Further south, an unusual subduction may have caused the growth of the Rocky Mountains in the United States, where the Farallon plate dove at a shallow angle below the North American plate. This low angle moved the focus of melting and mountain building much farther inland than the normal . Scientists hypothesize that the shallow angle of the subducting plate increased the friction and other interactions with the thick continental mass above it. Tremendous thrusts piled sheets of crust on top of each other, building the broad, high Rocky Mountain range.