Note the title of this article has the word "single" in it. Does that mean the anchor material is not redundant? Yes it does. The idea of a single solid anchor means the anchor is so big that if it were to fail there are probably bigger problems that caused it to fail that are going to kill you.
Let's say you are tied into a huge boulder and it suddenly starts tumbling towards you. What caused that boulder to start tumbling towards you? A land slide is the main thing that come to mind. If you don't die from the fall you'll be crushed or suffocate.
There are two types of single solid natural anchors: big fu-riendly rocks and big friendly trees.
A tree is a solid anchor when it has the following three properties: six inches in diameter, alive, and well rooted.
The diameter of a tree is easy to calculate. Just cut down the tree and measure! If it's 6 inches or greater, then it would have worked as an anchor!
That was a joke. Please don't actually do that. Geez don't take everything so literally.
Why 6 inches? I don't know why that exact length was picked. Probably because it is really strong.
If it has green leaves on it then it's probably alive. If it's just a piece of a tree floating in a pothole then it's probably dead. If it's anything in between you'll have to use your own judgement.
This means the roots are in the ground. Impossible to actually see but there have only been a few times where I wasn't absolutely sure.
If it has all three of these qualities then you can trust it to hold you on rappel. Let's quickly discuss some almost optimal trees.
What if you have a tree that is 5.9 inches in diameter, alive, and well rooted? Or a dead tree that's 36 inches in diameter and very well routed? Is it still safe to rappel on either of those? I don't know. Um do what your heart tells you. Don't trust everything you read on the internet.
When tying webbing around trees the strongest place for the webbing to go is at the base of the tree. The base of the tree is also the place where it makes the rappel starts hardest for people and it can make the pull more difficult.
If the tree is strong enough you can use a cinch wrap to put the webbing higher up in the tree to eliminate the tough start and tough pull. If the tree is not strong enough you can use courtesy rigging to make the rappel start a little easier while making the pull easy as well.
Rocks don't have exact requirements like trees, unfortunately. I'll go over a few things to think about and call it good.