( Primarily ( , ( it ( ( 's ( ( ( the forces ) ( of ( ( waves and ) gravity ) ) ) ( that ( cause erosion ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Primarily)) (, ,) (NP (PRP it)) (VP (VBZ 's) (NP (NP (DT the) (NNS forces)) (PP (IN of) (NP (NNS waves) (CC and) (NN gravity))) (SBAR (WHNP (WDT that)) (S (VP (VBP cause) (NP (NN erosion))))))) (. .)))
Primarily, it's the forces of waves and gravity that cause erosion.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))