Stratigraphy is a branch of geology that focuses on the layering of rocks.

So when you see cutaways of rock faces,

because of erosion or blasting for a new highway or whatever else, all those horizontal lines

running across the newly exposed faces are layers of primarily sedimentary and volcanic rock.

The layers are called strata. The way they form

these layers is called stratification, and the study of such layers and their layering is called

stratigraphy. I should note that there are also stratigraphers who work in the world of archaeology,

rather than geology, and their field

operates on very similar principles to those of the geological stratographer. For instance,

it can generally be assumed that when you take a look at a dig site or a cutaway, the stuff

that is higher up that's closer to the surface of the earth will be

newer, and the things that are further down will be older. That's called the principle of

superposition. It's also generally assumed that a layer within these formations will extend

to cover a large area, so you could conceivably trace one layer across thousands of

square miles, and as long as that area wasn't obstructed when it was at the surface. And even if that

layer is now divided by some lake or canyon, you can still generally treat that layer as being a

continuous body. That's called the principle of lateral

continuity. Both of these principles and several others that are held to be generally true

across both geographic and archaeological stratigraphers are heavily reliant on the understanding

that what was once the surface of the earth is now buried under more recent

surfaces. And by assessing these distinct horizontal regions, we can look back in time and learn

about previous periods and test our hypotheses about how things were, from how people lived,

...