This is a very new trail system with aspirations for a freeride section to be built in the future. The trail is currently being built by the Northwest Trail Alliance and has been built specifically for mountain biking so the flow is great and the grade is manageable.
follow directions to L.L. Stub Stewart State Park just south of the town of Vernonia. The mountain bike trail system is in the very back of the park at the Clayhill Horse staging area.
http://www.singletracks.com/item.php?c=1&i=4805