Stratigraphy is a branch of geology that studies rock strata with an emphasis on distribution, deposition, age and evidence of past life.
Nicolas Steno, William Smith, Georges Cuvier, Alexandre Brongniart, and James Hutton developed the basic rules for the science of stratigraphy.
The question is, how can geologists decipher the available record?
Applying the principles of relative dating to these rock exposures (also called "outcrops"), we can reconstruct the sequence of events that created the geologic features which we see.
Events can be the deposition of a sedimentary layer, the eruption of a lava flow, the intrusion of magma to form a batholith, a fault (break) in the rock that shifts one side relative to the other side (and causes an earthquake), a fold that bends and distorts rock layers, or any number of other geologic processes.
The laws of physics and chemistry that governed geologic processes in the past are the same as those that govern processes now and in the future.
The geologic timescale is a chronology (calendar) of events on Earth based on obtaining ages of past events.
When we determine how old an event is in years (dates), we refer to this as absolute time.