( ( ( ( ( ( Using ( ( the Law ) ( of Superposition ) ) ) ( , ( ( the ( rock ( layer OXD ) ) ) ( is ( beneath ( rock ( layer DM ) ) ) ) ) ) ) and ) , ) ( therefore ( , ( is older ) ) ) ) . )
(ROOT (S (S (S (VP (VBG Using) (NP (NP (DT the) (NN Law)) (PP (IN of) (NP (NNP Superposition)))))) (, ,) (NP (DT the) (NN rock) (NN layer) (NNP OXD)) (VP (VBZ is) (PP (IN beneath) (NP (NN rock) (NN layer) (NNP DM))))) (CC and) (, ,) (S (ADVP (RB therefore)) (, ,) (VP (VBZ is) (ADJP (JJR older)))) (. .)))
Using the Law of Superposition, the rock layer OXD is beneath rock  layer DM and, therefore, is older.
(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))))) (. .)))