The principle of inclusions states that inclusions found in other rocks (or formations) must be older than the rock that contain them. Geologists call it relative dating — we know which one is older but do not know how old they are.
Further confusion arises when one publication or geologist says, for example, that the Kaibab Formation is 270 m.y. The same questions arise for the other rock units at Grand Canyon. This article will answer these questions by providing a short primer on geologic dating methods and how they were applied to Grand Canyon rocks.
The study of fossils and the exploration of what they tell scientists about past climates and environments on Earth can be an interesting study for students of all ages.