Sit back and relax: When the Susquehanna ran backwards

Environment

HARRISBURG, Pa. (WHTM) — Once upon a time, the Susquehanna River flowed in the opposite direction than it does now.

“Once upon a time” in this case means about 325 million years ago. The Appalachian Mountains did not yet exist. Instead, there were the Taconic Mountains, which formed from a collision between two tectonic plates about 440 million years ago. Over the millennia, as those early mountains wore down, the channel of the Susquehanna formed, with the water flowing north.

Then came the Alleghanian Orogeny, 325 to 260 million years ago. Orogeny, according to Oxford Languages, is “a process in which a section of the earth’s crust is folded and deformed by lateral compression to form a mountain range.”

This was when all the continental plates slammed together to create the supercontinent of Pangaea. What’s now the East Coast of the United States suffered massive stress. Flat seabeds got flexed, crumpled, and shoved up to create the Appalachian Mountains, which at the time were higher than the Rocky Mountains are today.

If you drive up to Penn State for the next football game, take a look where PennDOT cut away parts of the mountains to widen Route 22-322 a few years ago. The curved arcs of strata you see are evidence of that mountain-building period. Construction workers found a lot of fossilized seashells.

Now while 65 million years is just the blink of an eye in geological terms-seriously, it’s only as long as the period of time between the T-rex and us, the change was slow enough for the Susquehanna to keep eroding its channel. And at some point, when the Appalachians became higher than the Taconics, The Susquehanna changed direction and started flowing south.

Wait a minute…

If the Susquehanna started by flowing north and is now flowing south…

Does that mean the river is flowing backwards now?

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