Hartley is a beautiful park with some great hardwood forests, an open grassland marsh and a large rock bluff. Some trails are a bit rocky and rooty, which lead to some technical challenges, but they are not overly difficult. Many of the new reroutes are quite smooth and have nice flow to the design. There are several punchy climbs with enough rocks and roots to make choosing the right line important, but there aren't any lung busters, just rolling hills.
You'll need to ride either Rhamnus
or The Fisherman
to get to the start of the Guardrail Loop at the "Four Corners Intersection."
If you keep right at every intersection, you'll do a counterclockwise lap of the Guardrail, eventually ending up back at Four Corners.
Turning onto Blue Pots
will give you an inner loop that is a bit more difficult.