As sediment weathers and erodes from its source, and as long as it is does not encounter any physical barriers to its movement, the sediment will be deposited in all directions until it thins or fades into a different sediment type.This concept known as the Principle of Lateral Continuity allows us to assume that similar layers of rock or sediment that are separated by a valley or other erosional feature were once continuos.
This means that the oldest are the strata that are lying at the bottom.
However, age of deposition does not mean the age of artifacts found in that layer.
It implies that relative dating cannot say conclusively about the true age of an artifact.
Absolute dating, on the other hand is capable of telling the exact age of an item using carbon dating and many other techniques that were not there in earlier times.
There are many methods employed by these scientists, interested in the old, to get to know the age of items.
It is possible to tell the number of years ago a particular rock or archeological site had been formed.
The Principle of Original Horizontality states that due to the influence of gravity all sediment is originally deposited horizontally.
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.
Therefore, the piece, or inclusion, must be younger than the material it is included in. Aside from single-celled bacteria, most living organism reside at or very near the Earth's surface either in continental or oceanic environments.