These past several days, as I’ve been working with the western Siouian tribes and the White Buffalo (Calf) Legend, it occurred to me that during my work with the history of the Eastern Siouian tribes, that I had never heard of any White Buffalo stories or legends. Many traders and historians, John Lawson in 1701, the first among them, wrote of the various tribes they came in contact with, and not once have I seen any reference to any buffalo legend, let alone a White Buffalo legend. I certainly would have noticed this, as would ethnologists during the past hundred years or so.
Therefore, this would seem to suggest that the Eastern and Western Siouian tribes divided before the appearance of the White Buffalo Woman, generally thought to have been about 2000 years ago.
On the map below published by the Smithsonian, the Eastern Siouian tribes are shown. They are surrounded by other tribes which are not classified on this map, but which are now also believed to be Siouian as well, such as the Wateree and the Monacan.
Native Heritage Project 
Another thing you’ll probily run into if you already have not with the western sioux
is “who migrated?” The western sioux are adament that they have always been west and
never migrated from the southeast. While southern sioux believe that they did split and went from east to west. I was 1st made aware of this by former cheif of the chicora siouian tribe of Conway SC Gene Martin who’s made numerious trips out west to learn more of there language and ceromonies that the eastern sioux have lost through out the yrs, in order to reincorporate them into the tribal members today.