The lowermost layer of the soil is the parent rock.

Explanation:
The soil is formed from the weathering of rocks over a long period of time. It is composed of various layers, each with its own characteristics and composition. The lowermost layer of the soil is known as the parent rock, which is also referred to as the bedrock.

Weathered Rock:
- Weathered rock refers to the fragmented and decomposed rock material that is found above the parent rock.
- It is the result of physical and chemical weathering processes that break down the solid rock into smaller particles.
- Weathered rock is typically found in the upper layers of the soil, above the parent rock.

Parent Rock:
- The parent rock is the solid rock material that lies beneath the soil layers.
- It is the unweathered and unaltered rock that serves as the source material for the soil.
- The parent rock can be made up of various types of rocks including igneous, sedimentary, and metamorphic rocks.
- The characteristics of the parent rock, such as its mineral composition and structure, influence the properties of the soil that develops from it.

Subsoil:
- Subsoil refers to the layer of soil that is located below the topsoil and above the parent rock.
- It is typically composed of a mixture of weathered rock fragments, minerals, and organic matter.
- The subsoil plays an important role in the movement and storage of water, as well as providing support for plant roots.

Conclusion:
In conclusion, the lowermost layer of the soil is the parent rock. It is the solid rock material that lies beneath the soil layers and serves as the source material for the soil. The weathered rock is found above the parent rock, while the subsoil is located between the topsoil and the parent rock.