The dunes are built up from blowing sand that is either blown up from the beach (beach dunes) or from the sandy moraine on the plateaus (perched dunes). Most of the sand, gravel, and rocks in this area were deposited by the glaciers in big hills that you see along the lakeshore. The wind blows the sand from these into dunes. When you are seeing rocks in the sand, you are actually looking at glacial moraine - not dunes.