A bed is a single layer of sedimentary rock that is distinct from the layers above and below it. Beds can vary in thickness from millimeters to meters, and they can be made up of a variety of different sedimentary materials.

Here's why a bed is considered the fundamental unit:

* Distinct Layer: It represents a specific period of deposition, reflecting a particular environment or set of conditions.

* Basic Building Block: Multiple beds stack together to form larger units like formations.

* Identifiable: Geologists can easily recognize and map beds based on their lithology (rock type), texture, and other characteristics.

While the term "bed" is widely used, other terms like "stratum" and "layer" are also used synonymously.