Even before that, the Sioux were much further east, and I think I read that in the 1600s they may actually have been very near the East Coast. I’m not sure who shoved them into Crow territory but some tribe or tribes did. Everyone thinks of them as Plains Indians but the Plains culture was relatively modern, after the reintroduction of the horse to the Americas by the Europeans.
I skipped over to my library...
THE SIOUX, by Guy Gibbon says that the Sioux were driven out of the Great Lakes area by the Ojibwa, who were THE power in the North prior to the Seven Year’s War.
It’s a terrific book, by the way. Blackwell publishing produces some very nice, comparatively inexpensive volumes that are stuffed with information. Check ‘em out on Amazon.
Before that, most of the tribal elders believed they came from what is now the James River Valley in Virginia. Others thought it was from the Finger Lakes Region of New York. In any case, the Sioux were one of the tribes doing the pushing of weaker tribes west. And our tribe suffered the same fate at the hands of the Chippewa who pushed us out of northern Minnesota as they were displaced from what is now Michigan, Wisconsin and southwestern Ontario.
Interesting stuff. Many of the tribal divisions are purely arbitrary and partly white man's invention. A good friend of mine who is Navajo tells me that they are basically the same people as the Apache-- both tribes which originally moved from the south into what is now northern New Mexico and Arizona to raid the smaller, weaker tribes which had settled that region.
The only difference is that the Navajo accepted territory and an agricultural/ranching lifestyle in return for ending their periodic raids and invasions whereas their Apache cousins clung to that lifestyle for an extra half century or so. Thus they were designated as separate tribes by the white man, not by the Native people themselves. Our Navajo friend, who is well educated, even laughs at the term "Native people" as they expect they invaded what is now the southwestern United States from what is now Mexico some centuries before to escape the cruelties of the Aztec. The idea that they are somehow supposed to feel guilty about what America has supposedly done to Latin America is lost on them for this reason.
Not so different from the Normans (originally from Norway), Saxons (supposedly native to England but originally from Germany) and Norwegians who all fought for the English throne in that pivotal autumn of 1066.