A deep, steep-sided valley formed by weathering and erosion is called a canyon or gorge.

Here's why:

* Canyon: This term usually refers to a deep, narrow valley with steep sides, often carved by a river over a long period of time.

* Gorge: This term is similar to canyon, but can be used for smaller or shallower valleys.

While Valley Forge is famous for its historical significance during the Revolutionary War, it's not a canyon or a gorge. It's a valley with less dramatic terrain and a less steep slope.