A couple interesting things here: first, what was that loop circa 1890-1940? I can't think of any major trends that would cause a westward and northward shift of the population except for the industrialization and de-industrialization of the midwest, though in my mind, de-industrialization (and thus population shifts) only occured in the 1980s or so. Second, why did it shift so drastically in the 1970s? You would expect a massive shift in the 1840s and 1850s with the Oregon Trail and Manifest Destiny, but I can't think of any shift in US history that would cause that large. Third, I'm curious where this dot would be during the Revolution (probably somewhere in North Carolina, I would guess), or during Columbus.