Minecraft wiki entry for Wall, associated text: When a fence gate is placed next to a wall, it stoops down slightly to match the height of the cobblestone wall.
A stack of cobblestone made into cobblestone walls (which use only 60 of the 64 cobblestones) makes exactly enough walls to fence off one chunk. However, using a stonecutter can make 64 cobblestone walls without any cobblestone left, rather than 60.