Thread by @PAMapper

The Fall Line is an ancient geological feature. Lined by a string of waterfalls, it divides the hills of the Piedmont region from the Atlantic coastal plain.

Now, the line is a near-perfect boundary between left-trending & right-trending areas in Metro Philly.

Why? A thread:

The conventional wisdom about 2020 is that Biden lost ground in Philadelphia proper and made gains in the suburbs.

That’s true, but incomplete. For instance, swathes of Bucks and Delaware Counties swung right, while Northwest Philly neighborhoods like Roxborough got bluer.

Comparing the 2012 & 2020 coalitions, the Fall Line comes into focus as a more accurate demarcator.

Biden improved on Obama’s 2012 margin above the Fall Line by 9.5%.

Below the Fall Line Trump exceeded Romney’s margin by 10.4%.

That’s a starker disparity than city vs. suburbs.

The Fall Line has little effect on our lives today.

But for the indigenous Lenape people and the first European colonizers, it was crucially important.

The Fall Line was the limit of rivers’ navigability, where waterfalls made transportation further inland much more difficult.

Colonization entailed the transformation of the natural landscape in service of industrial capitalism. The waterfalls along the Fall Line proved useful, as they could power mills and factories.

Philadelphia, located at the Falls of the Schuylkill River, made use of this power source. Meanwhile, the Delaware River was a key transportation corridor, as it was navigable up to its crossing of the Fall Line at Trenton.

As the city below the Fall Line grew increasingly crowded in the late 18th and early 19th Centuries, wealthy Philadelphians built vacation homes above the Fall Line, in neighborhoods such as Chestnut Hill.

Later, canals, dams, train tracks, and roadways would eliminate the Fall Line as an obstacle to transportation. But the vestiges of its significance remain with us in the form of place names like Falls Township in Bucks County and the East Falls neighborhood of Philadelphia.

Still, segregation by class and race became self-perpetuating. Polluting industries like the oil refineries in South Philly and Marcus Hook located below the Fall Line, and the affluent followed new modes of transportation, like the Pennsylvania Railroad’s Main Line, above it.

For decades, Democrats have performed much better below the Fall Line, which is poorer, less educated, and more diverse, than above it.

But in the last 2 cycles, Democrats have increasingly made appeals to college-educated voters, while ceding the less well-educated to Trump.

The portion of Metro Philadelphia below the Fall Line is still much bluer than the portion above it, but they are moving towards each other. And this affects where Democrats go for votes.

In 2012, Obama netted 2.2x more votes below the Fall Line than he did above it. For Biden in 2020, that ratio was 1:1.

If this trend continues, the center of political gravity for Democrats in the Philadelphia region will soon shift decisively above the Fall Line.

Hope this contributes something to your understanding of Pennsylvania’s political geography. Hat tip to Ben Forstate whose analysis of Blue Mountain was the inspiration for this thread.

A few notes on methodology:

I drew the Fall Line on DRA as best I could based on geological maps and Google street view. The line I drew couldn’t be perfect, because precinct boundaries don’t always line up with geology.

DRA’s earliest dataset is 2012/2016 President, so the precinct level swing map is comparing that to 2020 President.

2012 election results in Delaware County were only available by municipality, so for the comparisons of vote totals and margins, Delco municipalities are counted as either entirely above or below the Fall Line…

But even within municipalities that straddle the Fall Line, the tendency is precincts above trending left and precincts below trending right. I’m confident that the disparity I found would only be enhanced if I had access to precinct-level data in Delaware County.

Re: the critique that I'm overgeneralizing: that's the point. Above vs. below the Fall Line is a better overgeneralization than city vs. suburb. To be clear, geology is not predictive of election outcomes. But in this case it neatly captures how political coalitions are changing.