The study of the Earth's crust strata and especially rocks is called geology.

Here's a breakdown:

* Geology is the scientific study of the Earth, its materials, processes, and history.

* Stratigraphy is a branch of geology that focuses on the study of rock layers (strata) and their relationships in time and space.

* Petrology is the study of rocks, including their formation, composition, structure, and occurrence.

So, while geology encompasses the study of the entire Earth, stratigraphy and petrology are the specific branches that focus on the crustal strata and rocks.