"Horizon C" is not a scientifically recognized term or concept. Therefore, it's impossible to tell you what it is made of.

It's possible you're referring to a specific term used in a particular field, or perhaps it's a fictional concept. Please provide more context or clarify your question!

For example, you could say:

* "What is Horizon C made of in the context of soil science?"

* "What is Horizon C in the fictional universe of [name of book/game/movie]?"

With more information, I can help you find the answer!