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.

14th June 2019

notesPosted by Vince Simmonds Fri, June 14, 2019 05:52:10

Notes by Vince.

Just after the breakthrough into An Unexpected Development in August 2018, we’d had a discussion regarding the possible origins of the cave system along with other geomorphological processes and events, i.e. Pleistocene frost and ice damage. At the time I started to put together the following notes:

Notes on geomorphology. Is there a hypogenic origin for Hallowe’en Rift?

Cave development can occur in deep-seated conditions, without direct recharge from the surface, by recharge to the cave-forming zone coming from depth. This type of speleogenesis is termed hypogenic (or hypogene). The concept of hypogene speleogenesis does not necessarily mean cave development at great depth but refers to the origin of the cave-forming agency from depth. Hypogene speleogenesis is defined as the formation of solution-enlarged permeability structures by water that recharges the cavernous zone from below, independent of recharge from the overlying or immediately adjacent surface.

The following elementary cave patterns are typical (although not necessarily exclusive) for hypogene speleogenesis:

· Single passages or rudimentary networks of passages;

· Cavernous edging along transverse hypogene conduits;

· Network maze;

· Sponge-work maze;

· Irregular isolated chambers;

· Rising, steeply inclined passages or shafts;

· Collapse shafts over large hypogenic voids and breccia pipes.

Network maze caves of hypogene origin are known in limestones, dolomites and gypsum, in mixed limestone-dolomite-gypsum strata, and in conglomerates. A common feature of network mazes is a very high passage density.

Rising, steeply inclined passages or shafts are outlets of deep hypogene systems in which the “root” structure remains unknown in most cases. Possibly formed by rising thermal waters charged with CO2 and H2S.

Composite 3D systems are comprised of various elementary patterns at different levels, such as irregular chambers, clusters of network or sponge-work mazes and rising, subvertical conduits and other morphs connecting them.

Hypogenic features may become relict but still, remain within contemporary systems, for example, in a system where original confinement was breached and the flow pattern reversed from upwelling to descending (Klimchouk, 2012).

Ref: Alexander Klimchouk. Speleogenesis, Hypogenic in The Encyclopaedia of Caves. Elsevier, 2012, p748-765

Nick made the following comments (first reported 21/08/2018):

“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.”

Duncan and Tav also made some valid comments noted while carrying out a survey of the cave.

These thoughts might be more salient following a recent paper by Smart and McArdle published in the UBSS Proceedings Volume 28 (1) 2019, p65-102, suggesting a hypogenic origin for Denny’s Hole.

It is obvious that further investigation and research is required.

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