Larson,
Have you found other pieces of that material? Sometimes a sign of a different aged tool is the use different material. i.e. If the later plains indians that made most of what you find used all those nicer materials for scrapers, tools and points, and assuming they had access to more of it, why would they use that grainy, hard to knap material? It might be something earlier or later (in your case, I'd say earlier) and they didn't have access or know about other deposits.