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A Question of Limestone

1 What is this feature and how was it formed? How does it become this?

2 Why is there no sign of the river which eroded this valley?

3 What map evidence suggests the area is underlain by limestone? Image produced from the OS Get-a-Map service. Image reproduced with kind permission of the OS and OS of Northern Ireland

4 This is the largest known example in the UK. What is it and how was it formed?

5 This is the top of Britain’s highest waterfall. Why is it seen by very few people?

6 What features of the structure of carboniferous limestone are visible in this photo?

7 Why is the only limestone visible in this photograph underneath the big boulder?

8 What is happening here? Check out photo 5.

9 What are these shallow circular depressions?

This is limestone pavement. After glaciers had scoured the rock, it was exposed to chemical weathering by mildly acidic rainwater. This enlarged the joints in the rock to form grykes. The blocks of rock between the grykes are called clints. Where rainwater has lain in pools on the clints, there are solution hollows and where rainwater has flowed off the clints, there are channels called runnels. After prolonged weathering, the pavement will erode until it resembles the second photo on Slide 1. clint gryke runnel Solution hollows

This is a dry valley. Although it has a ‘V’ shaped cross section and was clearly eroded by water, there is no river flowing in it at present. At the end of the Ice Age when ice blocked all the underground passages in the rock, the limestone became temporarily impermeable. Meltwater had to flow over the surface and carved valleys such as Watlowes in the photo. After the ground thawed, water was able to make its way underground again and the valley was left dry.

limestone pavement intermittent drainage gorge swallow hole

This is Britain’s largest stalactite : the so- called Sword of Damocles in Ingleborough show cave. Stalactites form in underground caves where water rich in dissolved calcium carbonate drips from joints intersecting the cavern roof. Evaporation of the water results in the deposition of a tiny amount of calcite. This process, repeated over a very long time, produces a stalactite.

When Fell Beck disappears underground at this swallow hole which is known as Gaping Gill, it plunges for over 100 metres into an underground cavern. Fell Beck rises on the impermeable Yoredale rocks of Ingleborough Hill. It flows downhill until it meets Carboniferous Limestone. At this point it disappears underground via the swallow hole of Gaping Gill. Once underground, it makes its way through the permeable limestone until it reappears beside Ingleborough cave on the impermeable rocks of the valley floor.

joint bedding plane Limestone has cracks which run horizontally (bedding planes) and cracks which run vertically (joints). The bedding planes represent periods of interruption in the deposition of the rock. The joints formed when crustal movements placed the rock under stress.

The large boulder is an erratic which was deposited on top of the limestone pavement by a glacier during the Ice Age. It has protected the limestone underneath and prevented it from weathering. All around, where the limestone has not been protected, chemical weathering has lowered the surface of the pavement. The height of the plinth of limestone beneath the erratic represents the amount of weathering which has occurred since the last ice Age.

On the May Bank Holiday, the Bradford Pothole Club sets up a winch at the top of Gaping Gill. Members of the public can be lowered free of charge deep underground into a huge chamber the size of York Minster. It is an awesome experience. P.S. It costs £8 to be winched back to the surface!

These little ‘craters’ are shake holes and they are commonly found in limestone country. They form when percolating rainwater enlarges joints beneath the surface. Surface deposits (often boulder clay) fall into the enlarged joints creating depressions which may range in size from 2m to 15 m across.