They provided a physical barrier that protected North America from invasion.
Explanation:
The North American Cordillera is the northern portion of the American Cordillera which follows the western margin of America. The North American Cordillera includes several subordinate mountain ranges, intermontane valleys and high plateaus in the west of North America with the Great Plains being the eastern limit in a considerable part of its journey from north to south. The North American Mountain Range passes through 3 countries: Canada, Mexico and the United States including Alaska.
It is composed in turn of the following main mountain ranges:
Brooks Range: North of Alaska and in Yukon.
Alaska Range: South of Alaska and in the Yukon.
Rocky Mountains: In Canada and the United States.
Pacific Coastal Chain: From the Kenai Peninsula (Alaska) to Sierra La Laguna (Baja California, Mexico), but its delimitation may vary according to the authors.
Cordillera de las Cascadas: In Canada and the United States.
Sierra Nevada: In California, USA
Sierras Madre System: From the US-Mexico border area to Central America
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