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

Rocks and minerals found on Earth are different than on Mars, but the idea of a mountain made of layers is familiar to scientists.

CNN: Rover Curiosity near Mars landing

Further examination revealed a crater rich in shocked quartz and iridium, minerals that are commonly found at impact sites. (These are also the telltales in distant layers of ejecta that the rock in question has come from an impact.) Most important, the rocks above and below Shiva date it to 65m years ago.

ECONOMIST: Mass extinctions