[dirtboy]

Fairy tale nonsense! - What was the rest of the river bed doing while this cut was made through this very selectively uplifted hard material?

Doing what rivers do - erode and send out tributaries. The tributaries eroded away the softer rocks on both sides of the basaltic formation. Just as in Pennsylvania, the ridges are resistant to erosion and the valleys are softer rock that eroded away due to tributaries. The underlying geology drives the result of erosion.

This is some of the most obvious and basic stuff in Geology, and applies around the world. The fact that you are even debating it points out the futility of bothering to engage you in such. Later.

[editor-surveyor]

"erode and send out tributaries."

Suddenly you believe in 'trickle-up?'