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, what was formed through erosion over long periods of time?

Available choices: (i) Columbia River Gorge; (ii) a hydraulic system; (iii) swamp vegetation; (iv) Mississippi River delta; (v) the Moon; (vi) Calcium carbonate; (vii) the Himalayan mountains; (viii) permineralization;
The answer is:
(i)