From the Wikipedia page 
Arborglyphs, dendroglyphs, silvaglyphs, modified cultural trees, or aspen carvings are tree carvings made in the bark of aspen trees by shepherds, many of them Basque and Irish American, throughout the Western United States. They have been documented across northern California and in areas such as Boise, Idaho and Steamboat Springs, Colorado. There is an astronomical component to the designs.
A project run by the USDA Forest Service in 1997 to record and study arborglyphs in the Fremont National Forest of Oregon is documented here.