Idioms
"From sea to shining sea" is an American idiom meaning from the Atlantic Ocean to the Pacific Ocean (or vice versa). Many songs have used this term, including the American patriotic songs "America, The Beautiful" and "God Bless the USA". In addition to these, it is also featured in Schoolhouse Rock's "Elbow Room". A term similar to this is the Canadian motto A Mari Usque Ad Mare ("From sea to sea.")
"Purple mountain majesties" refers to the shade of the Rocky Mountains, which Kate looked at while writing the poem.
Read more about this topic: America The Beautiful