STONE B (No. 7 SW circle)

• Conglomeritic texture, clasts supported.
• Clasts up to 10cm and include limestone, chert, quartz crystals and a black grain.
• Alteration to clasts causing reaction rims. These rims are often left behind with the rest of the clast having been weathered out.
• Possible clast orientation seen (actual reading unworthwhile as stone not in situ).

Fig 8. Close up of conglomerate with clearly visible reaction rims.

STONE C (No. 9 SW circle, better example in Great Circle)

• Sandstone with layers of large clasts, possibly indicating an erosion surface or a period of faster water current.
• Cross bedded.
• Beds are no longer than 1cm

Fig. 9 Possible erosion layer within Butcombe Sandstone.

STONE D (Many examples in Great Circle)

• Limestone, no large clasts visible.
• Stones invariably flat and square possibly reflecting jointing in original bed.
• Pitted surface and always covered in lichen (flat nature of stone means no one face exposed to preferentially grow lichen.

Fig 10 Pitted surface of Limestone stone

STONES OF THE COVE

GR: ST 601634

The Cove is made of three stones. Two of these were probably one piece as the shapes of the stones indicates they once fitted together, and they are lithologically identical.

• Conglomeritic texture. Clasts exceed 20cm in places.
• Overall white grey in colour
• Clasts are angular to sub angular and include chert, limestone and sandstone.
• Highly fossiliferous. Crinoids, corals and shell fragments.
• Matrix is lime rich and varies in size from silt to sand grains.
• Probably Carboniferous limestone found locally in the cores of the Mendip Hills.

Fig 11. Close up on the surface of the stones of The Cove

CLASTS PROVENANCES within the Dolomitic Conglomerate.

Sandstone: Devonian (Old Red Sandstone).

Chert: Carboniferous (Quartzite Sandstone Group).

Limestones: Carboniferous.

One of the diagnostic features of the silicified Dolomitic Conglomerate are the reaction rims found on a large number of the clasts. Mostly they appear as light grey/ pink, extremely hard material that seems to remain when the inner parts of the clasts erode away. They are found in clasts from the mm scale to those on tens of centimetres. These are more than likely to be the result of the metamorphism.

MAP OF THE STANTON DREW SITE

A map of the Great Circle, the north east and the south west circle has been produced including alterations to the position of stones. It includes the lithology of each stone and notable features (see appendix 3). The original map used was from Morgan 1887.

PROVENANCE

It was necessary to concentrate on locating the silicified conglomerate as it is the most distinct lithology present at Stanton Drew. The location of its source could serve to determine the origin of the silicification.

The first promising sample found in the field was in the possible location for Oaky Hole (GR: 564613). Unfortunately the piece was not in situ but indicate the correct area may have been found. This area is at the north eastern end of Harptree Combe. The fact it was not in situ precluded it from being one of the test samples. In all cases no definite outcrops were found but the samples were pulled up by fallen trees or in other circumstances where it was obvious they had not been moved far from their outcrop.

Fig 12 Possible location of Oaky Hole.

The tested samples are summarised in the next location and their localities recorded on map B and the hand specimen details are summarised in the field notebook (appendix 2).

GEOCHEMISTRY

The following descriptions are the controls and samples used in the geochemical analysis.

CONTROL SAMPLES:

CONTROL A: Piece from Megalith.

This sample was not taken directly from the stones by myself. It is a piece from the University collection. Its notes claim that it is from a megalith at Stanton Drew collected by J.A. Kemp and suggest that it is similar to the West Harptree Silicified Conglomerate. It is used as a control to the possible source samples and to determine if hydrothermal activity has altered the conglomerate in anyway.

GENERAL FEATURES

• Silicified conglomerate, clasts of quartzite and limestone.

MATRIX

• Variable Size. Seems to be clasts supported with some silt layers but is mostly coarse grained.

CLASTS

• Notable rim alterations on a few clasts approx. 5-10mm.
• Larger clasts exist up to 2cm
• Sandstone and limestone

CONTROL B: Rhyolite.

As there is reasonable doubt as to the lithology of Stone A on the SW circle it is necessary to include a control sample of rhyolite to ascertain whether it is an igneous rock. With the obviously high quantity of quartz it is most likely to be rhyolitic if it is of igneous origin.

Rhyolite is the volcanic equivalent of a granite. It is essentially composed of alkali feldspar and quartz. Quartz will probably be present as tridymite or cristobalite. Augite and hornblende are common minerals (Klein and Hurlburt, 1993).

CONTROL C: Unsilicified Dolomitic conglomerate.

This is used as a control sample to see what alteration has occurred between the unsilicified conglomerate to the silicified conglomerate in terms of overall element content and REE abundance. It is likely that the movement of hydrothermal liquids will have altered he chemistry of the conglomerate in someway if it is hydrothermally altered as opposed to contact baking with no liquid movement involved.

Location: Wells Way Inn car park.

GR: 545 560

GENERAL FEATURES

• Hard but friable in places
• Weathered surface, dark pinks and reds
• Fresh surface lighter pink and reds
• Fairly homogenous

MATRIX

• Red sand sized grains, some even coarser.
• Crystals up to 5mm

CLASTS

• Large subangular clasts up to 30mmx15mm.
• Limestone, sandstone, quartzite and a darker crystalline material.

SAMPLE A

Found in Garrow Bottom

GR: 555553

GENERAL FEATURES

• Very Hard
• Red to light pink colouration
• Apparently unfossiliferous
• Breaks in block fashion
• No reaction with acid

MATRIX

• No definition of clasts
• Quartz rich with some mica

CLASTS

• Some clasts may have been weathered out where there are now small geodes partially infilled with quartz. Some red staining of the quartz.
• Geodes up to 8mm. Not all holes have geodes in them
• Some possible remnants of clasts, blackened area, rounded edges, discrete from matrix

SAMPLE B (Blackened edges)

Sample B on Harptree Combe map

GR: 560556

GENERAL FEATURES

• Weathered surface, very black
• Fresh surface, reveals conglomeritic texture.
• Light to dark pink with white and orange patches
• Hard
• No evidence of fossils

MATRIX

• Sand sized grains, mainly sandstone but with a notable calcareous content that fizzes with HCl
• Some sandy areas
• Mainly pink in colour
• Contains quartz and part-filled geodes with well formed crystals up to 8mm. Reflect grains

CLASTS

• Notably clasts supported. Mostly coarse grains, angular to sub angular grains.
• Red colour: sandstones, Grey colour: reacts with acid, limestone, Quartz crystals: medium sized with some smaller.
• Small reworked material of these clasts make up the matrix
• Infrequent large clasts a few cm long. Sandstone

(geode containing)

This appears to be the most like the 'rhyolitic' stone seen in the SW circle. Sample B on Harptree Combe map.

GR: 561558

GENERAL FEATURES

• Extremely hard.
• Fresh weathered surface light pink to red.
• Weathered surface, darker pinks and reds.
• No evidence of fossils.

MATRIX

• Grains generally indistinguishable but apparently fine grain to sand size.
• Variable in colour: yellow to dark red.
• No fabric, possibily overwritten by metamorphism.
• Massive.

CLASTS

• Varies from 1/2mm to 30-40mm.
• Back and pink clasts.
• Sandstone: coarse grain.
• Some clasts lost to weathering, especially smaller ones.

• Geodes infilled by quartz. Mostly complete infill, others partial. Clear and euherdal crystals.
• Pattern in colour surrounding geodes: black rim to geodes then a diffuse red colouration up to 10mm thick.

• Two types of reaction rims on clasts.
• One discrete white colour, few mm thick and uniform around weathered out or present clast.
• One black rim with diffuse black granular rim.

SAMPLE D

Found in Harptree Combe

GR:559555

GENERAL FEATURES

Semi Quantitative Analysis

The easiest way to compare the spectrum of the samples is to directly over lay them and determine whether peaks of abundance of elements match. Recent analysis carried out on the ICP-MS may have left a contamination of 74M and 56Ba. This explains the high levels of these two seen in the semi quantitative analysis. Spectrums of these abundances can be seen in appendix 4.

Quantitative Analysis

Abundances of rare earth elements in each of the controls and samples were attained in ppb. 64Gd has been ignored due to it being notoriously unreliable in ICP-MS work. A calibration curve was introduced. The internal standards were ⁴⁴Ru and 75Re. Graphs were plotted for comparison of the samples and controls and are found in appendix 6.