One of only two instances of the letter X appearing as a "hillside letter." The other is in 'Dixie' in St George, UT. There are no stand-alone 'X' hillside letters.
It is actually an AIR MARKER. It was intended as an aviation navigational aid. It was created by members of Air Explorer 13 of Phoenix. Years to construct and paint = 1950 - 1956. Each letter is 100 ft high. Its total length is equal to the height of the Empire State Building.Building.
http://en.wikipedia.org/wiki/Hillside_letters