When a river goes down a vertical, or near vertical slope a waterfall results.

Severn-Break-Its-Neck waterfall

They often form due to a change in rock type.  If the river is flowing along on top of some really solid, hard rock and then comes to something softer that breaks up more easily, the abrasion will rub away the hard rock very slowly, while the softer rock will more easily be broken and washed away.

The result, a vertical drop for the river – a waterfall.

As the water is now falling at this point, the impact, at the bottom of the drop, soon gouges out loads of the softer rock forming a deep pool – a plunge pool.

Plunge pool below Pistyll Rhaeadr

The water swirls in the pool, digging out the soft rock from underneath the harder rock.  Sometimes you can walk round behind the curtain of falling water.

But be careful, the hole under the hard rock gets bigger and bigger and eventually the lack of support could cause the overhang to collapse.

The debris lands in the plunge pool and the water now falls down from a different point slightly further upstream.

This cycle of undercutting and rockfall can repeat many times.  Each time the waterfall’s position moves upstream slightly.  But this is only happening on the river’s path.  The land either side is still at a higher level.

Immediately downstream from Severn-Break-Its-Neck waterfall

So downstream of the waterfall the valley sides can have a vertical drop into the river.