Why do sediments in plateau valleys often lie flat even when nearby mountain rocks are deformed?

In plateaus formed by crustal shortening, the underlying basement rocks and surrounding mountains are often strongly deformed due to the tectonic forces that created them. However, the sediments found in the valleys between these mountains typically lie flat because they are young deposits that accumulated after the primary tectonic deformation occurred.1

These valleys act as basins where streams and rivers transport and deposit eroded debris from the adjacent mountains. Because these sediments are deposited into these existing depressions, they settle horizontally, creating the broad, flat surfaces characteristic of these specific plateau types. These features are often preserved in dry climates where the rate of erosion is slow enough to prevent the removal of the accumulated debris.1