The lowest level of rock that usually forms the original source of topsoil is bedrock.

Here's why:

* Bedrock is the solid, unweathered rock that lies beneath all other layers of soil. It's the foundation upon which all other soil components are built.

* Over time, bedrock is broken down by weathering processes like freezing/thawing, wind, water, and biological activity. This broken-down material, along with organic matter from plants and animals, forms the basis of soil.

* Topsoil is the uppermost layer of soil, rich in organic matter and nutrients, that supports plant growth.

So, while bedrock itself isn't topsoil, it's the source material from which topsoil is derived.