Jackie and I were running on a trail in the Presidio a while back when we were surprised by this huge wooden spire that had appeared.
It was pretty startling, and then intriguing. Turns out it's a work by Andy Goldsworthy. I like his work, and even more so his attitude toward his work and his process.
I watched Rivers and Tides some time ago. I was most struck by a scene where Andy Goldsworthy is carefully stacking rocks. He builds them up to a certain level, but the rocks suddenly collapse into an indiscriminate pile. He hangs his head in frustration for a moment but then says "This is my work".
You can find Spire by entering the Presidio at the Arguello Gate. Make a right at the gate and follow the road down the hill. After maybe a quarter mile, you'll see on overlook on your right. Look left and you should see the spire.
It's a ways from downtown, but if you catch the 38 Geary from downtown, get off at Arguello, and walk north, up the hill, about six blocks. That'll put you at the Arguello gate. Cabbies might not know where the spire is, but they should know the gate.
Finally, it seems that Andy Goldsworthy has inspired some imitators near the spire: