from Black Elk Speaks:
"Then as we walked, there was a heaped up cloud ahead that changed into a tepee, and a rainbow was the open door of it; and through the door I saw six old men sitting in a row.
"The two men with the spears now stood beside me, one on either hand, and the horses took their places in their quarters, looking inward, four by four. And the oldest of the Grandfathers spoke with a kind voice and said: ‘Come right in and do not fear.’ And as he spoke, all the horses of the four quarters neighed to cheer me. So I went in and stood before the six, and they looked older than men can ever be--old like hills, like stars."
The rock formation that was blasted away to make the four Presidents on Mt Rushmore was called the Six Grandfathers.