<Tr> <Th> Orogeny </Th> <Td> Taconic, Acadian, Alleghanian </Td> </Tr> <Tr> <Th> Age of rock </Th> <Td> Ordovician - Permian </Td> </Tr> <P> The Appalachian Mountains (French: les Appalaches), often called the Appalachians, are a system of mountains in eastern North America . The Appalachians first formed roughly 480 million years ago during the Ordovician Period . They once reached elevations similar to those of the Alps and the Rocky Mountains before experiencing natural erosion . The Appalachian chain is a barrier to east - west travel, as it forms a series of alternating ridgelines and valleys oriented in opposition to most highways and railroads running east - west . </P> <P> Definitions vary on the precise boundaries of the Appalachians . The United States Geological Survey (USGS) defines the Appalachian Highlands physiographic division as consisting of thirteen provinces: the Atlantic Coast Uplands, Eastern Newfoundland Atlantic, Maritime Acadian Highlands, Maritime Plain, Notre Dame and Mégantic Mountains, Western Newfoundland Mountains, Piedmont, Blue Ridge, Valley and Ridge, Saint Lawrence Valley, Appalachian Plateaus, New England province, and the Adirondack areas . A common variant definition does not include the Adirondack Mountains, which geologically belong to the Grenville Orogeny and have a different geological history from the rest of the Appalachians . </P>

Which mountains are higher the rockies or the appalachians