In order to access Marina, riders must either come from the Golden Belt loop, or a park access road. Marina Loop, in turn, is the only access to the third loop (Hell Creek).
Marina starts a wonderful section named IMBA built by a Trail Care Crew led by Nat Lopes. Post Rocks is an optional loop defined by ~25 stone fence posts built by pioneers from (note lack of trees). Oh and forego the Bike Wash at about mile 7. Mile 8-10 is Brokebike Mountain. Too much fun to put into words with rips, drops, and rolls. Mile 10.5 passes under the Hell Creek Bridge.