<P> The geology of the Rocky Mountains is that of a discontinuous series of mountain ranges with distinct geological origins . Collectively these make up the Rocky Mountains, a mountain system that stretches from Northern British Columbia through central New Mexico and which is part of the great mountain system known as the North American Cordillera . </P> <P> The rocky cores of the mountain ranges are, in most places, formed of pieces of continental crust that are over one billion years old . In the south, an older mountain range was formed 300 million years ago, then eroded away . The rocks of that older range were reformed into the Rocky Mountains . </P> <P> The Rocky Mountains took shape during an intense period of plate tectonic activity that resulted in much of the rugged landscape of the western North America . The Laramide orogeny, about 80--55 million years ago, was the last of the three episodes and was responsible for raising the Rocky Mountains . Subsequent erosion by glaciers has created the current form of the mountains . </P> <P> The rocks in the Rocky Mountains were formed before the mountains were raised by tectonic forces . The oldest rock is Precambrian metamorphic rock that forms the core of the North American continent . There is also Precambrian sedimentary argillite, dating back to 1.7 billion years ago . During the Paleozoic, western North America lay underneath a shallow sea, which deposited many kilometers of limestone and dolomite . </P>

How did the u.s. rocky mountains come to look the way they do today