Avast plain of rolling hills are actually folds of monoclines domes and basins anticlines and synclines horsts and grabens

A vast plain of rolling hills are actually folds of domes and basins.

Explanation:

The symmetrical anticlines that have round shape are referred as Domes. This portion of the crust pushes in an upward direction due to the volcanic activity that occurs under the crust of the Earth.  

The folds of domes that are overlapping always face inwards. The term Basins is the antonym of domes. Basins are usually symmetrical depressions of the crust of the Earth. The overlaps of folds in basins always face outward. Hence the actual folds that are visible in a vast plain of rolling hills are domes and basins.

81 is a composite number.