Depending on how fine you're looking to go, another possibility is to keep sifting dirt down to the point where you can put some in a container and wrap a woman's stocking over the opening of the container, then sift through that. You'll basically end up with dust.
-Brian.
Just don't use fish-net stockings.
Actually the mesh size of average stocking will pass particles quite a bit larger than dust, but it might still be fine enough to work as a scale road surface.
