Signs Taken for Wonders

The best I've got -- and it's more of a starting place than an answer -- is The Lore of the Land, edited by Westwood and Simpson. It's an encylopedia of British folklore, with a substantial bibliography, but the downside is that the organization isn't all that ideal for your purposes; you'd have to look up specific caves etc., and then check out their bibliographical references. That might lead you to something of more use, though.