Disclaimer: my knowledge of working on slopes is more theoretical than practical. My own
land is very flat.
That said, it seems to me that if you've got ruts, it's because water is flowing in from the surrounding area, so if you want to stop the erosion you're going to have to address the area uphill, not just pile stuff into the ruts.
Also, since you've got exposed rocks already, maybe you can find ways to make them productive without soil. Many
trees and shrubs, as well as sprawling plants like squash, will happily grow in the cracks between stones. This could be either a stopgap measure until you have
enough soil (or organic matter) to cover the stones or a longer-term way to make the poor soil productive. As for the clay, comfrey will grow in very heavy clay and produce your organic matter on location, without your having to bring it in, if you can wait a few years.
Good luck!