A four mile loop in the Marin Headlands including the beach detour. It starts at the roundabout and goes up to Hawk Hill and Nike missile control site. Then it plunges down the coastal road to the Black Sands Beach parking lot. If you go down to the beach (which you should) you then have a steep climb back up and take Upper Fisherman's Trial back down into Rodeo Valley before heading uphill again on Julian trail to the roundabout. Great views back over Rodeo Valley in addition to the dramatic descent down the coast mean this is worth doing once. Around a third of the hike is pretty close to the road which is the only real downside of this loop.