( ( The glacier ) ( ( ( has since ) ( retreated ( behind ( ( the rock ) ( visible ( in ( ( the center ) ( of ( the photograph ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN glacier)) (VP (VBZ has) (ADVP (IN since)) (VP (VBN retreated) (PP (IN behind) (NP (NP (DT the) (NN rock)) (ADJP (JJ visible) (PP (IN in) (NP (NP (DT the) (NN center)) (PP (IN of) (NP (DT the) (NN photograph)))))))))) (. .)))
The glacier has since retreated behind the rock visible in the center of the photograph.
(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.