The bottommost layer of soil is called bedrock. It's not technically soil, but rather the solid rock that lies beneath the soil layers.

Here's why it's the bottom layer:

* Formation: Bedrock is the parent material from which soil forms. Over time, weathering and erosion break down the bedrock into smaller particles, creating the soil above.

* Immovable: Bedrock is a solid, hard layer that is largely immobile. It doesn't move or break down easily, making it the foundation for the soil layers above.

* Depth: Bedrock can be found at varying depths depending on the geological history of the location. In some places, it may be just a few inches below the surface, while in others, it could be hundreds of feet down.

Here's a simplified explanation:

Imagine a cake. The bedrock is like the cake pan, providing the base for the other layers. The soil is like the cake batter and frosting, formed on top of the pan.

While bedrock itself isn't soil, it plays a crucial role in soil formation and defines the lower boundary of the soil profile.