The confusion arises because a long tree lying at right angles to the cliff face is visible for a long time. However, a tree trunk lying parallel to the cliff face would be exposed for only a short time before it falls down the hill. This natural bias can be neutralized by finding a second cliff facing approximately at right angles to the first one. The two cliffs would each have a bias in the orientation of the trees, but those biases ought to cancel each other and give a true picture. Many similar