Okay, I think I get it. You're saying that this structure isn't something that was used historically in France. Just not a French thing, right?
I don't think that's a problem, if you're presenting this book as fiction. First of all, even though these structures aren't French in origin, a small group of people could have built a few in a certain sector.
Second of all, just because it isn't likely doesn't make it implausible. For example, most modern-day fiction is set in a made-up town. That town doesn't exist in real-life, but it's plausible. There's nothing in that fictional town that is impossible in real-life, so we, in our suspension of disbelief, can assume that there really is a town with that name in America. There aren't actually any cairns in France, but there's no reason there couldn't be. It's a "what if." What if a group of people had built these in a region in France?
I myself could accept it, but if you want to be historically accurate, consider relocating your setting and just taking some Frenchmen with you.
