There were indeed northward Indian migrations post-contact, starting for the Tuscaroras (an Iroquois people) after a war in 1713, when they moved north to join distant kindred among the Five Nations by 1715. The movement continued piecemeal late into the century up the "Tuscarora Path Valley" in PA, heading particularly towards lands held by the northern tribes (which eventually were referred to as the Six Nations when granted rights to land in PA in the 1780s). A good story on this can be seen at: http://tuscaroras.com/pages/history/path_valley_1.htmlhttp://tuscaroras.com/pages/history/path_valley_1.html http://tuscaroras.com/pages/history/1784_treaty_six_nations.htmlhttp://tuscaroras.com/pages/history/1784_treaty_six_nations.html http://www.vmnh.org/indhist.htmhttp://www.vmnh.org/indhist.htm
The NC-VA-MD tribes seem to have disappeared under the weight of white settlement, probably with significant marriage out among whites and other tribes as they dispersed through the 1700s.Basically, there seem to have been attractions in PA for anyone with ties to the tribes granted rights by the 1784 Treaty of Six Nations, especially if they had close kin who'd already moved up over the preceding decades.
We don't really know much about the Barlows and virtually nothing about Susannah's supposed Indian or part-Indian mother.It's possible there were other PA connections among the Barlows -- for instance, there's a town named Barlow just below Gettysburg in PA and right above Taneytown in MD -- maybe that family already had footholds in PA and that drew John and family further north.With all the border changing back then, I'm not too surprised to hear Pipe Creek is in MD!I do wonder how the family managed to get up to the Cambria Co area back then -- it looks like moutainous and difficult land between there and the PA-MD state line.
Re my family's location in Australia:I was born in Indiana, but have lived here for the past 30 years, becoming a citizen in 1992. My children and I call Oz our home, extending quite a few family lines into the antipodes:)