Powerful torrent in the river’s flow
The beauty of waterfalls has inspired poets and attracted visitors for centuries, and these compelling spectacles of nature are at their most dramatic at winter’s thaw
Spray swirls into the air as, swelled by winter’s rain and melting snow, High Force waterfall crashes down a cliff of stark grey stone into the churning pool below. The roll of the water reverberates around the gorge as the river adjusts to its new position, sweeping past the rocks that crowd its edges as it resumes its unstoppable journey to the sea. Named from the Norse word ‘foss’, meaning waterfall, High Force is one of the most spectacular in Britain. It is formed as the River Tees plunges 72ft (22m) from the high, heather-filled moors of the northern Pennines into the limestone basin below. Mesmerising to watch, as the water bucks and whirls, it is no surprise that High Force has attracted visitors for hundreds of years. The awesome power or torrid beauty of waterfalls, as they plummet over sheer cliffs or tumble gently down hillsides, has inspired writers, painters and poets, and in legend they are home to supernatural beings and magical events. Yet, even the biggest and most romantic of waterfalls are gradually disappearing due to the very processes that created them, providing a fleeting glimpse in geological time.
Ancient rock
Waterfalls are the result of a complex mixture of geological events, born out of the types of rock present in an area and the physical changes they have undergone. “Although the rocks involved in waterfall formation will be many millions or even billions of years old, the processes that create them may be much more recent, perhaps only a few thousand years,” says Dr Edward Cole, a geographer at Bristol University. “Waterfalls in the UK are often found in mountainous areas where the gradient of the land is steeper and rainfall higher. This means they are prevalent in the Scottish Highlands, Snowdonia and the English Lake District,” he explains. “In some places, such as the Brecon Beacons, Yorkshire Dales or Antrim Glens, the underlying geology means you find a lot of notable waterfalls in a relatively small area, whereas there are very few waterfalls in South East England because the rock there consists mostly of porous chalk and other unsuitable types.” Although a vast sheet of water pouring down a tall rock face forms many people’s idea of a waterfall, this need not be the case. “There is no official, hard and fast rule on what constitutes a waterfall,” explains Edward. “Any distinct step in a river or stream can count, even if it’s only one or two metres high. Neither is there a minimum amount of water involved.”
Step formation
As rivers wind their way from high ground to low, they follow the path of least resistance, trying to smooth their way to the sea and even out the gradient along which they flow. However, along their course, they may encounter sharp drops in height, known as knickpoints, which is where waterfalls tend to form.
One of the most common causes of knickpoints is the river itself, and the erosive force of the water as it travels, breaking up the rock beneath and churning up sediment. If the bed over which a river flows is made up of more than one type of rock, the softer one will erode more easily and quickly than the harder one. This creates a difference in height along the path of the river: a knickpoint. Water drops over the lip of the knickpoint, and a waterfall is formed. “A good example of this process taking place is Hardraw Force in the Yorkshire Dales, formed by harder limestone bands alternating with layers of softer rock,” says Edward. “Thornton Force, also in the Dales, has a layer of limestone over much older, softer rocks. You can scramble up to the place where they meet and span at least 170 million years of geological time between your thumb and fingers.” As water falls down the knickpoint, it creates a plunge pool below, which gets bigger as more of the softer rock is worn away. Undercut by its own pool, the overhanging lip of rock will eventually collapse. The waterfall disappears, only for the process to start again at the new knickpoint where harder and softer rock meet. In this way, waterfalls move upstream, cutting a narrow gorge in their wake.
The gorge left by High Force is just under half a mile (700m) long, and the waterfall is retreating by 5-6mm every year. It is formed where a band of hard dolerite, which is a type of igneous rock formed from solidifying lava, meets softer sandstone, shale and limestone, clearly seen as layers in the surrounding cliffs.
Forces of nature
Erosion is not the only way in which knickpoints can develop. Fault lines, caused by the movement of tectonic plates, can also create a gap in the river’s course. “The Highland Boundary Fault in Scotland gives rise to waterfalls such as the Falls of Bracklinn and Leny, both near Callander,” says Edward. “A small but notable example of a fault-formed waterfall is the wonderfully named Folly Dolly Falls, near Huddersfield.” Glaciation has also played a big part in forming waterfalls. Large glaciers erode rock more deeply than smaller tributary glaciers, so when the ice melts, it leaves a steep drop in the course of the tributary valleys, where hanging valley waterfalls form. Examples include the three streams known as Sour Milk Gill in the Lake District and Britain’s highest waterfall, Eas a’ Chual Aluinn, meaning The Splendid Waterfall of Coul. This descends some 656ft (200m) down a remote mountainside in Assynt, in the north-west of Scotland, and can be visited via a difficult walk or by taking a much less strenuous boat trip up the nearby Loch Glencoul. “There are also coastal waterfalls, where streams reach sea cliffs created by changes in sea level, and fall over the edge,” says Edward. “The Hebrides are rich in these types of waterfall, such as Eas Fors on Mull or the Mealt Falls near Kilt Rock on Skye, where the wind can be so strong it blows the water back up the cliff face. There’s also a good example at Speke’s Mill Mouth in North Devon.”
Sight and sound
The lie of the ground and the shape of the surrounding rock affect the way in which water moves down a fall, with height, any jutting ledges or outcrops of stone and the quantity of water all playing a part.
“…the purl of a little valley fall About three spans wide and two spans tall Over a table of solid rock, And into a scoop of the self-same block; The purl of a runlet that never ceases In stir of kingdoms, in wars, in peaces” Thomas Hardy, ‘Under the Waterfall’
“The volume of water flowing will depend on the size of the catchment area and the rain within that area,” he says. “Waterfalls are often impressive, especially after heavy rain, but they have a different appeal in low water when the gentler side of their character can be appreciated. “The surroundings also affect the noise. Some waterfalls can be heard from a long way away, whereas others, for example, if hidden in a deep gorge, seem much quieter until they are approached closely, and the noise booms imposingly. Sometimes waterfalls create their own natural amphitheatre, with excellent acoustics, where sound echoes around the surrounding cliffs. At Hardraw Force, there is a bandstand where a brass band contest and other musical events have been held since the 1880s.” Some British waterfalls are plunge falls, where water plummets down a cliff face without making any contact with the rocks behind it. A walkway can form behind the curtain of water, as at Sgwd yr Eira, the Falls of Snow, in the Brecon Beacons, once used to move sheep and cattle across the river
and still used as a footpath today. Horsetail waterfalls, such as Aber Falls in North Wales, are also steep, but the water keeps some contact with the rock as it falls, often spreading out in the shape of a horse’s tail. Other waterfalls have two or more distinct drops close together, such as Pistyll Rhaeadr in the Berwyn Mountains of Wales. Streams can also tumble into a valley in an almost continuous series of cascades. Cautley Spout, near Sedbergh in Cumbria, descends in this way for some 656ft (200m) in the wilds of the Howgill Fells, a track leading to its base from the ruins of a nearby iron age settlement. Other waterfalls travel more sedately down a series of broader steps, often in the flatter part of a river’s course. Aysgarth Falls in Wensleydale are a group of three such falls along a mile-long stretch of the River Ure.
Changing faces
Waterfalls are shifting, ever-changing entities, which are by their nature, impossible to pin down. “Waterfalls’ shapes are often complex and difficult to categorise,” says Edward. “Countless waterfalls change shape if the volume of water increases, when individual steps can merge into a more continuous fall. Others may change from a single flow to become ‘segmented’ in high water, with a second stream of water appearing. At High Force, another, smaller fall occasionally appears on the north side of the central rock after prolonged rainfall.”