There is a certain amount of folklore from both Scotland and Ireland concerning using depresions in both cups and on standing stones to pour milk and other offerings into to ensure fertility and good milk yields and appease the local spirits such as the Gruagach.
I would recommend having a read of Paul Bennetts article Called "Cup & Ring Art - It's Folklore, Myths and the Shamanic Persepective".
I'll see if I can find an on-line copy and post a link.
If not I can send it to you
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