Here's how they form:

* Tensional forces: The Earth's crust is stretched and pulled apart.

* Faulting: The stretching creates fractures in the crust, called faults.

* Block uplift: The rock blocks between these faults are uplifted, creating the mountain.

These mountains are often characterized by:

* Steep, straight slopes: Due to the abrupt uplift along the fault lines.

* Flat tops: The uplifted blocks often retain their original, horizontal layers.

* Graben and horst: The valleys (graben) are formed by the blocks that sink, while the raised blocks (horst) form the mountains.

Examples:

* Sierra Nevada Mountains (California): These mountains are a classic example of fault-block mountains.

* Wasatch Mountains (Utah): Another prominent example in the western United States.