That would tie in with the fact that the book was written by Geoffrey Grigson who was living in Wiltshire at the time. The book is called "Gardenage" and features a whole chapter on Grottoes. Do we know of any other landscape features that made use of ancient sites?

I am quite a fan of grottoes, perhaps when i have finished visiting all of Cornwalls stones etc I will try to hunt out the "caves of verdure" as Grigson calls them.