<P> Human population is not very dense in the Rocky Mountains, with an average of four people per square kilometer and few cities with over 50,000 people . However, the human population grew rapidly in the Rocky Mountain states between 1950 and 1990 . The forty - year statewide increases in population range from 35% in Montana to about 150% in Utah and Colorado . The populations of several mountain towns and communities have doubled in the last forty years . Jackson, Wyoming, increased 260%, from 1,244 to 4,472 residents, in forty years . </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> <P> In the southern Rocky Mountains, near present - day Colorado, these ancestral rocks were disturbed by mountain building approximately 300 Ma, during the Pennsylvanian . This mountain - building produced the Ancestral Rocky Mountains . They consisted largely of Precambrian metamorphic rock forced upward through layers of the limestone laid down in the shallow sea . The mountains eroded throughout the late Paleozoic and early Mesozoic, leaving extensive deposits of sedimentary rock . </P> <P> Terranes began colliding with the western edge of North America in the Mississippian (approximately 350 million years ago), causing the Antler orogeny . For 270 million years, the focus of the effects of plate collisions were near the edge of the North American plate boundary, far to the west of the Rocky Mountain region . It was not until 80 Ma these effects began reaching the Rockies . </P>

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