( They ( ( are ( formed ( on ( ( ( ( essentially horizontal ) , ) early ) ( Quaternary sediment ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBP are) (VP (VBN formed) (PP (IN on) (NP (NP (RB essentially) (JJ horizontal)) (, ,) (RB early) (NP (NNP Quaternary) (NNP sediment)))))) (. .)))
They are formed on essentially horizontal, early Quaternary sediment.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NNS sediments)) (VP (VB settle) (PRT (RP out)) (PP (IN of) (NP (NN water)))))) (, ,) (NP (PRP they)) (VP (VBP form) (NP (JJ horizontal) (NNS layers))) (. .)))
When sediments settle out of water, they form horizontal layers.