I suspect that in the Midwest they are talking about peagravel and tar ~ although I have experienced "gravel roads" but many of them were "paved" with local gravel so they looked pretty much like dirt anyway ~ then there's "crushed rock", and I haven't seen that term showing up in any of these articles. Gravel roads ride one way, and crushed rock roads ride differently.
On the western side of Michigan where the counties discussed are located, there isn’t any “local rock” to speak of. Gravel roads are just that, gravel.