The ocean-facing side of Lower Cape Cod used to extend at least a quarter mile further out into the ocean, but there has been persistent erosion each year since the mid-1800s. Of course, this is highly visible during big storms, but even on quiet days, you can literally watch it happen, sometimes pebble by pebble. The video below is a quick demonstration.
Meanwhile, though, the patterns left in the dunes provide a beautiful display of the layers of glacial till, clay, and other materials that comprise them. More details follow below.