( ( ( The humus ) ( that ( ( is present ) today ) ) ) ( ( is ( ( decayed ( organic matter ) ) ( from ( ( the ( gradual revegetation ) ) ( of ( the ( exposed bedrock ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NNS humus)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (ADJP (JJ present)) (NP (NN today)))))) (VP (VBZ is) (VP (VBN decayed) (NP (JJ organic) (NN matter)) (PP (IN from) (NP (NP (DT the) (JJ gradual) (NN revegetation)) (PP (IN of) (NP (DT the) (VBN exposed) (NN bedrock))))))) (. .)))
The humus that is present today is decayed organic matter from the gradual revegetation of  the exposed bedrock.
(ROOT (S (NP (NP (DT The) (JJ decayed) (NNS remains)) (PP (IN of) (NP (VBG living) (NNS organisms)))) (VP (VBZ is) (VP (VBN called) (NP (NNS humus)))) (. .)))