Fact 1: Most canyons are formed by flowing rivers through erosion over long periods of time.

Fact 2: Deep canyons and valleys are carved out.

Given the two facts above, answer the question "What are formed by flowing rivers through erosion over long periods of time?" with the following options: 
- deserts
 - a hydraulic system
 - deep valleys
 - pyramids
 - Calcium carbonate
 - patch reefs
 - buildings
 - Sediment


 

   deep valleys      