The Old Ruminator
Well-known member
A useful PDF a little out of date now but understandable fot the layman.
A Distinction Between Epigenic and Hypogenic Caves
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The Old Ruminator
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Useful for us as we are digging in a hypogenic cave.
The Old Ruminator
Well-known member
Our old dig at Vurley. Epigenic but not following the classic Mendip model.
Vurley.
Here we have a closed karst basin ( one of several ) aligned along The South West Overthrust with associated faults. The Vurley basin is the closest to Cheddar Gorge and has a low col leading into a valley in the escarpment. At one time the basin was much deeper and was probably formed by solutional activity. During periglacial conditions it was sealed by permafrost allowing a glacial lake to form. The cold dry weather during the ice age allowed easterly winds to bring fine wind-blown soil to settle in the depression. This is known as loess and at Vurley lies seven metres deep overlying the Oxwich Head Limestone. Collapse over solutional features have formed dolines along the line of basins. At Vurley aggressive water seeping through the loess has formed multiple fissures and collapse in the limestone down to a depth of sixty metres. At this point one of the major faults has been encountered allowing further epigenic speleogeneses to take place. Various mis fit vadose trenches are still forming in the solid limestone and many of the rocks in the upper cave exhibit sharply eroded disolved features . The main fault rift in Vurley occurs at around sixty six metres depth with heavily eroded areas. A linear minor fault forms the chamber below the first pitch giving way to a passage running down dip with vadose trenches in the floor. A major fault is then met almost at right angles to the previous one. This has primarily been formed by drip and the tiny mis fit stream. Unlike many Mendip Caves this is not the result of vadose stream erosion . In fact the vertical section of Vurley Swallet ( not a swallet in the true sense ) is almost self contained with just the small vadose passage that has been blasted out to reach it. Drip below here in Winter is quite heavy and constant though the hydrological componant has been altered by the addition of a pipe through the loess cover. A fairly high wet weather stream can now enter the cave but again this is modern and mis fit. The cave continues down through Clifton Down Limestone passing though a well eroded chert bed somewhere near the one hundred metre depth. In all probability water in the cave drains to Cheddar Risings but unlike the typical Mendip swallet this will be along strike rather than down dip. Therefore the deep phreatic switchbacks we see will be replaced by boulder and mud filled rifts.
Its possible that Vurley can be aligned with sequential horizons in Reservoir Hole and at Cheddar Risings. A deeper phreatic/vadose conduit could also be encountered draining other karst basins back along the overthust. Nevertheless Vurley is unique on Mendip. Epigenic in origin but formed by aggressive groundwater draining through the loess in a large karst basin. Unfortunately these conditions mean that the cave suffers from higher than usual concentrations of Co2 much to the detriment of future diggers.
Vurley.
Here we have a closed karst basin ( one of several ) aligned along The South West Overthrust with associated faults. The Vurley basin is the closest to Cheddar Gorge and has a low col leading into a valley in the escarpment. At one time the basin was much deeper and was probably formed by solutional activity. During periglacial conditions it was sealed by permafrost allowing a glacial lake to form. The cold dry weather during the ice age allowed easterly winds to bring fine wind-blown soil to settle in the depression. This is known as loess and at Vurley lies seven metres deep overlying the Oxwich Head Limestone. Collapse over solutional features have formed dolines along the line of basins. At Vurley aggressive water seeping through the loess has formed multiple fissures and collapse in the limestone down to a depth of sixty metres. At this point one of the major faults has been encountered allowing further epigenic speleogeneses to take place. Various mis fit vadose trenches are still forming in the solid limestone and many of the rocks in the upper cave exhibit sharply eroded disolved features . The main fault rift in Vurley occurs at around sixty six metres depth with heavily eroded areas. A linear minor fault forms the chamber below the first pitch giving way to a passage running down dip with vadose trenches in the floor. A major fault is then met almost at right angles to the previous one. This has primarily been formed by drip and the tiny mis fit stream. Unlike many Mendip Caves this is not the result of vadose stream erosion . In fact the vertical section of Vurley Swallet ( not a swallet in the true sense ) is almost self contained with just the small vadose passage that has been blasted out to reach it. Drip below here in Winter is quite heavy and constant though the hydrological componant has been altered by the addition of a pipe through the loess cover. A fairly high wet weather stream can now enter the cave but again this is modern and mis fit. The cave continues down through Clifton Down Limestone passing though a well eroded chert bed somewhere near the one hundred metre depth. In all probability water in the cave drains to Cheddar Risings but unlike the typical Mendip swallet this will be along strike rather than down dip. Therefore the deep phreatic switchbacks we see will be replaced by boulder and mud filled rifts.
Its possible that Vurley can be aligned with sequential horizons in Reservoir Hole and at Cheddar Risings. A deeper phreatic/vadose conduit could also be encountered draining other karst basins back along the overthust. Nevertheless Vurley is unique on Mendip. Epigenic in origin but formed by aggressive groundwater draining through the loess in a large karst basin. Unfortunately these conditions mean that the cave suffers from higher than usual concentrations of Co2 much to the detriment of future diggers.
The Old Ruminator
Well-known member
Part of a summary of Quantock Caves in Somerset. ( still in preperation )
The Quantocks.
We have come to realise that many very large cave systems in the world have a differant origin. This we call Hypogenic being almost the antithisis of the standard form of speleogenesis we call epigenic. The latter has surface derived water often contained as a phreatic conduit under hydrostatic pressure but then becoming vadose as water tables drop in the local catchment. Both Cheddar and The St Dunstans Well catchment exhibit this classic phreatic - vadose transition. The St Dunstans origins being probably in the Hoxnian interglacial period 400,000 years ago. Both catchments have caves with later periglacial cryogenic speleothems.
The Quantocks have no known open cave systems. All have been found by quarrying or mining. Holwell Cave is a classic maze structure formed by hypogenic solution. The rising thermal waters opening tubes and chambers along the faults and bedding planes at the juncture of the Devonian limestone and slate. We see similiar at Cothelstone.
The heavy spar lode has offered free convection and recharge from depth. At the water table cave formation has taken place because as the warmer water cools it becomes more aggressive. Also as air spaces are formed more speleogenises takes place driven by condensate from the thermal water. With the containing ceiling of Triassic rock there is no recharge from the surface and the water disperses in an upper aquifer. The condensate can go further creating open voids like dome pits, cupolas and dendritic passages. Essentially caves here have developed between a lower aquifer and upper aquifer ( high permeability zones ) which equilizes the flow pattern allowing recharge from many different points and so aiding cavern development. Residue from speleogenises is not removed in most hypogenic structures so settles within the confines of the dissolved portions of limestone. We see further evidence of dissolution on the boulders within the fill.
The Quantocks.
We have come to realise that many very large cave systems in the world have a differant origin. This we call Hypogenic being almost the antithisis of the standard form of speleogenesis we call epigenic. The latter has surface derived water often contained as a phreatic conduit under hydrostatic pressure but then becoming vadose as water tables drop in the local catchment. Both Cheddar and The St Dunstans Well catchment exhibit this classic phreatic - vadose transition. The St Dunstans origins being probably in the Hoxnian interglacial period 400,000 years ago. Both catchments have caves with later periglacial cryogenic speleothems.
The Quantocks have no known open cave systems. All have been found by quarrying or mining. Holwell Cave is a classic maze structure formed by hypogenic solution. The rising thermal waters opening tubes and chambers along the faults and bedding planes at the juncture of the Devonian limestone and slate. We see similiar at Cothelstone.
The heavy spar lode has offered free convection and recharge from depth. At the water table cave formation has taken place because as the warmer water cools it becomes more aggressive. Also as air spaces are formed more speleogenises takes place driven by condensate from the thermal water. With the containing ceiling of Triassic rock there is no recharge from the surface and the water disperses in an upper aquifer. The condensate can go further creating open voids like dome pits, cupolas and dendritic passages. Essentially caves here have developed between a lower aquifer and upper aquifer ( high permeability zones ) which equilizes the flow pattern allowing recharge from many different points and so aiding cavern development. Residue from speleogenises is not removed in most hypogenic structures so settles within the confines of the dissolved portions of limestone. We see further evidence of dissolution on the boulders within the fill.
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The Old Ruminator
Well-known member
Porifera sponge fossils in Devonian Limestone exposed by hrpogenic solution. ( Quantocks )
The Old Ruminator
Well-known member
Void and debris associated with baryite at a preferential horizon in Devonian Limestone. Quantocks. Since excavated.
The Old Ruminator
Well-known member
Rising hypogenic feeder. Quantocks.
cavingbiker
Member
What is the scale?Porifera sponge fossils in Devonian Limestone exposed by hrpogenic solution. ( Quantocks )
The Old Ruminator
Well-known member
Yes most incorrect for a dissertion. Each fossil is about 2cm long. Done in a rush prior to todays digging. Got part buried with a half ton boulder on my back. Recovering at home. Some sort of video may emerge.
The Old Ruminator
Well-known member
Hypogenic cave fill. Everything here has been dug out.
The Old Ruminator
Well-known member
Preferential horizon. Rock above mud below. A metre wide section of a much wider area.
The Old Ruminator
Well-known member
A small vugh about 200 mm across associated with baryite.
The Old Ruminator
Well-known member
Hauling dug fill. Everything here has been excavated as it was full to the " roof " .
The Old Ruminator
Well-known member
Hypogenic maze cave at Holwell.
The Old Ruminator
Well-known member
Including Pen Park Hole at Bristol. Perhaps even Lamb Leer.
