The Mattaponi Trail runs a mile through forest and marsh to an unremarkable stretch of York River sand, and the instruction is to look down rather than out. Erosion works the cliffs and drops what they were holding: shells, shark teeth, whale bone, coral. The common prize is Chesapecten, an extinct scallop that lived here millions of years before the beach did. The supply renews itself, which is why the walk is still worth making.