Question

What is the difference between contour lines on a mountain and contour lines in a broad flat valley

Answer 1

You have elevation differences on mountains and elevation differencesin the valley. The valley is just not as pronounced.

Answer 2

The contour lines on a mountain are going to be much closer together, since the elevation changes much quicker than in a flat valley, where the contour lines will be further apart because there isn't much elevation change.