The Badlands of South Dakota are elevated, eroded plateaus and valleys composed of undeformed sedimentary strata. These are not mountains, because

there's no deformation of rocks or structures.

Explanation:

Mountain refers to a elevated portion of the Earth's crust, but has an exposed bedrock.

It should be noted that the Badlands of South Dakota are elevated, eroded plateaus and valleys composed of undeformed sedimentary strata but they aren't mountain because there's no deformation of rocks or structures.

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