Ingleton: Thornton Force

© Craven & Pendle Geological Society

Thornton Force and Unconformity
Thornton Force

Thornton Force

This one of the most important waterfalls of the Yorkshire Dales National Park geologically speaking of course. Located on the River Twiss, the waterfall plunges off Carboniferous Great Scar Limestone that was laid down in a clear sub-tropical sea 330 million years ago. The water bounces onto rocks belonging to the Lower Ordovician (Arenig) Ingleton Group laid down some 500 million years ago. The time gap between the horizontally bedded limestone and the steeply dipping turbidite sandstones is about 170 million years.

Mountains eroded away

Submergence of land

Uplift of limestone

End Moraine blocks Kingsdale

Recently more research on the age of the Ingletonians suggest that they may be Precambrian after all! There are dark greenish-grey cobbles lying on the underside of the limestone outcrop. This is the classic Carboniferous basal conglomerate that marks an ancient land surface; probably representative of an early Carboniferous beach. The significance of this locality was first spotted by John Playfair at the end of the 18th century during his preparation of his book Illustrations of the Huttonian Theory of the Earth (1802). He described the section in terms of evidence to prove Hutton's theory that geological processes of the past were the same as those of today continued over immense periods of time.

Formation of Thornton Force

The diagrams show how the geological feature of Thornton Force was formed.

The top diagram represents a time when Ordovician (Precambrian?) mud and sand was deposited into a deep ocean. These sediments, often of a turbiditic nature, were uplifted and folded into mountains which were subsequently eroded down over millions of years.

The next diagram below shows how the land was submerged beneath a warm shallow Carboniferous sea.

In the following diagram there has been some uplift of the Limestone and ice flowing down Kingsdale has further eroded Kingsdale valley.

The final diagram shows the end moraine at the head of Kingsdale - a major obstruction to the natural drainage. The River Twiss had to cut a new course in order to bypass the moraine. The right-hand edge of the moraine can be seen in the photograph at the top.