( ( ( ( ( Erratics , ) or ) ( rocks ( dumped ( by ( retreating glaciers ) ) ) ) ) , ) ( ( littered ( the landscape ) ) . ) )
(ROOT (S (NP (NP (NNS Erratics)) (, ,) (CC or) (NP (NP (NNS rocks)) (VP (VBN dumped) (PP (IN by) (NP (VBG retreating) (NNS glaciers))))) (, ,)) (VP (VBN littered) (NP (DT the) (NN landscape))) (. .)))
Erratics, or rocks dumped by retreating glaciers, littered the landscape.
(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.