( ( As ( rocks ( reach ( the surface ) ) ) ) ( , ( they ( ( ( ( ( roll downhill ) ( under ( ( the influence ) ( of gravity ) ) ) ) and ) ( collect ( in ( the depressions ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN As) (S (NP (NNS rocks)) (VP (VBP reach) (NP (DT the) (NN surface))))) (, ,) (NP (PRP they)) (VP (VP (VBP roll) (ADVP (RB downhill)) (PP (IN under) (NP (NP (DT the) (NN influence)) (PP (IN of) (NP (NN gravity)))))) (CC and) (VP (VBP collect) (PP (IN in) (NP (DT the) (NN depressions))))) (. .)))
As rocks reach the surface, they roll downhill under the influence of gravity and collect in the depressions.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))