British artist Andy Goldsworthy has returned to the Presidio again and again, leaving an intruiging work behind him each time. First was 2008’s Spire, a 90-foot steeple of cypress near the Presidio Golf Course. This, and his next piece, Wood Line, a sinuous shape near Lovers’ Lane made with downed eucalyptus branches, are destined to merge, slowly, with the surrounding forest. His third installation here, Tree Fall, exists inside the fort’s old powder magazine, a one-room stone structure in the Main Post’s parking lot, and his fourth, Earth Wall, is hidden in a courtyard off the Officers’ Club.