Absolute dating vs realtive dating
In order for any material to be included within in the rock it must have been present at the time the rock was lithified.For example, in order to get a pebble inside an igneous rock it must be incorporated when the igneous rock is still molten-- such as when lava flows over the surface.This is due to the fact that one or both of the objects may have been moved or redeposited into a different location.In other words, they may no longer be in their primary context.
In other words, as sediment fills a depositional basins we would expect the upper most surface of the sediment to be parallel to the horizon. Using this principle we can than assume that sedimentary layers which have been deformed/folded must have been deformed after all affected layers have been deposited.
The principle of cross-cutting states that any geologic feature that crosses other layers or rock must be younger then the material it cuts across.
Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.
If conditions are right the remains of the dying organisms can then be preserved as fossils within the rock that formed from sediments that covered the remains.
Since, all sedimentary rock is formed through the gradual accumulation of sediment at the surface over time, and since the principle of superposition tells us that newer sediment is deposited on top of older sediment, the same must also be true for fossils contained within the sediment.
When the bones of our early ancestors are found in the same geological strata as those of other animals that are known to have lived only during a specific time period in the past, we assume that these ancestors must also have come from that time.