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

Fact 2: The Columbia River Gorge is a spectacular river canyon cutting through the volcanic rock of the Cascade Mountain Range.

Given the two facts above, answer the question "What was formed through erosion over long periods of time?" with the following options: 
- Columbia River Gorge
 - a hydraulic system
 - swamp vegetation
 - Mississippi River delta
 - the Moon
 - Calcium carbonate
 - the Himalayan mountains
 - permineralization


 

 Columbia River Gorge        