Grey Cloud Dunes SNA is one of those places you can hardly believe is in a metro area. Located in Cottage Grove, this highly protected Scientific and Natural Area (SNA) features two sandy terraces along the river. The first rises 40 to 60 feet above river-level with remarkable crescents and blowouts sculpted by southwesterly winds. The second, 110 feet above the Mississippi, has a dramatic cut face. Dunes on both terraces rise in waves, cresting 10 to 20 feet above the swales.