In folded Appalachians, why can one rock layer become a ridge while its neighbor becomes a valley?

Answer: Contrasting rock resistance

Contrasting rock resistance ✓ — Right. Folding supplies the striped geometry, but erosion edits the stripes. Resistant sandstone and conglomerate tend to remain as ridges, while more easily eroded shale, slate, limestone, or carbonate rocks become valleys. That is the same big lesson as the fault-damage story: topography records rock strength, not just uplift.

Younger layers always rise — No. Age alone does not make a layer climb above its neighbors. In the Valley and Ridge, folded layers repeat across the landscape, and the present relief largely reflects which rocks resisted erosion. A younger soft layer can be cut down while an older hard layer remains high.

All folds erode evenly — The opposite is the point. If folds eroded evenly, the landscape would not show such sharp parallel ridges and valleys. Differential erosion turns a folded stack into a barcode of highs and lows. This is why a map of topography can reveal hidden geology even where the original mountain-building event is ancient.