cdandlp.com has a large enough picture of Ian & Sylvia's album's backside so that one can read it. The sleeve notes give this as their source:

Also from the Cecil Sharp collection of Southern mountain folksongs of British origin comes the version of The Greenwood Sidie (Cruel Mother - Child No. 10) heard here. Ian and Sylvia's treatment emphasizes the sense of mystery and eerieness that informs this tale of infanticide. "The song seemed to lend itself to the use of parallel fifths in the harmony," the duo said, "and seems to us to be all the more powerful for this treatment."