Musical movement. In the mid 1960s the popularity of a number of British rock-and-roll ("beat") groups spread rapidly to the U.S., beginning with the triumphant arrival of Liverpool's Beatles in New York in 1964 and continuing with the Rolling Stones, the Animals, and others. Building on 1950s U.S. models, these groups incorporated such local musical traditions as skiffle (acoustic drummerless ensembles), dancehall, and Celtic folk