Foolow: Sinkhole or Subsidence? EarthCache

In December 2013, a hole  49m wide and 40m deep opened up in a field adjacent to where you are now standing. It was reported to be the largest sinkhole ever to appear in the Peak District and featured heavily in the national press. This EarthCache brings you close to the site of the hole and examines if this really is a sinkhole.

Please note, as of 2020, the road along Bretton Edge has been temporarily closed and the bridleway and footpath that crossed this area are also temporarily closed. This is due to further instabilities in the ground and the danger that further large sinkholes could occur and swallow the footpath or road. Therefore please complete this EarthCache from the waypoint given, not from the listed coordinates unless you are lucky enough to arrive and find the footpath has reopened. At the moment it is closed until April 2025 but this could be prolonged. Take some binoculars and look for holes and work, it is all part of the topic of the EarthCache and quite interesting as the scale of what is happening here is a lot more than imagined when this was published.

In recent years, the word ‘sinkhole’ has been reported frequently in the media. In many cases however, these were not natural geological phenomenon at all, but related to the collapse of artificial cavities or to the failure of materials capping mine shafts or wells. These should be termed ‘collapse subsidence’, ‘crown-holes’ or ‘mine entrance collapse’. Others have been related to burst water mains and pressurised sewers, or simply compaction of loose fill. But for public use, ‘sinkhole’ now seems to mean all these. The media reported the hole here as a sinkhole, but is that really the case at Foolow? This EarthCache will help you to try and work out the truth.

The term ‘sinkhole’ has come to encompass a whole range of unrelated processes that result in the collapse of the ground. In a strict geological sense, it refers to a natural surface depression caused by dissolution of soluble rocks at depth. Other more local names are often used; for example, ‘shakehole’ or ‘swallow hole’ is typical in northern England while ‘swallet’ is common in parts of the southwest. More academic literature, especially in Europe, tends to use the term ‘doline’, which is derived from the Slovene ‘dolina’ meaning valley or plain.

In the UK, natural sinkholes occur more frequently in northern England. The most vulnerable area is around Ripon, North Yorkshire, where the underlying rock is gypsum calcium sulphate. Gypsum dissolves faster than the more common limestone (calcium carbonate) as water runs through it, leaving cavities whose roofs eventually fall in. At least 30 major collapses have occurred in the last 150 years around Ripon some of which have affected property and infrastructure.

Natural sinkholes are caused by the dissolution of soluble rocks at depth. These include limestone, dolomite, chalk, gypsum and halite. Evaporites, predominantly halite and gypsum are by far the most prone to dissolution. Carbonate rocks are less soluble, although this largely depends on rock purity, while the solubility of dolomite is typically lower than that of pure limestones. Sinkholes occur in all these lithologies. How and where they form depends on bedrock geology (lithology, bedding and structure), the presence, type and thickness of superficial cover, and local factors such as drainage and topography. In this area the underlying rock is limestone. Natural sinkholes in ordinary limestone caves are relatively rare, as most limestone caves are actually pretty stable over 100,000-year timescales. Indeed, some caves in the UK are over 700,000 years old. So whilst collapses will occur, you can image why they won't be that regular.

Another view of the hole that opened up near to Foolow.

Suffusion and dropout sinkholes are more common and form in superficial deposits overlying the soluble rock, typically limestone. Suffusion sinkholes occur where sediment is gradually washed down into solutionally enlarged fissures in the limestone below, causing gradual subsidence. Dropout failures form similarly, but in more cohesive soils able to bridge voids until they become unsupportable, resulting in sudden failure and collapse. It is these unexpected events which often make headlines. Thousands of suffusion sinkholes dot the limestone outcrops of the Yorkshire Dales where Devensian glacial till has been washed into bedrock fissures.

Geological conditions cause more spectacular and dangerous sinkholes to appear in overseas locations than in the UK. Florida is particularly vulnerable. Some central and western parts of the state have a limestone “karst” landscape riddled with underground caverns capped with thin roofs liable to collapse; extensive urban and suburban development has exacerbated the risk by changing the natural pattern of subterranean water flows. People have died simply because sinkholes have opened up and swallowed entire houses. The deepest urban sinkhole in recent years opened in Guatemala City in May 2010 following a tropical storm. This created a cylindrical hole 20m across and a huge 100m deep. There the cause was water flowing underground through soft and unconsolidated volcanic deposits. However, nowhere in Britain has loose volcanic strata like those in Guatemala. However, smaller examples do occur in the UK on thick soils and in upland peat bogs.

The Guatemala City sinkhole opened without warning and was 100 metres deep.

 In England, the majority of the holes reported in the media are actually due to the collapse of old shafts or mines. These should really be referred to as subsidence, not sinkholes. The UK is peppered with man-made underground cavities including mines, wells, tunnels, cellars and store-rooms, culverts, and drains. When these collapse they can cause subsidence. Ancient chalk workings often cause trouble. From medieval times until the 19th century, chalk was dug intensively in parts of southeast England – to lime soil, to obtain flint and as an ingredient in brick making. Nowadays we would probably transport the chalk from surface outcrops, which might be a long way away, but back then it was often easier to sink a shaft through the sandy or clay soil covering the chalk and haul it up. The resulting cavity, known as a denehole, typically had several excavated chambers that were reached down an excavated shaft 20m deep. Later the top of the shaft was capped or blocked and forgotten. A sinkhole that appeared last winter in the grounds of Rainham Mark Grammar School in Gillingham, Kent, had ancient bricks at the bottom – suggesting that it was a chalk denehole topped with a brick arch that had collapsed.

According to the press, England’s record rainfall over the winter of 2013/14 triggered the appearance of ‘sinkholes’ (or perhaps subsidence) in unprecedented numbers. “Terrifying holes … are opening up all over Britain,” reported the Daily Mail. Or “Brits suffer sinkhole hell”, to quote The Sun. A few weeks after this hole opened up here at Foolow, sudden 'subsidence' caused more than 25 large holes to open up across southern Britain. The British Geological Survey was inundated by enquiries, an unprecedented situation when they usually hear about no more than half a dozen sinkholes during a year. 

To look at the problem from a very basic level, sinkholes and subsidence can be triggered in four different ways -

Method 1 - Heavy rainfall or surface flooding - the additional weight of saturated ground may increase the loading on the surface material capping a cavity, which then collapses; and more water flowing through a cavity can flush out the looser sediments filling it, creating a void into which the cap collapses.

Method 2 - Natural deterioration - Cavities may collapse due to the deterioration of the structure holding them up. In an old shafts or mines, this could be due to the use of timber supports that have rotted or other weakening of the roof supports they were used long ago. It could be a passageway was blocked or dammed long ago, and it bursts. Such an occurrence happened nearby in 2007 when Stoney Middleton suffered a major flood after a dammed tunnel in Glebe Mine burst. It could also be the natural deterioration of limestone roofing in a cave being corroded by acidic water.

Method 3 - Loss of underground water - In some parts of the world, drought or groundwater abstraction can cause sinkholes by changing the level of the water-table. This removes the buoyant support water provides to a cavity, and so lead to collapse.

Method 4 - Construction and development - Construction and development are also potential triggers, either by modifying surface drainage or altering loads imposed on ground without adequate support.

A scene in Stoney Middleton following flooding caused by a dam failing in a Glebe Mine tunnel

To establish the cause of this sinkhole, we must look at the local geology, and one thing to consider is the local history of swallow holes. The nearby Stoney Middleton Dale is known for limestone caves, and the underlying geology of this area is limestone. So how does the water get into the caves in the first place? Perhaps it enters at a higher level. Swallow holes are often the source of where all the water that flows through the caves goes underground. There are several obvious swallow holes, but others keep appearing. One was found recently in the garden of a house in Eyam, and is currently being dug by cavers, one of whom is the householder!

The most prominent local swallow hole is Waterfall Swallet, to the north of the road from Eyam to Foolow, by the sharp bend in the bottom of the dip. A huge volume of water can disappear underground here, and it is very spectacular in flood conditions. A low crawl entrance on the right of the path down into the shakehole leads into a cave which can be followed to a depth of 140ft below the entrance. The stream disappears into boulders and mud, and there is no room to dig. It reappears 200ft lower at the Boil-up in Glebe Mine, from where it flows out to daylight via Moorwood Sough, the mine drainage level which emerges to daylight by the church at Stoney Middleton.

Swallet Falls

Little Waterfall Swallet lies immediately east the main swallow hole. It can also engulf a large stream, but efforts to follow it have so far proved inconclusive. Like the main stream, it reappears at the Boil-up in Glebe Mine. Other swallow holes have almost certainly been buried over the years. Crosslow Cavern was a natural cavern reached via a 60ft mineshaft, but this is now blocked. The next significant swallow hole is Hungerhill Swallet, a large shakehole nearer to Eyam on the north side of the road. Excavated in the 1980s, a spectacular cave 250ft deep was found. It includes a large pitch of 130ft, big chambers with impressive calcite formations, but also some very tight squeezes and loose boulder chokes. The stream is almost certain to emerge in Glebe Mine, though probably not from the Boil-up.

In 1846 local historian William Wood described the Pippin Swallow, but it was lost. When Glebe Park was built on the site of Glebe Mine, it was found that a shallow valley had been unfilled with what was, in the mid-1800s, mining waste. The stream that ran down the valley had been culverted, into a swallow hole. The enclosure was known as Pippin Close. Excavation was attempted, but nearby houses were threatened if the hole was deepened any further and it was abandoned. The swallow was filled in with concrete, but it solved a long-standing mystery.

 The village of Foolow was originally a Lead Mining village. South of the village, the Watergrove mine was active from the 18th century until 1853. Water was a problem over this period; both soughs and pumping engines were used. In 1837 a Fairbrother beam engine was installed. Its 80 ft (24 m) chimney stood until 1960. There are hillocks in the north of the parish that mark the sites of other mines. There are also sinkholes present in this vacinity - including the one you are visiting for this earthcache. It is probable that mining, whether lead or other mineral, has been an occupation for the people of Foolow since at least the 15th century. Robert Roworth, of Folowe, a miner, appears as owing £4 to Thomas Calton of Chesterfield, in a legal record of 1470. Looking even further back, Roman coins were found in a waste hillock in the New Close in Eyam when the council estate was built in the late 1940s.

Interesting to note, is that Old Mill Dam Mine at Great Hucklow was originally worked for lead until 1885. At that time miners discarded other minerals they found as waste, but it is now these spoils and discarded veins that are rich pickings, financially viable in the 21st century mineral market. A hundred years after its closure the new Milldam Mine opened in 1985 to extract fluorspar and barytes with lead ore as a by-product and this has now become Glebe Mine, and is home to the UK's only remaining indigenous fluorspar mining company. It was in danger of closure but was recently purchased by INEOS Fluor, a market-leading specialties chemicals manufacturer. Glebe Mines had been supplying INEOS Fluor with fluorspar, the raw material vital to the manufacturer’s fluorochemical production at its Runcorn site, for over 50 years. It employs 60 people at its site in Stoney Middleton in the Peak District, and operates as British Fluorospar. They still work the Great Hucklow vein, with a network of underground tunnels linking the veins offering the valuable mineral.

A view inside the modern mines operating in the area.

Following the vein can be fortuitous to the landowners above, as no matter how deep the miners dig as royalties have to be paid. Therefore it is not surprising that where economically viable, the active mine may buy other old mine land in the area. The land where the hole occurred has been refered to as Nether Slates Mine, and British Fluorospar have taken responsibility for filling in the the hole. They have done this gradually using material mined out locally during current mining operations in the area. However, it is not considered that the sinkhole is related to any recent fluorspar mining as modern health and safety practices ensure it should remain safe both below and above ground. The open access area the earthcache co-ordinates bring you to is known as the 'Silence Mine Heritage Site'. The land is managed by the local parish council and excavations took place between 2008 to 2010 to uncover a few interesting relics of the past. Any spoil heaps in the area will have been re-worked in the 1960s to remove the Fluorospar previous disregarded by the miners in years gone by.

Geology of Eyam & Foolow Edge

The above diagram should answer any questions you have about the local geology of the area. You are stood on Eyam Edge, which runs above Eyam, Foolow and across to Great Hucklow. You are standing directly on mudstone here, with Limestones below the mudstone, and sandstone if you climb higher up the edge. The millstone grit doesn't occur here, but does on other higher edges such as Froggatt Edge.

Please e-mail me the answers to the below questions via my profile. I do read all answers and try to reply to them all; I may not reply immediately so please do not wait for a reply before posting your find. Please send your answers at the time you submit your log, or within a few days of your visit. I do check answers have been sent for every log, if you do not send any answers your log may be deleted. If possible, please use the email service rather than the messenger as it is easier to manage due to the volume of messages I receive.

The background information you need is contained within the listing, you should read it before you visit, and then again during/after your visit. Your visual observations of the site and area in combination with the information on the listing will give you all the information you need to know to answer the questions. As of 2020 you can't get to the open access site which I appreciate will limit the detail for certain questions, however they have now been adapted. I still believe visiting here and completing the EarthCache provides a valuable Earth Science lesson as the instabilities here are ongoing making this an active site, not a historic one, and this is actually quite unusual in a remote area of the Peak District and raises some interesting questions. Remember, the farmer has to work this land for his livelihood!

Question (1)

Please take a walk around the open access site (2020, view it from the waypoint given - you need to look across the fields but you can still make out some features such as the shape of the land visually, but you can also Google 'Silence Mine, Foolow' and find photos taken on site when access was allowed). Examine the area around you. The actual sinkhole was in the adjacent field, lower down the slope - it was fenced off but you should still be able to see where it occurred. You can also cast your eyes out into the wider area around you. What evidence can you see to support suggestions that mining may have taken place here around here in the past? You are looking for anything that does not appear to be part of the natural landscape. This may include building foundations and tools, at the time of publication there were also some signs that might help on site. But I am more interested in the shape of the land, anything that might not be natural such as evidence of old mined material that looks unnatural (spoil heaps for example). Please tell me what, if anything you spot, feel free to include photos on your log.

Question (2)

We know the lead mining operated in the area long ago, this open access area is now called the 'Silence Mine Heritage Site' - back then, miners operated manually using simple tools and little mechanisation existed. In those days miners wouldn't have moved the spoil far from the mine so evidence would remain, spoil heaps of a volume that might fill the hole that opened up perhaps? However, just because mining occurred in the area, this doesn't mean this is automatically the cause. We also know the underlying land is limestone, and water can naturally dissolve limestone forming caverns underground, and we know swallow holes, tunnels and caves exist in this general area. From your examinations of the area in question 1, and the information throughout the cache page (which presents all the relevant evidence), explain to me which you believe was the most likely cause of the huge hole that occurred here - (natural) sinkhole or (man-made) subsidence? Please give me a percentage for each option (e.g. sinkhole 50% likelihood / subsidence 50% likelihood) and a brief few words to justify your decision. I'm not looking for 50/50, you need to make a considered decision one way or the other.

Question (3)

Further to your answer in question 2, there are four 'trigger' methods described in the listing, which do you believe was the most likely culprit here and why?

Question (4)

We know from the local geology diagram above, about the layering of the rock in this area. The limestone is deep beneath mudstone here and so the limestone is not visible. However, miners could dig through the mudstone from here to reach it. Please look for an area of exposed mudstone. Please describe the properties of the mudstone (hard/soft/crumbly/rough/smooth etc). Do you think the properties you have described make the ground vulnerable to sinkholes or subsidence? Please do not dig up the hillside. (During the site closure from 2020 you may need to look this up online)

Question (5) - optional

You may enjoy climbing further up the footpath towards the top of the edge, examining the exposed stone as you go. Can you find where the layer changes to sandstone? You could describe the properties of this sandstone compared to the mudstone lower down. Please take an elevation reading on your phone or GPSr so we can accurately note where the sandstone layer begins if you find it. 2020 alternative question Look across at the site in front of you where the icon is placed. Can you see any holes that have opened up? Another one could open up at any point so you might see something on your visit. Please note in your log if you see anything interesting and include a photo if you wish.

Photo Opportunity! (2020 - Now Mandatory)

Under revised guidelines a photo can now be a mandatory logging requirement. As you may need to do some research online to complete this EarthCache with the site now more distant, please include a photo of yourself at the waypoint with the edge and the listed coordinates in the background to prove you visited the site.

Please enjoy your visit to Foolow's Sinkhole or Subsidence earthcache.