It sounds like you're describing a plateau. From dictionary.com a plateau is "a land area having a relatively level surface considerably raised above adjoining land on at least one side, and often cut by deep canyons."
A plateau is a flat topped mountain or elevated area with a flat top. A mountain is tall. elevated, pointed structure.While mountains and plateaus are both elevated, plateaus are flat, whereas mountains slope.
Mountains are generally much higher. However, the real difference is that plateaus have a flat area on top. This flat area can be quite large, even mile across. A mountain has little or no flat space on its peak or summit.
A plateau has a flat top, and a mountain has a peak.
applacation mountains
A mesa is a small mountain with a wide, flat top.
The word you need may be 'Plateau'
A flat area is called a plane. A flat area the is elevated higher than all the surrounding land is called a plateau.
There are actually three different names that can be used to describe a high, flat topped mountain or hill. These names are plateau, mesa, and butte. These flat top mountains are also called Tepui (or tepuy) in the native language of the Pemon, the original inhabitants of the Gran Sabana area in Venezuela. It means "house of the gods".
A plateau or mesa.
Anywhere reasonably - hills or mountains, have flat, or reasonably flat, tops.
Mountains are natural elevations of the Earth's surface that are higher and steeper than hills, with peaks and valleys. Plateaus are flat elevated landforms that are higher than the surrounding area, with a relatively flat on top. Plateaus are generally larger in area than mountains and have more uniform surfaces compared to the rugged terrain of mountains.
A plateau.