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Why do weaker rocks often form valleys in folded mountain regions?

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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.

In folded mountain regions, the formation of valleys and ridges is primarily driven by the process of differential erosion. As exogenetic forces—such as water, ice, and wind—act upon the landscape, they encounter rocks with varying degrees of resistance.1

In these regions, the folding of sedimentary rock layers creates a series of parallel structures. Rocks that are more resistant to weathering and erosion remain as prominent ridges, while the weaker, less resistant rocks are worn away more rapidly. Over time, this differential wearing away of materials results in the development of valleys underlain by these weaker rock units, while the more durable strata stand out as ridges. This pattern is a classic example of a structural landform, where the final topography is a direct reflection of the interaction between tectonic deformation and the varying physical properties of the underlying rock.21