Complex sentence: By the 1960s, the post-hippie music that was labelled "progressive" (i.e. "progressive pop") would later come to be known as progressive rock, pushing the genre into greater complexity while retracing the roots of romantic and classical music.
Split into simpler sentences: By the 1960s, the post-hippie music that was labelled "progressive" (i.e. "progressive pop") would later come to be known as progressive rock. It directly followed the development of rock and roll, pushing the genre into greater complexity while retracing the roots of romantic and classical music.