My brother and sister-in-law and I were remembering an unpleasant event fondly, as one does once it’s safely over. A few years ago, they’d been here in Baltimore and were heading back on I-95 to Philadelphia. The trip, which usually takes an hour and a half to two hours, took five because a snowstorm had moved over I-95 and stayed there. In our reminiscences, we noted that storms, rain or snow, seemed to follow I-95, that is, I-95 seems to be the line between one kind of weather and another. “Were we making that up?” we wondered. Why would weather follow an interstate? I had an epiphany: maybe because I-95 follows the Atlantic Seaboard Fall Line. I am obsessed with the Fall Line, mostly because the name is so pretty.

Interstate 95 is the white line in this picture. It runs the length of the East Coast—a terrible, kill-or-be-killed road, but that’s neither here nor there—and connects the East Coast’s major cities. It follows the Fall Line and the cities are dotted along the Fall Line.

The cities and roads are where they are because of what the Fall Line is: a more or less invisible, small, underground cliff—an escarpment—that marks the old edge of the continent.

On the Fall Line’s west side, the high side, are tough crystalline rocks; on the east side, the low side, are soft, easily moveable sediments. Rivers running out of the Appalachians east to the Atlantic crossed the cliff, and where they did, made falls. The early settlers couldn’t get their boats up the rivers past the falls, so they unloaded there and stayed put. Cities grew along the Fall Line, roads connected the cities. It’s all so logical.