Simplify the following text for a Grade 4 native English-speaking person.
Original: Rock and roll ( often written as rock & roll or rock 'n' roll ) is a genre of popular music that originated and evolved in the United States during the late 1940s and early 1950s , primarily from a combination of the blues , country music , jazz and gospel music . Though elements of rock and roll can be heard in country records of the 1930s , and in blues records from the 1920s , rock and roll did not acquire its name until the 1950s .
Simplified: Rock and roll is a form of rock music developed in the 1950s and 1960s .