Rocks initially formed as layers of sediment laid down in an ancient sea can be deciphered by knowing that older layers are found beneath the younger layers.

BBC: Mars south pole

Natural gas formations are thousands of feet below drinking water aquifers so for contamination to occur the fracking solution would have to move through multiple layers of rocks.

FORBES: EPA's Fracking Hysteria

That there are not could be explained by the fact that the accumulation of sediment in most rocks is so slow that the two layers are, in effect, superimposed.

ECONOMIST: Mass extinctions