( ( According ( ( the law ) ( of superposition ) ) ) ( , ( we ( ( assume ( that ( ( ( the ( rock unit ) ) ( located ( on ( the bottom ) ) ) ) ( is ( older ( than ( the ( top layer ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (VBG According) (NP (NP (DT the) (NN law)) (PP (IN of) (NP (NN superposition))))) (, ,) (NP (PRP we)) (VP (VBP assume) (SBAR (IN that) (S (NP (NP (DT the) (NN rock) (NN unit)) (VP (VBN located) (PP (IN on) (NP (DT the) (NN bottom))))) (VP (VBZ is) (ADJP (ADJP (JJR older)) (PP (IN than) (NP (DT the) (JJ top) (NN layer)))))))) (. .)))
According the law of superposition, we assume that the rock unit located on the bottom is older than the top layer.
(ROOT (S (S (NP (NP (JJ Deeper) (NNS layers)) (PP (IN of) (NP (NN rock)))) (VP (MD must) (VP (VB be) (ADJP (ADJP (JJR older)) (PP (IN than) (NP (NNS layers)))) (ADVP (RBR closer) (PP (TO to) (NP (DT the) (NN surface))))))) (, ,) (NP (DT this)) (VP (VBZ is) (NP (NP (DT the) (NN law)) (PP (IN of) (NP (NN superposition))))) (. .)))