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How does erosion change the shape of a mountain?

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A car drives along the Trans-Canada Highway with Mount Sir Donald's sharp, snow-capped peak rising above forested slopes in the Selkirk Mountains, British Columbia. — mountain Mount Sir Donald in the Selkirk Mountains, British Columbia, looms above a segment of the Trans-Canada Highway.

Erosion acts as a destructive force that progressively modifies and reshapes the form of a mountain range over time. While constructional processes like tectonic uplift build mountains, the external shape of these landforms is a constant interplay between that uplift and the forces of erosion.1

Erosion alters mountains through several key processes:

Agents of Change: Weathering breaks down rock in situ, while erosion transports this material away using water, wind, and glacial processes.123
Glacial Reshaping: In glaciated ranges, ice drastically alters valleys. Glaciers remove debris and erode bedrock to transform V-shaped stream valleys into characteristic U-shaped valleys. They also carve out cirques, and the headward erosion of these features can leave behind sharp, jagged ridges known as arêtes and pointed peaks called horns.4
Continuous Modification: Because erosion begins as soon as a landform is created and continues indefinitely, a mountain's shape is never static; it is a direct consequence of the ongoing destruction of its previous form.1