There is only one letter (such as N or P) on hills?

Question

While my friend was traveling from state to state, she noticed that there was only one letter (such as N or P) on hills.
Why is that? What does that mean?

Thanks for answering.
Would the "one letter" represent a ranch or a university near by?
What's the reason a town or a high school put one letter made of rock on the hills?

Answer 1

Usually, its the first letter of the nearest high school or town. Such as an N for Norman High School. In Ruidoso, NM, there is a giant R made with stones.