Why can a heavily dissected plateau lack an obvious flat surface?

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plateau The Columbia Plateau is uniformly covered with basaltic lava flows and spans an area of about 100,000 square miles in Idaho, Washington, and Oregon.

A heavily dissected plateau may lack an obvious flat surface because it has been extensively eroded by rivers, which carve deep canyons and valleys into the landscape.1

In such regions, the original flat surface is replaced by a complex topography of steep, narrow ridges and deep incisions. While the plateau may no longer appear flat to an observer on the ground, it is still geomorphically defined by the uniform elevation of its highest ridges and peaks. This high, uniform elevation remains the primary characteristic that distinguishes these dissected areas from other mountainous terrain.1