What is a dune? What is different about the dunes on the beach and dunes on the farm?

A dune is a large mound of hay. The difference between the hay dune on the farm, and the and the dunes on the beach, is that the dunes on the beach are made of sand.

"'Dear William,' " Sarah read to us by lantern light that night. " 'Sliding down our dune of hay is almost as fine as sliding down the sand dunes into the sea.' "