In reading Earth history, these layers would be read from bottom to top or oldest to most recent.
If certain fossils are typically found only in a certain rock unit and are found in many places worldwide, they may be useful as index or guide fossils in finding the age of undated strata.
The idea of millions and billions of years is difficult for children and adults to comprehend.
The Law of Superposition, which states that in an undisturbed horizontal sequence of rocks the oldest rock layers will be on the bottom, with successively younger rocks on top of these, helps geologists correlate rock layers around the world.
Now, look for a card that has either a T or C written on it.
Since this card has a common letter with the first card, it must go on top of the TC card.
Specific rock formations are indicative of a particular type of environment existing when the rock was being formed.
For example, most limestone represents marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or riverbed.