Are there other trees close around it that you can identify? It looks like almost all the leaves laying around it are in the red oak group, but leaves can blow around. Bark has similarities to red oak also, besides looking like walnut. Split a piece and look for the ray flecks of oak, although there are people who can identify oak from end grain pictures. If there are any twigs attached to it that you can split to check for chambered pith, that would be a good way to see if it's walnut.