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How can valleys filled with sediment become part of a plateau?

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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.

Valleys filled with sediment can become part of a plateau through the process of crustal shortening, which occurs in regions where tectonic forces cause the Earth's crust to thicken and uplift.12

In these environments, mountain ranges are created along the margins of the plateau. As these mountains undergo erosion, streams and rivers transport the resulting debris into the neighboring valleys. When drainage is internal—meaning water does not flow out of the basin—this sediment accumulates and fills the valleys. Over time, these deposits create broad, flat surfaces within the basins.1

Because the young sediment deposited in these valleys typically lies flat, the resulting landform is a high-elevation plateau defined by these broad, sediment-filled valleys surrounded by eroded mountains and hills. Essentially, these plateaus are mountain ranges that have been buried in their own debris. This type of landform is most likely to survive in dry climates where the rate of erosion is slow.1