"The frontier ends and disconnection begins. Cause and effect? How the dickens do I know? I spent my earlier hob-raising years out where you're headed, Denver and Cripple Creek and Colorado Springs, while there was still a frontier, you always knew where it was and how to get there, and it wasn't always just between natives and strangers or Anglos and Mexicans or cavalry and Indians. But you could feel it, unmistakeably, like a divide, where you knew you could stand and piss would flow two ways at once."
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"The frontier ends and disconnection begins."
The Professor is discussing a liminal space. Perhaps the liminal space that matters in Against the Day. I can't say for certain, having not read this book previously (and, yes, I can see how reading it first and then doing this project would be far more informative). If the frontier is the boundary to the unknown then it is also a boundary to the known, depending on which side you arrive at it. Like pissing at the top of the Rockies, which Heino is making reference to here, things flow from this space into (and out from) two distinct places: "civilization" and "nature." When discussing being "disconnected," Heino must mean disconnected from the natural, chaotic side of the frontier, perhaps obvious in that civilization (and, in the context of what Pynchon has written so far, white society) is being centered. The object of civilization (and white society!) is to destroy the boundary by forcing the center to become the whole. Frederick Turner's essay claimed that taming nature and pushing the frontier was part of the ambitious drive which characterizes Americans. I disagreed with that in many ways but, yes, in a lot of ways, usually after the first pioneers desperate enough to brave the frontier, people followed with ideas of exploitation (who also didn't have to do the work of taming the frontier and so are "disconnected" from the entire experience and idea of it. They never see the boundary).
"you always knew where it was and how to get there"
Heino explains how the frontier wasn't a place that needed to be recognized by sight or by the types of people on either side of it: the frontier was an idea, although one that was nearly tangible, which the mind could simply recognize. Like walking from a warm room to a cool one. You just knew where the boundary was and could probably tell on a daily basis how much it was moving.
"between natives and strangers or Anglos and Mexicans or cavalry and Indians"
Just in case the reader wasn't making the connection of some of the main ways the frontier divided people, by race and culture, Heino makes it plain while stating "These weren't the only ways to tell though! Something more mystic about it!" But it lends credence to the theme of centering white society and how one side of each of those pairs would continue to exist basically unchanged even after "the frontier" disappeared while the other half, having been decentered and disconnected from the wild side of the frontier, would have to completely change to cope with their new white world.