( ( ( ( ( ( The force ) ( of gravity ) ) , ) ( which ( ( holds us ) ( to ( the Earth ) ) ) ) ) , ) ( ( causes ( ( eroded rocks ) ( to ( ( ( roll off ) ( elevated areas ) ) ( into ( valleys ( and ( river beds ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN force)) (PP (IN of) (NP (NN gravity))) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ holds) (NP (PRP us)) (PP (TO to) (NP (DT the) (NN Earth)))))) (, ,)) (VP (VBZ causes) (S (NP (JJ eroded) (NNS rocks)) (VP (TO to) (VP (VB roll) (PRT (RP off)) (NP (JJ elevated) (NNS areas)) (PP (IN into) (NP (NNS valleys) (CC and) (NN river) (NNS beds))))))) (. .)))
The force of gravity, which holds us to the Earth, causes eroded rocks to roll off elevated areas into valleys and river beds.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))