At the height of the swinging ‘60s, British musicians found a channel for their angst by listening hard to imported blues records from Chicago and working out how to do it themselves. At first, the sound they came up with was rhythmic and needle-sharp: beat groups such as The Rolling Stones and The Animals covered greats like Chuck Berry, Nina Simone and John Lee Hooker with a theatrical intensity that helped set them apart from contemporary pop. As guitar amps became taller and louder, homegrown guitar heroes including Fleetwood Mac’s Peter Green and Eric Clapton distilled that same essence into monolithic riffs, like in Cream's incendiary version of "Crossroads." These sonic advances gave weight and scale to a musical form that originated in black America’s pain and, in the hands of stadium giants such as Led Zeppelin, would go on to become a sonic signature of ‘70s classic rock.