BTW, I've managed to drive around all these places several times over the years, so I'm familiar with this stuff. There's a recent Discover presentation of how the glaciers ground down Michigan and created the Great Lakes except for Ontario and Superior, and I'm drawing on that to describe subsurface conditions. Rest assured, there's limestone not too far below those sand and gravel deposits. If it were worthwhile they could bulldoze the gravel out of the way and get at the stone.
I suspect it's cheaper to buy crushed rock from Indiana, Illinois or Ohio though. (Ground water table problems are far different).
I probably should have explained that I lived in Western Michigan (Kent & Ottawa county, just south and west of Montcalm Counted mentioned in the article) for 30+ years. When I was a teenager my parents had a dirt/gravel driveway. Less than a quarter mile down the road it turned from a pea gravel “paved” road to gravel. The roads were all gravel for miles and miles. I rode the school bus and my bicycle on these roads a lot. Smooth or pleasant were not words anyone woud ever use to describe them.
There are water table challenges there, too. Our house well was at 15 feet. That was the second water. I’ve been told much of the reason that Dutch immigrants settled in the Holland, Michigan area is because the wet conditions were much like those where they had lived in the Netherlands. They were familiar with how to manage the water whereas other immigrants were not.
There are gravel pits in Western Michigan. Rock is highly prized, but NOT inexpensive (compared, at least, to Oklahoma where I live now).