Assertion: blue rock | /r/CapableOf | become riff-based
Facets:
value: by the 1970s
type: TEMPORAL
Source sentences:
text: By the 1970s, blues rock had become heavier and more riff-based, exemplified by the work of Led Zeppelin and Deep Purple, and the lines between blues rock and hard rock "were barely visible", as bands began recording rock-style albums.