Thomas Hardy had a trysting stone in one of his books - don't ask which one ...
Tryst-stone gets its own entry in the dictionary and is a formal use for the prehistoric markers. Plse see http://www.baltimore-ireland.com/heritage/prehistoric.html , and also, closer to home, http://www.themodernantiquarian.com/site/2443 and http://www.themodernantiquarian.com/site/3331