Mountains formed by blocks of crust moving along a fault are called fault-block mountains.

Here's a breakdown of why:

* Fault: A fracture in the Earth's crust where there has been movement.

* Block: A large section of crust that is moved by the fault.

* Mountains: Elevated landforms formed by the upward movement of the crust.

This type of mountain formation is often associated with extensional forces, where the crust is being pulled apart. As the blocks move apart, one block may be uplifted, creating a mountain, while the other may subside, creating a valley.