Britannica Sources used in this answer

How do folds in rock layers create ridges and valleys?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A car drives along the Trans-Canada Highway with Mount Sir Donald's sharp, snow-capped peak rising above forested slopes in the Selkirk Mountains, British Columbia. — mountain Mount Sir Donald in the Selkirk Mountains, British Columbia, looms above a segment of the Trans-Canada Highway.

Folds in rock layers create ridges and valleys primarily through the interaction between tectonic deformation and erosion. When compressional forces act upon sedimentary rock layers, they warp the strata into a series of alternating anticlines (upward folds) and synclines (downward folds), forming what are known as fold belts.123

The resulting topography is dictated by the varying resistance of these rock layers to weathering. As erosion acts upon these folded structures, more resistant rock layers remain to form prominent ridges, while weaker rock layers are worn away more quickly to create valleys. This process often results in a landscape characterized by long, parallel ridges and valleys, such as those found in the Valley and Ridge province of the Appalachian Mountains. In these regions, the geologic structure serves as a primary control on the orientation and form of the landscape, as the differential erosion of folded strata continuously shapes the terrain over time.24