The most straightforward definition is that a natural anchor is any simple anchor point that nature provides.
The class know-it-all in the front row raises her hand and asks, "but Mr. Martin, isn't a crack a natural anchor?"
A crack is a crack. We actually have to put something inside the crack before we have a piece. It is a natural spot to place an anchor, but it is not a natural anchor point. No, instead a natural anchor is anything that is already there. The most common examples of natural anchors are trees, bushes, boulders, pinches and thread-throughs.
Before you elect to use a tree as an anchor point, you should make sure that it is "Five-and-Alive." In other words, that it is at least five inches in diameter, five feet tall, has a good root-base and is alive. You should be wary of trees that could have a root-base in dirt or sand and on top of the rock. An anchor with this kind of structure could easily fail.
In the mountains and in the desert, it is not uncommon to use bushes and shrubs that clearly don't meet the Five-and-Alive standard. These are primarily used as rappels to get down obscure gullies or to get off the backside of a peak, so the tendency is to try to avoid leaving too much gear. The tendency is to want to only leave webbing or cordage.
When you elect to use these less-than-stellar natural anchors, consider equalizing a number of them together. If you're tying your cord around a desert bush that is comprised of a number of finger-sized sticks, you'll probably want to equalize this with similar bushes. Depending on the size and density, I would want at least two of these, if not more.
And lastly, when it comes to bushes and shrubs as anchors, use common sense. Don't put your weight on something that might blow out. You could always back up the first person (usually the heavier person) on rappel with a loose gear anchor. If all goes well, the second person could tear down that anchor and then descend. If the equalized bush anchor didn't come apart during the first rappel with the heavier climber, it's reasonable to believe that it wouldn't come out with the second climber either.
Boulders can be absolutely fantastic natural anchors. But there are a few things to look at before committing to a boulder. First, make sure that it is in good contact with the ground. Boulders on sandy or sloping surfaces should be considered suspect. Second, make sure that it won't wobble or roll toward the edge. Every boulder should be checked by pushing and pulling on it to confirm it's position. And lastly, if there is any possibility of movement, don't use it. The last thing you need is a boulder falling down on top of you.
Pinches and Thread-Throughs
Pinches are places where two large boulders come together so tightly that you can wrap cordage or webbing around them. Thread-throughs are places where there is a hole in the rock that you can something through to tie-off.
It is not uncommon for people to simply miss these opportunities while trying to build an anchor. They simply aren't as intuitive for most people as the other natural anchors out there. If you can keep the fact that these exist in mind and you look for them, you'll find them.
Like boulders and trees and bushes, it's important to make sure that pinches and thread-throughs are sturdy enough to handle the stress of being an anchor. This is particularly important in sandstone or in other soft and friable rock-types.
In the following video, the Canadian Mountain Guide, Mike Barter demonstrates a quick and dirty improvised anchor.
Ultimately, the great value to natural anchors is that they don't require much gear. And since they don't require much, you'll have plenty to use on your next lead.
Class dismissed. Now go build some natural anchors!
--Jason D. Martin