( ( Eklutna ( Lake ( The ( Eklutna Glacier ) ) ) ) ( ( ( carved ( this valley ) ) ( as ( it ( ( retreated , ) ( ( ( leaving ( horizontal scarring ) ) ( on ( the ( rock formations ) ) ) ) ( as ( evidence ( of ( its passage ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Eklutna) (NNP Lake) (NNP The) (NNP Eklutna) (NNP Glacier)) (VP (VBD carved) (NP (DT this) (NN valley)) (SBAR (IN as) (S (NP (PRP it)) (VP (VBD retreated) (, ,) (S (VP (VBG leaving) (NP (JJ horizontal) (NN scarring)) (PP (IN on) (NP (DT the) (NN rock) (NNS formations))) (PP (IN as) (NP (NP (NN evidence)) (PP (IN of) (NP (PRP$ its) (NN passage))))))))))) (. .)))
Eklutna Lake The Eklutna Glacier carved this valley as it retreated, leaving horizontal scarring on the rock formations as evidence of its passage.
(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.