Canyons and planation surfaces

The world is full of vast flat areas that geologists call planation surfaces. They often have a steep edge called escarpment which canyons cut into. One example of this is found in Texas.

How do you think the canyons could erode while the flat field next to them was left untouched? It could be explained if the whole area was flooded and the canyons eroded backwards when the water flowed of the edges.

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