Here's why:

* Crust splits apart: This is the fundamental process that creates rift valleys. The Earth's tectonic plates move apart, causing the crust to fracture.

* Valley formed: The splitting of the crust leads to a depression in the landscape, forming a valley.

* Steep narrow sides: As the crust continues to pull apart, the sides of the valley become increasingly steep.

* Flat floor: The valley floor is often relatively flat, as it is filled with sediments and volcanic material.

Famous examples of rift valleys include:

* The Great Rift Valley in Africa: This iconic valley system stretches for thousands of kilometers, showcasing the dramatic effects of continental rifting.

* The Rio Grande Rift in North America: This rift valley extends from Colorado to Texas, featuring volcanic landscapes and unique geological features.

Let me know if you have any more questions about rift valleys or other geological formations!