There isn't a single "horizontal layer of soil." Instead, soil is made up of multiple layers, called horizons, stacked on top of each other. These horizons vary in their composition and properties.

Here are the main soil horizons:

* O horizon: The uppermost layer, consisting of organic matter like leaves, twigs, and decomposing plant and animal material.

* A horizon: Also called topsoil, this layer is rich in organic matter, minerals, and nutrients. It's where most plant roots grow.

* E horizon: This layer is often found beneath the A horizon and is characterized by the leaching of minerals and organic matter.

* B horizon: This layer, also known as subsoil, is enriched with minerals that have leached down from the layers above. It can be dense and less fertile than the topsoil.

* C horizon: This layer is made up of weathered parent rock material from which the soil above has developed.

* R horizon: This is the bedrock, the unweathered parent rock that lies beneath the soil horizons.

The presence and thickness of each layer can vary widely depending on factors like climate, vegetation, and geological history.