(S (S (BASENP (JJR Later) (NNS bluestones)) (VP (VBD were) (VP (VBN encircled) (PP (IN by) (NP (BASENP (DT the) (JJ imposing) (NN sandstone) (NNS monoliths)) (PP (IN of) (BASENP (NN sarsen) (NNS stones)))))))) (PERIOD .))