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Geological Processes

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Breaking the Barrier: formation of Thornton Force

Thornton Force – the most famous of the Ingleton Waterfalls. Stephen Oldfield

As well as cutting channels and gorges, glacial meltwater carried and dumped large amounts of debris, effectively altering the shape of the landscape.  At the end of the Devensian glaciation, this material was built up in a massive terminal moraine barrier at the southern end of Kingsdale, just above the position of the present day Thornton Force.  Meltwater then ponded behind this barrier to form a lake with a flat sediment floor, and this lake can easily be imagined today by looking north along Kingsdale towards Braida Gath Farm.

Looking north-east along the former lake floor of Kingsdale, where water was held back by the terminal moraine barrier of Raven Ray. Stephen Oldfield

Eventually the lake over-topped the Raven Ray barrier, which had effectively blocked the former river’s course to the west.  A new route was therefore cut through the moraine and its underlying Great Scar Limestone, resulting in the formation of Ingleton’s most famous waterfall.

The beck above Thornton Force – showing the point at which the meltwaters broke through the Raven Ray barrier. Stephen Oldfield

The waterfall is not in its original position following the breach, but it is believed to have retreated into the hillside by about 60 metres (180 feet).  The large limestone boulder in the plunge pool may have once plugged the notch in the lip of limestone through which the 14 metre plunge now cascades.  In the distant future the force may run dry except in times of flood, as water is already sinking into the limestone fissures in the gorge above, some of which emerges as ‘Little Thornton Force’ to the west of the main fall.

Both the uplifting of basement slates by the North Craven Fault and the erosive power of meltwater have left one of the most famous geological localities in Britain.   Three layers of rock are classically displayed above the plunge pool.  Firstly the basement slates which have been folded into almost vertical alignment – then a thin layer of ‘conglomerate’ which is a mixture of ancient boulders and smaller fragments encased in a limestone matrix.  This was once a pebbly beach or ‘boulder bed’ on the slate before it sank beneath the waves, allowing limestone to be deposited on top over many millions of years. Finally, a thick bed of Great Scar Limestone was laid down on top, when sea levels were shallow but consistent during the Carboniferous.

Looking down Thornton Force from the top, where the beck cuts through the Great Scar Limestone. Stephen Oldfield
Above the plunge pool where the basement slates, conglomerate boulder bed and Great Scar Limestone form a classic three layered unconformity- with no Silurian beds in between. Stephen Oldfield

Intrepid explorers can, with great care, cross the bed of conglomerate and sit beneath the falling curtain of water without getting wet, but no doubt wondering at the sheer insignificance of their own measly lifespan in this place of wonder.  Touch the boulder bed with your thumb – stretch your fingers to touch the overlying Great Scar Limestone and you are covering a time gap of some 170 million years.  Multiply a lucky human lifespan of say 80 years by 2 million, 125 thousand and you are getting fairly close!  Thornton Force is one of those places where you are encouraged to question the meaning of it all.

Spanning the massive time gap between the conglomerate and the overlying Great Scar Limestone. Stephen Oldfield

References:

Crutchley, David (1981) Geology of the Three Peaks  (Dalesman)

Waltham, Tony (1987) Yorkshire Dales: Limestone Country (Constable)