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Billion-year-old rock breaks into a jagged pattern in Glacier National Park, Montana. Rocks in the park are primarily sedimentary layers deposited in shallow seas over 1.6 billion to 800 million years ago. During the tectonic formation of the Rocky Mountains 170 million years ago, the Lewis Overthrust displaced these old rocks over newer Cretaceous age rocks. Since 1932, Canada and USA have shared Waterton-Glacier International Peace Park, which UNESCO declared a World Heritage Site (1995) containing two Biosphere Reserves (1976).