The Appalachian Mountains (/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.