Introducing Stratigraphy:

The facts of life:

The process of "reading the sedimentary record" to reconstruct Earth history is:

Cretaceous stream deposits near Drumheller, AB - Not a layer cake.
Not a layer cake: It is naive to think of sedimentary strata as being like a layer cake - of uniform thickness and extending out in all directions. In fact, the sedimentary layers that geologists use to read Earth history are:

No single section records continuous sediment accumulation. We reconstruct geological history only through correlating strata widely separated in space and time. This must be approached through multiple lines of evidence, because no one type of evidence is without its biases. Possibilities include: