The Lakota kept track of events by drawing symbolic pictures of them on hides, cloth or paper. These are known as winter counts, since that's when they usually had the time to do this. Large families often had their own count keepers, who in turn consulted with the elders of the tribe for the appropriate event and symbol.

The Smithsonian has an online exhibit covering ten winter counts during the 18th and 19th centuries, which shows the symbols and offers an explanation of each. The earliest drawing is dated to 1701.

http://wintercounts.si.edu/