( ( This chunk ) ( ( is ( ( surrounded ( by ( ( rocks and ) dirt ) ) ) ( that ( ( the glacier ) ( leaves behind ) ) ) ) ) . ) )
(ROOT (S (NP (DT This) (NN chunk)) (VP (VBZ is) (VP (VBN surrounded) (PP (IN by) (NP (NNS rocks) (CC and) (NN dirt))) (SBAR (IN that) (S (NP (DT the) (NN glacier)) (VP (VBZ leaves) (PRT (RP behind))))))) (. .)))
This chunk is surrounded by rocks and dirt that the glacier leaves behind.
(ROOT (S (NP (DT A) (NN glacier)) (VP (VBZ leaves) (PP (IN behind) (NP (JJ bare) (NN rock))) (SBAR (WHADVP (WRB when)) (S (NP (PRP it)) (VP (VBZ retreats))))) (. .)))
A glacier leaves behind bare rock when it retreats.