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Quiz yourself on River Processes and LandformsGet your flashcardsBuild my revision planTrack my progress⛓️ How Waterfalls and Gorges Form
Figure 3: Waterfall Formation — from initial step in hard rock to gorge formation as the waterfall retreats upstream
Waterfalls form wherever a river flows from hard rock onto soft rock — the differential erosion of these two rock types creates a step that becomes progressively more dramatic over time. The formation process is one of the most frequently examined sequences in GCSE Geography.
The river crosses a geological boundary where resistant rock (e.g. hard dolerite, granite) sits immediately above weaker rock (e.g. limestone, shale, sandstone). Initially, a small step or rapid exists where the two rock types meet.
The softer rock beneath erodes much faster than the hard rock above. Hydraulic action and abrasion attack the softer rock at the base of the step, while the hard rock above resists erosion and remains in place. The step gradually becomes more pronounced, then vertical, forming a waterfall.
Erosion of the soft rock continues behind and beneath the waterfall. This forms a rock shelter or cave behind the falling water, and the hard cap rock extends outward as a dangerously unsupported overhang above it. Hydraulic action is most powerful here — water plunges into the cavity and repeatedly compresses and releases the air within cracks in the rock.
Eventually, the overhanging hard rock is unsupported over too great a span and collapses. This collapse is often triggered by freeze-thaw weathering in the joints of the hard cap rock, which progressively weakens it until gravity pulls it down.
The falling water and the collapsed debris both drive abrasion at the base of the waterfall. Boulders are swirled around by turbulent water, acting like a drill, deepening a circular plunge pool. The abrasive action of swirling boulders also smooths the walls of the pool.
With the overhang collapsed, the new face of the waterfall is slightly further upstream than before. The entire process then begins again: undercutting → overhang → collapse → retreat. The waterfall migrates steadily upstream over hundreds and thousands of years.
As the waterfall retreats upstream, it leaves behind a steep-sided, narrow valley called a gorge — the scar of where the waterfall once stood. Over geological time, if the gradient flattens enough, the waterfall may eventually disappear, leaving only the gorge as evidence of its former existence.
Named UK Example: High Force, River Tees, North Pennines
- Location: River Tees, County Durham / North Yorkshire border, North Pennines AONB
- Drop: 21 metres — the largest waterfall in England by volume of water
- Geology: Hard dolerite of the Whin Sill (an intrusive igneous rock) overlies softer limestone and shale
- Gorge: The gorge downstream of High Force is approximately 700 metres long — representing thousands of years of upstream retreat
- Evidence of process: A plunge pool is clearly visible at the base; the cave behind the falls shows active undercutting
- Low Force: 1.5 km downstream, Low Force waterfall sits on the same Whin Sill — a younger, less retreated stage in the same process
Quick Check: Explain why a gorge forms downstream of a retreating waterfall. Use the sequence of formation in your answer.
A gorge forms because as the waterfall retreats upstream, it leaves behind the valley it carved during its retreat. The retreat process — hydraulic action undercutting the softer rock, the hard cap rock overhang collapsing, the waterfall stepping back — occurs over and over. Each cycle moves the waterfall slightly further upstream, abandoning the steep-sided rock face it previously occupied. The accumulated rock faces from each stage of retreat form the walls of the gorge. The gorge is therefore a record of the waterfall's upstream migration — longer gorge = more retreat = longer period of formation. High Force on the River Tees has a gorge approximately 700m long behind it.
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