Residual mountains are those caused by

Residual mountains are caused by denudation. Denudation is a process that involves the wearing away of the Earth's surface by various agents of erosion, such as wind, water, and ice. Over time, these agents of erosion can remove large amounts of material from the surface of the Earth, including rock and soil. Residual mountains are formed when erosion removes the softer, less-resistant rock layers, leaving behind the harder, more-resistant rock layers. These harder rock layers then form the core of the mountain, while the softer layers are eroded away, creating valleys and other landforms. Examples of residual mountains include the Appalachian Mountains in the eastern United States and the Scottish Highlands in Scotland. These mountains were formed over millions of years through the process of denudation and continue to be shaped by erosion to this day.