This is a 2 minute instrumental written by Jimmy Page, which was based on a traditional folk song called "Blackwater Side." The most popular version of "Blackwater Side" was recorded by Bert Jansch, but a British folk singer named Anne Briggs also did a version. She was a big influence on Led Zeppelin and on many British singers, but she stopped recording when she was 27 and never became famous. Briggs is sometimes incorrectly credited as the author of another song on the Led Zeppelin
album, "Babe I'm Gonna Leave You
," but that was written by an American folk singer named Anne Bredon.