sentence2_structure: A glacier<>leaves<>behind bare rock<>when it retreats$$$it<>retreats<>
sentence1: Eklutna Lake The Eklutna Glacier carved this valley as it retreated, leaving horizontal scarring on the rock formations as evidence of its passage.
sentence2: A glacier leaves behind bare rock when it retreats.
question: What does a glacier leave behind when it retreats?