Thornton Force – 170 million years in 14 metres

Thornton force is a 14m waterfall on the River Twiss; located in the Ingleton Waterfalls group in Yorkshire, UK. The waterfall drops from horizontal limestone (deposited 330 million years ago) onto dipping sandstone deposited 500 million years ago. This means while the water falls 14m over the fall, it passes an age gap of 170 million years. This type of contact between rocks is known as an angular unconformity.

An unconformity represents a contact between two rock units where there is a substantial age difference between the rocks (usually caused by a break in the geological record). An angular unconformity is a type of unconformity seen when horizontal strata are deposited on older tilted (also known as dipping) layers.

Originally, Ordovician (485 – 444 million years ago) mud and sand would have been deposited in deep water. The origin of this sediment was likely to be in turbidites (a deposit formed by a turbidity current, which is rapidly moving the water that moves down a slope under the sea). This mud and sand would have been uplifted before being folded into mountains and then eroded to form a flat surface. The onset of marine transgression lead to the flooding of the land by a subtropical sea leading to deposition of limestone horizontally 330 million years ago. When the sea regressed, further uplift and erosion of the valley by glaciers occurred. Source: Thornton Force Thornton force is a 14m waterfall

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