At its deepest, happy valley is close to 330 m (a bit over 1000 feet) deep, and it once was at least as high as the mountains around the valley. let us call the thickness of rock removed from the valley each year, your answer in the previous blank, 0.33 m. then the depth of the valley, 330 m, divided by the erosion rate, 0.33 m/yr, yields the number of years it took to hollow out happy valley, . this should be a large number.