Because the text on the scanned version of this 1991 report is not easy to read, I have done an OCR scan and here it is again.
RSN 18 ignimbritic tuff-lava
Description by RK Harrison: “This large, dark blue-grey, hard, flinty (? partly worked artifact) shows a white weathered crust up to 5 mm thick. The thin section shows a complex structure of very finely banded welded tuff (compressed foliated shards cemented by fine silica) with composite quartz grains and strings of dusty leucoxene, separated by patches of much finer grained, finely fluxioned glassy lava with patches of granular quartz, This specimen appears to represent a complex of originally viscous glassy lava and welded vitric tuff, all presumably of rhyolitic composition.”
The dimensions and general appearance of the rock are shown in Plates 35 & 36.
Comparison of this ignimbritic rock with other Welsh Ordovician volcanic rocks suggests that it is more closely related to the known ignimbrites of North Wales than to those of the Mynydd Preseli area of Dyfed. On the other hand none of the ignimbrites available for comparison provided a sufficiently good match to enable a link to be made with one specific locality. At the time. this conclusion appeared to be disaappointingly vague and out of line with some of the more dogmatic slatements made in respect of Stonehenge rocks. In the light of more recent work, however, Mr Harrison's cautious approach has been fully justified.
If the ignimbritic tuff-lava is indeed an erratic of North Wales origin and the ice which conveyed it is as old as it appears, then careful consideration must be given to the value of implying direct derivation from individual exposures or outcrops which exist at the present day. In the 247 Ma which may have elapsed since this Pliocene glaciation occurred, the mountainous parts of Wales (both north and south) have undergone substantial changes of relief. While, therefore, it may be justifiable to compare the Stonehenge rocks with suites of Welsh Palaeozoic igneous and metamorphic rocks on a regional basis, it may not be possible to give precise locations without having more information about the Pliocene relief and surface geology of Wales than is currently available. (Note: Kellaway was fascinated by the idea of a very large Pliocene Glaciation which occured when the landscape looked very different to that of today. Most of his ideas are well supported by modern research, but not this one!)
The weathered zone seen on the surface of the ignimbritic boulder is similar to other weathered exteriors of specimens examined by Mr R. Sanderson (Appendix Ill). This type of weathering is to be seen at the present day in peaty or humic environments and could have occurred before the rock was moved in ice from Wales to Salisbury Plain. Reporting on some of the ophitic dolerites from Stonehenge, Mr Harrison commented that "In order to ascertain the differences between the altered outer crust of pale dolerite and the fresher, blue-grey interior, X-ray diffraction photographs were taken of powders drilled from the respective zones in ENQ 2301, the outer crust here being about 5 mm thick. Mr B.R. Young reports that both samples contain chlorite, quartz, feldspar, pyroxene and amphibole. The crust contains more quartz and less feldspar than the interior. Kaolinite was not detected. Superficial leaching by acid water has broken down the feldspar, leaving residual silica."
If the weathering does date from before the translation of the rock to Wiltshire then the stone must have been conveyed either by hand or in ice. If the bleaching and weathering took place after arrival at Salisbury Piain then the boulder might have been carried by fluviatile or fuvioglacial transportation, though the weakness of the rock (due to shearing) argues strongly against fluviatile transport from Wales, or rounding in any medium other than ice.
When found, the weathered boulder had been thrown away with chippings and other waste material. An attempt had been made to dress one end of the boulder but this, in Mr Newall’s opinion, had failed because of the sheared condition of the rock. It would appear that this small boulder, already deeply weathered, would never have been of any practical value. To suggest it had been carried from North Wales lo Wiltshire only to be tested and thrown away as worthless would imply an astounding lack of common sense and understanding of the properties of rocks on the part of the men who built Stonehenge. If, however, the bluestones were recovered locally from material scattered on the surface of the Chalk or were present in solution cavities, then the presence of inferior material is comprehensible. Having gathered up all the available bluestones, both from natural sources and from abandoned Neolithic structures, the Bronze Age builders of Stonehenge used the large ones for constructional purposes and tested the smaller boulders for the manufacture of implements. Those which were unsuitable were thrown away.
It is worth noting that Sir A. C. Ramsay (1863) observed that the foreign stones of Stonehenge include rocks which are petrologically similar to those of North Pembrokeshire and that others resemble rocks from Caernarvonshire and the Lilandeilo Flag district of Montgomeryshire west of the Stiper Stones. This appears to be the first published suggestion that the bluestones may include rocks similar to these found both in southwest and North Wales.
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From: 1991: "The older Plio-Pleistocene glaciations of the region around Bath." In Kellaway, GA (ed) Hot Springs of Bath, pp 243-41.
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