Led Zeppelin,in particular,Jimmy Page, has collaborated with English folk singer songwriter Roy Harper,so it's interesting to speculate that Page had a keen interest in medieval English folk music...as obviously demonstrated throughout Stairway to Heaven !
English folk singer Sandy Denny has been featured on The Battle of Evermore. Gallows Poll smacks of folk music if you strip it down.
Led Zeppelin's folk period only lasted for two years on Led Zeppelin III (1970) and Led Zeppelin IV (1971) with a small smattering of folk rock. IMHO it is the period in which the band had folk(folk rock) on their radar!