An Anvil?

      I've been sure, for some time, that I had a resident Thrush; I've never seen it - BUT - have seen the evidence.  Apparently my "half step"; build against the time I May not be able to manage the full step.  If it's not a "Thranvil"  (Thrush's anvil) for snail bashing I don't know what is.
       The only "cheating" is to gather the Shells slightly closer together.