The Appalachian Mountains (Listeni/ˌæpəˈleɪʃɨn/ or /ˌæpəˈlætʃɨn/, 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 and once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded. The Appalachian chain is a barrier to east-west travel as it forms a series of alternating ridgelines and valleys oriented in opposition to any road running east-west. 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 provinces. 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. For a century, the Appalachians were a barrier to the westward expansion of the British colonies. The continuity of the mountain system, the bewildering multiplicity of its succeeding ridges, the tortuous courses and roughness of its transverse passes, a heavy forest, and dense undergrowth all conspired to hold the settlers on the seaward-sloping plateaus and coastal plains. Only by way of the Hudson and Mohawk Valleys, Cumberland Gap, and round about the southern termination of the system were there easy routes to the interior of the country, and these were long closed by powerful Native American tribes such as the Iroquois, Creek, and Cherokee, among others. Expansion was also blocked by the alliances the British Empire had forged with Native American tribes, the proximity of the Spanish colonies in the south and French activity throughout the interior.