Joanne channelFrom Middlesbrough (North Yorkshire)
Some fossils, called index fossils, are particularly useful in correlating rocks. Aside from single-celled bacteria, most living organism reside at or very near the Earth's surface either in continental or oceanic environments. Note the lunar dust clinging to Shepard's space suit. Using these key or index fossils as markers, geologists began to identify a particular layer of rock wherever it was exposed. So to use igneous rocks specifically the radioactive elements in those rocks to date fossils we have to use a concept called relative dating.
Specific rock formations are indicative of a particular type of environment existing when the rock was being formed. Each card represents a particular rock layer with a collection of fossils that are found in that particular rock stratum. This is equivalent to the organism dying in carbon-dating - no more carbon gets added to the organism. For example, if a valley is formed inside an impact crater , the valley must be younger than the crater.