Hallowe'en Rift, Mendip Hills

Excursions [and other notes] involved in the exploration of Hallowe'en Rift; a cave, so far, formed within Triassic Dolomitic Conglomerate.

The exploration of Hallowe'en Rift was started in 1982 by Trevor Hughes with other members of the Bristol Exploration Club, then during the early 1990's Vince Simmonds and other, mostly, local diggers were active at a number of locations within the cave, including the start of the present dig with Graham Johnson in December 1991. The current phase of exploration was commenced in 2009, with the majority of the early work being carried out by Vince Simmonds and Alex Gee, now the regular team includes Rob 'Tav' Taviner, Graham 'Jake' Johnson, Nick Hawkes, Matt Tuck, Jonathon Riley, Paul 'Brockers' Brock, Roz Simmonds, Duncan Price and Mike Moxon. There has been occasional help from others including Mike Willett, John 'Tangent' Williams, Pete Bolt, Bob Smith, Callum and Hazel Simmonds, with regular guest appearances by that well-known antipodean, Ray Deasy.

21st August 2018

digging 2018Posted by Vince Simmonds Thu, August 23, 2018 07:47:09


A trip to take more photographs and have time to get a good look around and make some observations of my own. It was a chilled and peaceful time spent in the cave.

There is a change in the sediments that partially filled the low bedding sections on the approach to the breakthrough into An Unexpected Development. Initially, the sediments are mostly sandy silt with occasional cobble and boulder-size fragments of fractured calcite flowstone. The fractured calcite flowstone becomes more frequent and after the drop down through the draughting rift becomes the dominant component of the sediment fill. It was suggested that the damage was caused by earth movements but, after close examination of the sediments and some documentary research, this is unlikely, and the cause of the fracturing is through frost and/or ice.

Most of the passage in Hallowe’en Rift is shallow below the surface and root growth has been noted in several areas, there are snail shells in the extension to An Unexpected Development and some rare bat droppings.

During the Pleistocene, interglacial and warmer interstadial periods produced calcite flowstone deposition in the cave. Glacial or stadial periods caused periglacial activity in the cave, during which the calcite layers were fractured by frost heave and some redistribution by solifluction occurred.

Hallowe’en Rift was shallow enough for ice to form in the cave during glacial periods. During the build-up of ice and it’s subsequent thawing, ice can flow and slide, thereby stalactites and curtains can be sheared off the roof and stalagmites can be tipped over or sheared off their bases and displaced. Lumps of calcite enclosed in ice can be deposited on inclined surfaces or be left in precarious positions, i.e. at positions which would not be stable if deposited by falling.

Ice related damage covers a wide range of phenomena:

· Missing ceiling formations of older generations;

· Sheared-off stalactites and curtains, deposited on top of floor speleothems;

· Broken and deposited stalagmites;

· Sheared-off stalagmites which have shifted from their base but still stand upright;

· Cracked conical stalagmites;

· Tilted and leaning stalagmites;

· Moraine-like piles of floor flowstone;

· Precariously placed ceiling deposits.

In addition to speleothem damage, freezing and cave ice can leave other traces:

· Cryoturbation in cave sediments;

· Solifluction deposits;

· Transport of gravel without evidence of flowing water;

· High collagen content of fossil bones’

· Loss of uranium due to ‘leaching’;

· Scratch marks on cave walls.

Further observations and comments

“The polished nature of the dolomitic conglomerates was noted throughout most of the cave with hard limestone/dolomitic pebbles and crystalline red marl matrix having been eroded equally. This erosion pattern is in marked contrast to the dolomitic conglomerates in Home Close where the softer matrix is eroded preferentially compared to the limestone pebbles that stick out as knobbly lumps. The polished erosion pattern is consistent with a base of a streamway or a passage full of water as opposed to slow dripping of water. As similar polished conglomerates are clearly seen down the new pitch, as well as in the roofs of the horizontal passages which are phreatic in shape and have well developed scalloping, the logical conclusion is that water that initially formed the pitch was upward flowing. Undoubtedly there has been a limited amount of inflow from above later in the history of this cave’s development but it is relatively insignificant in terms of passage dimensions although highly significant for the development of the formations.

Scalloping in the roof, An Unexpected Development. Direction of flow is left to right.

The only other passage development of notable magnitude has been by a group of nutters using explosives.” (Hawkes, 2018)

“From a speleogenesis point of view, possibly excluding the aven below the Tuck Shop and a few minor modern runnels, the cave is phreatic in origin. The few scallops that could be found all pointed outward, and this, coupled with the lack of any inflow passages into the pitch strongly suggested that the cave had been formed by water rising-up the pitch under a head of hydrostatic pressure before flowing outwards along the bedding planes. We considered that the original outlet was along the choked bedding-plane connection between the platform at the head of the pitch which emerges in the crawl just before the breakthrough point and then flows out along the upper series bedding planes. Later, presumably as the water level dropped, the water flowed out via An Unexpected Development and the various passages comprising the Lower Series.

Where all this water ultimately derives from and where it's going remain a mystery, which is of course exactly how it should be.” (Price and Taviner)

The enigma of the where the water comes from has several possible answers; including from fluctuating sea levels and/or from rising thermal waters.


Nick Hawkes, Duncan Price, Robin Taviner (pers comms)

Joyce Lundberg and Donald A. McFarlane. 2007. Pleistocene depositional history in a periglacial terrane: A 500 k.y. record from Kent’s Cavern, Devon, United Kingdom. Geosphere, August 2007, pp 199-219

Stephan Kempe. 2004. Natural Speleothem Damage in Postojnska Jama (Slovenia), Caused by Glacial Cave Ice? A First Assessment. Acta Carsologica 33/1, 18. p265-289

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