The part of the soil profile made up of partly weathered rock is called the C horizon.

Here's a breakdown of the soil profile:

* O horizon: This is the top layer, made up of organic matter like leaves, twigs, and decomposing organisms.

* A horizon: This is the topsoil, a mixture of minerals, organic matter, and living organisms. It's rich in nutrients.

* B horizon: This is the subsoil, where minerals from the A horizon accumulate. It often has a different color and texture than the A horizon.

* C horizon: This is the parent material, partially weathered rock from which the soil is derived.

* R horizon: This is the bedrock, the solid, unweathered rock beneath the soil.

So, the C horizon is the layer where the process of soil formation begins. As the parent rock breaks down, it forms the basis for the other layers above it.