I guess since there is so much exposed rock, people tend to draw on it.
These are relatively recent (15th century) since there are horses.
Ancient Anasazi Indian electrical substation.
Large “G” – for Grand County High School – painted on the cliffs above town. To paint it, you need to climb to the top of the cliff, rappel down, and hang in midair on ropes while painting…… extremely dedicated. It has changed from what was probably considered vandalism in the 1930’s, to a town landmark that the owner of the property now maintains. (Picture is from our back yard).
Also the location of a hilarious edit about 8 years ago: http://www.deseretnews.com/article/635194402/Moab-all-abuzz-over-rock-writing.html