You usually find a river flowing along the bottom of a valley.
Water heads down the side slopes and then flows along the valley in the downhill direction. As it does so, it removes soil and other loose material so that, as time passes, the valley gets wider and deeper.
But in areas of limestone rock you are likely to come across valley shapes without the river.
If you’ve been following this series, you will already know that limestone is full of cracks, which get enlarged as the limestone dissolves and the water disappears into the ground.
But if the rock is limestone, how did the valley form in the first place?
Sometimes a dry valley is formed because an underground passageway caves in. The resulting valley is likely to slope rather randomly and may not have a clear downhill direction.
But many dry valleys look as though they should have a river in the bottom because they were shaped by a river in the past.
The holes in limestone gradually enlarge as the water flows through and more rock is dissolved. So turn the clock back a bit and the holes would have been smaller: maybe not big enough for the whole river to flow into the ground. So some water could have remained on the surface, cutting the valley shape as it flowed.
This is not the only possible explanation. What if the limestone had plenty of holes but they were blocked for a time. Can you think how this might happen?