Here's why:

* Bed: A bed is a distinct layer of sedimentary rock that is characterized by its texture, composition, and internal structure. It's the most basic building block of sedimentary sequences.

* Strata: Multiple beds together form strata, which are layers of sedimentary rock that are often visible in cliffs and outcrops.

* Formation: Several strata that share common characteristics are grouped together into a formation, a larger unit used in geological mapping.

So, while formations and strata are larger units, the bed is the foundation upon which these larger units are built.