Stratigraphy is the branch of Geology that studies the characteristics of rock layers and their mutual relationships. Observations on the composition and geometrical characteristics of different layers can reveal a host of information on the geological evolution of an area.

Let's consider this example from Colorado: a very weathered granite (the white/pink rock at the bottom) is overlain by a layered limestone (in red), typically formed in shallow marine environments. The interface between granite and limeston is rugged, a typical characteristic of so-called erosional surfaces.                      

Erosional surface juxtaposing a highly weathered granite (white/pink) and shallow marine lilmestone (red layered rocks)                    

  Detail of the erosional surface

This limited set of observations on the compositional and geometrical characterisitcs of the various layers allows us to draw some first order conclusions on the geological evolution of this area:

1. firstly, cooling of magma within the crust led to the formation of the granitic rock;

2. this granite, after crystallization, was transported to the surface of the crust (possibly because the overlying rocks were eroded away) and became itself victim of erosion and chemical weathering;

3. a sea level rise (called ingression) "drowned" the granite, so that the limestone could be deposited on top of it 

4. now these rocks are found at about 2000 m of altitude, implying a significant uplift following the deposition of the limestone. This uplift is related to the formation of the mountain ranges in the western United States.