Here's why:

* Formation: This layer is the bedrock or geological material from which the soil above it developed. It's the foundation upon which the other layers are formed.

* Composition: The parent material can be anything from weathered rock to glacial till or even volcanic ash. Its composition directly influences the soil's characteristics.

* Depth: The depth of the parent material can vary greatly depending on the region's geological history. In some areas, it may be just a few inches below the surface, while in others, it could be hundreds of feet deep.

Important Note: While the parent material is considered the "deepest" layer, it's not always visible at the surface. The other soil horizons (layers) are formed over time as the parent material breaks down and mixes with organic matter, water, and air.