( ( A ( till plain ) ) ( ( is ( ( ( an ( extensive ( flat plain ) ) ) ( of ( glacial till ) ) ) ( that ( forms ( when ( ( ( a sheet ) ( of ice ) ) ( ( ( becomes ( detached ( from ( ( the ( main body ) ) ( of ( a glacier ) ) ) ) ) ) and ) ( ( ( melts ( in place ) ) , ) ( depositing ( ( the sediments ) ( it carried ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (JJ till) (NN plain)) (VP (VBZ is) (NP (NP (DT an) (JJ extensive) (JJ flat) (NN plain)) (PP (IN of) (NP (JJ glacial) (NNS till))) (SBAR (WHNP (WDT that)) (S (VP (VBZ forms) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT a) (NN sheet)) (PP (IN of) (NP (NN ice)))) (VP (VP (VBZ becomes) (VP (VBN detached) (PP (IN from) (NP (NP (DT the) (JJ main) (NN body)) (PP (IN of) (NP (DT a) (NN glacier))))))) (CC and) (VP (VBZ melts) (PP (IN in) (NP (NN place))) (, ,) (S (VP (VBG depositing) (NP (NP (DT the) (NNS sediments)) (SBAR (S (NP (PRP it)) (VP (VBD carried)))))))))))))))) (. .)))
A till plain is an extensive flat plain of glacial till that forms when a sheet of ice becomes detached from the main body of a glacier and melts in place, depositing the sediments it carried.
(ROOT (S (NP (NNS Glaciers)) (VP (VB deposit) (NP (PRP$ their) (NN sediment)) (SBAR (WHADVP (WRB when)) (S (NP (PRP they)) (VP (VB melt))))) (. .)))