Ageologist notices that a river is eroding its valley at a constant rate. knowing the height of the valley walls, how coul
geologist figure out when the river started carving the valley?
a.
sit and observe the river for a few hours until the valley walls double in height.
b. fill up the river valley with rocks, and time how long it takes the rocks to wash out
c. divide the height of the valley walls by the rate of erosion.
od. count growth rings of trees growing on the valley floor.

The answer is C

Explanation:

In this example, a geologist is trying to figure out how long ago a river began carving its valley (that is, the age of the river valley). The age of the river valley is determined by dividing the height of the valley walls by the rate of erosion.

For example, assume the rate of erosion is 2 mm per year. If the valley wall height is 20 m (20,000 mm), then the age of the valley can be estimated as follows...

Age of river valley = 20,000 mm ÷ 2 mm per year

Age of river valley = 10,000 years

This calculated age is a good estimate for the river valley's age. But, the rate of erosion may not have been constant throughout the valley's formation. Also, other Earth processes could affect the rate. Geologists have to account for these factors to improve their estimates.