Residual mountains are called mountains of denudation because they are the remnants of much larger mountains that have been eroded and worn down by agents of denudation like wind, water, ice, and gravity over millions of years.

Here's a breakdown:

* Denudation: This is the process of wearing down and removing the Earth's surface through erosion and weathering. It's a slow but relentless process.

* Residual Mountains: These are mountains that have survived denudation, often due to their resistant composition (harder rocks) or their location (sheltered from erosion).

* How they are formed: Initially, there were much larger mountains formed by tectonic activity. Over time, these mountains were subjected to denudation. The weaker rocks were eroded away, leaving behind the harder, more resistant rocks that form the residual mountains.

Think of it like this: Imagine a giant sandcastle on the beach. The waves (like denudation) slowly erode the sandcastle, wearing it down. Eventually, only a few sturdy parts remain. These remnants are like residual mountains.

Here are some examples of residual mountains:

* The Black Hills of South Dakota: Formed from resistant granite that resisted erosion better than surrounding softer rock.

* The Ozark Mountains in the United States: Primarily composed of resistant limestone and dolomite.

* The Scottish Highlands: Formed from resistant metamorphic and igneous rocks.

So, residual mountains are called mountains of denudation because they are what's left after significant erosion, showcasing the power of denudation in shaping the Earth's surface.