( ( The rocks ) ( ( show ( that ( ( ( ( the north ) and ) ( ( south ( magnetic poles ) ) ( of ( the Earth ) ) ) ) ( have ( ( ( ( traded places ) , ) perhaps ) ( ( ( several hundred ) times ) ( during ( the ( past ( ( 160 million ) years ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS rocks)) (VP (VBP show) (SBAR (IN that) (S (NP (NP (DT the) (NN north)) (CC and) (NP (NP (JJ south) (JJ magnetic) (NNS poles)) (PP (IN of) (NP (DT the) (NN Earth))))) (VP (VBP have) (VP (VBN traded) (NP (NNS places)) (, ,) (ADVP (RB perhaps)) (PP (NP (QP (JJ several) (CD hundred)) (NNS times)) (IN during) (NP (DT the) (JJ past) (QP (CD 160) (CD million)) (NNS years)))))))) (. .)))
The rocks show that the north and south magnetic poles of the Earth have traded places, perhaps several hundred times during the past 160 million years.
(ROOT (S (NP (NP (DT The) (NN earth) (POS 's)) (JJ magnetic) (NNS poles)) (VP (VBP have) (VP (VBN switched) (NP (NNS places)) (ADVP (RB repeatedly)) (PP (IN in) (NP (DT the) (NN past))))) (. .)))