underestimation of both species and specimen numbers. Figure 4.5 shows examples of shells which have clearly been broken round their whorls into numerous fragments and are only held together by soil.
These were extracted from unsieved sediments and the apices may well have been lost if subjected to wet sieving as is the norm in mollusc sample preparation.
Figure 4.5. Shells showing concentric fracturing caused by percussion coring.
The graph paper has 2mm divisions
4.1.3.1 Hand auger coring
Coring using a gouge auger was performed at Gwithian on the shallower parts of the transect where it was anticipated that the underlying natural could be reached without too much difficulty. A 2.5cm diameter gouge corer (1m length) was used at the initial visit. The quantity of material obtained using this gauge of core is limited and all the hand cores except those at 287m and 400m were later repeated using a 5cm diameter corer (50cm length). Unlike percussion coring there was no obvious damage to mollusc shells caused by the hammering action required to drive the augers.
The stratigraphy and Munsell number of each core was recorded on a pro-forma on withdrawal from the ground. The sediments were then placed in labelled plastic bags, either as complete stratigraphic samples or with long sediments divided into portions, usually no more than 10cm in length.
4.1.4 Excavation
A single trench was excavated at Gwithian and five trenches at Gunwalloe. These will be discussed in detail in the relevant chapters.
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4.1.5 Mollusc samples
4.1.5.1 Bulk samples from mollusc columns
Samples for mollusc analysis were obtained wherever possible using mollusc columns taken in a standard way (Evans 1972, 41). In the present study these were obtained from the trench at Gwithian, and from each of the five trenches at Gunwalloe. At Strap Rocks a coastal cliff exposure was cleaned to obtain as vertical a surface as possible, and a small pit was dug at the base of the exposure to extend the column as deeply as possible. Stratigraphic boundaries were respected in deciding where to divide samples. It is normal when taking samples from a mollusc column to commence at the lowest level to reduce the risk of contamination from falling material from higher levels. This was not possible on these sand dune sites due to the instability of the sand and all samples were taken from the ground surface downwards; care was taken to ensure that after one sample was taken the upper surface of the next sample was cleaned of any extraneous matter as much as possible to avoid contamination.
Munsell colours were recorded on site while the sediments were still damp.
4.1.5.2 Core samples
Samples obtained from the hand auger cores along the Gwithian transect, as well as those obtained with the percussion gouge, were separated and bagged on site, again respecting stratigraphic boundaries and dividing thicker contexts. Munsell colours were recorded.
4.1.5.3 Modern molluscs
Samples of turf were taken from each of the Gwithian core sites to assess molluscs which were present in very recent times, although the number of living animals was found to be few. At each site a square approximately 20 x 20cm in size and 4‒5cm deep was cut using an archaeological trowel and placed in labelled plastic bags. Photographs of each location were taken so that the mollusc assemblages could be matched as far as possible to current microhabitat preferences.
One sample was obtained from the Red River leet by dipping with a scoop with a 0.5mm mesh into the sediment at the base of the leet. Approximately 500ml of sediment was retrieved from which the molluscs were extracted, but it is emphasised that mollusc numbers for this sample are not quantitative in regard to weight or volume of sediment; the sample is, nevertheless, valuable in assessing the current freshwater molluscs likely to be found in the area.
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4.2 Laboratory methods 4.2.1 Core samples
4.2.1.1 Percussion core samples
The cores containing the percussion samples consist of a plastic tube which must first be divided lengthwise into two halves. This was done using a circular saw set to a depth which just penetrates the thickness of the plastic without damaging the contained sediment. The tubes are then wrapped in cling film to keep them together and prevent dehydration.
When required the tube is unwrapped and the sediment divided into two halves by cutting lengthwise with a sharp knife and the two halves spread side by side. Once displayed it is necessary to decide how much contamination there is in the upper part of the core from material which has fallen into the bore hole between removing one length of core and inserting the next. This is usually fairly clear, as contaminating material frequently has an uneven mix of sediments of varying texture and colour and is different from new material in the lower part of the core. On occasions, especially in the sand cores at Gwithian, it was very difficult to determine the change from contamination to new sediment and a 'best guess' was made. In practice it was found that if the core was purely sand then 5‒15cm was contaminant, whereas if there were thick silt layers then up to 65‒70cm may be contaminant with subsequent marked compression of the silt (Figure 4.6).
Figure 4.6. An example of contamination of a percussion core.
The 2‒3m core from the 0.5m location, demonstrating the contamination of the upper 54cm of the core
The stratigraphic levels in a newly opened length of core were determined by inspection and recorded on a pro-forma, together with the nature and texture of the sediments, the abruptness of the interfaces, the presence of stones or other inclusions and the Munsell colours. One half of the core was then re-wrapped in cling film and stored as a potential archive for tests at a later date; however, it soon became clear that both halves of the core were necessary to maximise the number of molluscs in each sub-sample and all the results that follow include the whole core content. The sediments were removed into small labelled open containers in lengths determined by the stratigraphy, with thick contexts being divided into portions. The sandy sediments were air dried and weighed. In the case of organic silt the material was kept moist and the whole examined microscopically for molluscs; in practice it was easy to tease silty nodules in a Petri dish using forceps into sufficiently small units so that the molluscs could be extracted.
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Correction method for compression caused by percussion coring
The problems with compression caused by the vibrations of the percussion corer have been mentioned above, and it was necessary to find a method to provide corrected depths for each stratum within the core. If the core consists of similar sediment throughout its length, such as all sand or all organic silt/peat, then conversion is easy, as, once the length of contamination in the upper part of the core tube is deducted, then the remaining length can be extrapolated to cover the full tube length (Canti and Meddens 1998). However, if there is a mixture of organic sediments in the core there will be different degrees of compression, and a straightforward conversion cannot be applied. Sand compresses relatively little whereas organic silt was subject to marked compression, with up to 70cm of contamination in some core lengths.
A method has been calculated which to a large degree compensates for this. In several of the 1m core lengths in the present study the contained sediments were entirely sand, as at the 76m core location.
Compression was found to be fairly consistent, with 5‒15cm of contamination in each tube, with an average of 10cm per 1m length. Therefore, to obtain the correct thickness of each sand layer in the tube the measured thickness should be multiplied by a factor of 1.1 (there is, of course, no contamination in the 0‒1m core, as this starts at the ground surface).
In a sequence of cores with differing sediments the following method was applied:
1 Add the measured total of all the sand deposits (but excluding 0─1m, after deducting the contamination lengths.
2 Multiply that total by 1.1 to give a corrected total for sand; this is also done for each individual sand layer within the cores.
3 If bedrock has been reached, then the difference between the corrected sand depth and the depth of the bedrock is the thickness which must be accounted for by organic silt.
4 Calculate the factor by which the silt/peat must be multiplied to convert the measured thickness to the difference found in (3) and apply this to each organic silt layer.
5 The total of the corrected thickness for sand and organic silt should equate to the depth of the bedrock.
This method will not be applicable if the bedrock has not been reached as the depth to which the lowest levels of silt have been pushed by the percussion process is not known. In practice when the bedrock has been reached the average compression ratio has been found to be 1.9‒2.1 (i.e. the silt has been compressed to around half its ‘true’ thickness). A factor of 2 can therefore be applied in these circumstances and will provide corrected depths of acceptable accuracy.
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