Purely oral??? What about all those petroglyphs, pictoglyphs, and hide "calendars"? Granted it wasn't written language as we know it, but it was just as written down as the cave paintings in Altamira: a way to pass knowledge on to others. Because WE can't read it means nothing -- we can't read Linear A, either. However, there ARE some alive today who CAN read the glyphs.

(I'll quietly point out the Inukshuk as well.)