dighalloween

An exposure of Dolomitic Conglomerate in Harptree Combe showing good sequence of wadi deposits (refer to note on dolomitic conglomerate below).

With Alex. Well, the original plan was to clear out all the dumped spoil back to surface, but someone forgot a spanner to erect the tripod – so continue digging was our only option. As usual we cleared the base of Witches Cauldron before I headed off for the tube. It seemed particularly sticky today like trying to cave through ‘velcro’, there is not enough space to free your oversuit from the clag and in the more snug spots it can be a little ‘hard going’. Continued clearing the sediment at the end of the tube and did manage to get a better look at the rift but am still a little undecided about the best way to proceed – don’t want to leave myself in a position where collapse of the sediment overhead might pose an inconvenience. Air movement was not as obvious during today’s session.

A noticeable feature when describing either breccia and/or conglomerate is clast angularity, generally breccia is more angular while conglomerate is more rounded, of course it must be emphasised that the boundaries between the rock types is blurred and there may be a high degree of variability. The angularity or roundness of the clasts within breccia/conglomerate might be used as evidence of the amount of transportation that has occurred, i.e. more roundness might indicate greater transport/higher energy. Clast size might also be a factor, smaller size would require less energy to be transported a further distance.

Boulder of Dolomitic Conglomerate removed from Hallowe’en Rift, the purple-red colour is the result of high iron content and evidence of haematitization (see below).

On the Mendip Hills, the Dolomitic Conglomerate mostly comprises clasts of Carboniferous limestone cemented into a matrix of sandy marl or fine grained limestone debris, locally derived material from the Old Red Sandstone and Quartzitic Sandstone Group is also present. The rock clasts are angular to rounded and range in size from varying sized gravels to very large boulders. The Dolomitic Conglomerate forms bold crags, it might be eroded into gorges, and it can support underground drainage systems including caves and swallets and is the locus for extensive lead/zinc mineralization. The formation represents Triassic scree and outwash fans adjacent the ancient hills of Paleozoic rocks, and it fills ‘fossil’ wadis or gorges that had been cut into the hills. Recent erosion has partially re-excavated some of these gorges, as at Burrington Combe.

Exposure of surface weathered Dolomitic Conglomerate [slightly silicified] in Harptree Combe.

The conglomerate found on the Mendip Hills has, in many cases undergone considerable secondary changes, in particular silicification, haematitization and dolomitization. Haematitization is the conversion of the conglomerate into an ‘earthy’ iron ore known as ‘red ochre’. The presence of all gradations from unaltered conglomerate to impure haematite rock show that the formation of the latter is the result of metasomatic replacement of calcium carbonate by haematite.

The most widespread form of alteration is dolomitization, hence the name ‘Dolomitic Conglomerate’. All stages of dolomitization are present, both matrix and clasts showing varying degrees of alteration. Dolomitization is usually accompanied by hydration or partial hydration of the originally disseminated haematite to limonite so that macroscopically the colour has changed from red to yellow and yellow-brown. Dolomitization affects more than one level in the Dolomitic Conglomerate though it is most marked in the upper parts of the succession.

metasomatism is a metamorphic process [where rocks are changed by heat, pressure and fluids] in which the chemical composition of the rock is changed significantly, usually as a result of fluid flow.

References:

Green, G.W. et. al. 1965. Geology of the country around Wells and Cheddar. HMSO, London.

Keary, P. 2001. The New Penguin Dictionary of Geology, 2nd Edition. Penguin.

The bar to the left of centre in the image is about 300mm in length.

Solo trip. Just a quick Sunday morning jaunt to take some photographs of the end dig in order to update the online gallery before retiring to the Hunter’s Lodge Inn.

The image above shows Alex working his way along the approach tube from the base of Witches Cauldron to the dig site – a fine spot!

With Alex. Continuing the quest for passages measureless to mice! First job was to clear the spoil dumped at the base of the Witches Cauldron before returning to digging along the left-hand tube. The pot/rift edge noticed last session can be seen (take a look at the photo taken on 20th and shown on the post for that day) but some more forward progress still required before a ‘good old prodding’ is possible – might be a good idea to wear some goggles as crap falls into your face when lying on your back and waggling a bar around in an effort to dislodge a blockage above your head. There is certainly a change occurring bur more work still required. The dig has become a little squalid over the past week or so – the result of a strong drip, or lots of drips, obviously the surface above ground has become saturated and the water is permeating into the cave, emerged rather muddy, slightly knackered, but rather satisfied after a steady 2.5 hour session.