate the true geology of the Rockies you have to look back to the Precambrian era. 700 million years ago the Pacific Ocean covered most of western North America. The ocean flooded eastward several times depositing layers of silt and sand. When life like marine invertebrates and crustaceans emerged their dead corpses were added to the sediment layers.
What happened next occurred about 75 million years ago; two tectonic plates collided. Specifically the Pacific Plate hit the North American Plate and was subducted beneath it. The layers of sediment were squeezed, twisted and folded; older strata were pushed above and on top of younger strata (Geology of the Canadian Rockies).


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