Steve and Wagga have the place, but the specific answer(s) to the question may be more elusive. You may have to drill down more than a little bit.
By now, everyone knows the place. But who are those guys? We all know the four dead white guys who are the official residents: George Washington, Thomas Jefferson, Abraham Lincoln, and Theodore Roosevelt. In 1937, there was a bill in Congress to add Susan B. Anthony. There is something of a movement to add Ronald Reagan (presumably for his leadership of SAG). And there is an unofficial fifth face up there already.
tdtz got it with the four compass directions plus up and down. Known to the Lakota and others as the Six Grandfathers, this was the most sacred place in the sacred Black Hills, dwelling place of the six deities that figured so prominently in the visions of Black Elk, representing the four directions, the sky and the earth, before Doane Robinson and Gutzon Borglum took over.