Crushed limestone, with mileposts to boot. Connects many of the forest preserves below. It's wholly within the floodplain, so avoid it when the river level is high. (see stations: Des Plaines, Gurnee, etc)
The corners in the interior dirt section are educational... cornering theory is very useful here. There are also surprises in some of the corners, with some being hard dirt and others loose sand.. so much like road racing, memorizing the course would be very beneficial.
~1.8miles around west loop. ~1.7 miles around east loop. ~4.5 miles for a full circuit around both.