In the northern portion of the Evergreen woods, on a distant arm of the beach trail that begins at the F parking lot, greener mountain bike enthusiasts are constructing an elaborate mountain bike trail. On a steep downhill slope between the trail and the shore, student volunteers are clearing a path and carving features in the otherwise dense woodland. For thrill seekers, the trail will include several large jumps built out of natural materials. While many informal mountain bike trails already exist in the woods, this will be the most sophisticated. Student volunteers say they designed the trail to minimalize ecological impact, and replant the vegetation they remove.