Blazes!

In the old days, trails were often marked by chopping off a section of bark to show the light wood underneath, a "blaze." The practice dates back at least to pioneer times, and maybe even was used by Native Americans. Of course, the blaze injures the tree, and so blazing is considered Politically Incorrect these days. The US Forest Service quit doing it in 1960. You still often see blazes on old trees along trails, tho, as here. This is the Charity Valley trail in the Sierra Nevada, California.