Here's why:

* Stratigraphy is the study of layered rocks, specifically their sequence, composition, and age relationships.

* Layers (also known as strata) are individual units of sedimentary rock. They are often different in texture, color, or composition from layers above and below.

* Beds are larger, mappable units of sedimentary rock that consist of multiple layers.

So, when you see layers and beds of sedimentary rock, you are observing the fundamental building blocks of stratigraphy. These layers provide valuable information about the geological history of an area, including:

* Past environments: Sedimentary rocks are formed from the accumulation of sediments, which are derived from various environments (e.g., rivers, oceans, deserts). The types of rocks and their layering patterns can reveal information about these past environments.

* Relative age: The order of the layers, from oldest to youngest, provides a relative age sequence.

* Fossil evidence: Fossils found within the layers can provide insights into the evolution of life and the age of the rocks.