Illustration from the article by Rose et al, 2010
I have dealt with this topic before, and I am still somewhat puzzled. It's referred to by the authors as an "oversized" clast -- I am not sure what that is supposed to mean, since in glacial and fluvioglacial deposits you can get clasts of all shapes and sizes --- but yes, finding it in river gravels in Essex must have been quite a surprise! But it is not a gigantic boulder -- its dimensions are just 23 cm x 8 cm x 7 cm. So it is similar to, but smaller than, the famous Newall Boulder on which we have spent so much time.
The "Ardleigh Boulder" from Fig 2 of the article by Rose et al.
The Newall Boulder, slightly larger but of similar shape, with multiple facets and abraded edges. But this one has suffered considerable human damage since it was released from the ice........
Small glacially modified boulder from Cilgwyn -- approximately the same size.
This one is made of dolerite
J. Rose, J.N. Carney, B.N. Silva & S.J. Booth (2010)
Netherlands Journal of Geosciences — Geologie en Mijnbouw 89 (2 |), pp 137 - 146
https://www.cambridge.org/core/journals/netherlands-journal-of-geosciences/article/striated-far-travelled-clast-of-rhyolitic-tuff-from-thames-river-deposits-at-ardleigh-essex-england-evidence-for-early-middle-pleistocene-glaciation-in-the-thames-catchment/70E04344BEEC550AA135F38130748AFD
Abstract
This paper reports the discovery of an in-situ striated, far-travelled, oversized clast in the Ardleigh Gravels of the Kesgrave Sands and Gravels of the River Thames at Ardleigh, east of Colchester in Essex, eastern England. The morphology, petrography and geochemistry of the clast, and the sedimentology of the host deposit are described. The striations are interpreted, on the basis of their sub-parallelism and the shape and sub- roundedness of the clast, as glacial and the clast is provenanced to Ordovician rocks of the Llyn and Snowdonia regions of North Wales. On the basis of clast frequency within the Colchester Formation gravels of the Kesgrave Sands and Gravels it is inferred that glaciers reached the Cotswold region of the Thames catchment. Floe-ice transport during spring flood is invoked for movement from the glaciated region to eastern England. The paper discusses the possible age of the glaciation and recognises that it is difficult to be more precise than a cold stage in the early Middle Pleistocene (MIS 18, 16 or 14). Attention is drawn to the possibility of glaciation associated with a diamicton in the region of the Cotswold Hills known as the Bruern Till, but stresses the need for new work on this deposit.
"a fine-grained vitric tuff, consisting of tightly packed shards of fine-ash size that have devitrified to microcrystalline quartz-feldspar mosaics. The other pyroclastic constituents are small and very sporadic, angular to euhedral crystal fragments of plagioclase feldspar and (very minor) quartz. Only one ‘exotic’ constituent was seen, a silt-size, rounded clast of microcrystalline quartz, possibly representing a fragment of silicified rock."
The clast is referred to as a "rhyolitic tuff" -- a term which can also be used to refer to many of the rocks of the Fishguard Volcanic Group in north Pembrokeshire. We might as well refer to it as "bluestone" in order to flag up the geological similarity. But this one is supposed to have come from Llyn or Snowdonia -- I am not sure how strong that provenancing is, but according to the authors, the detailed analyses done by the IGS points to:
But the suggestion is therefore that it has come from NW Wales, not NE Wales, which one might expect if the suggestion is that it has somehow been incorporated into glacial and / or fluvioglacial deposits in the Thames catchment a very long time ago...... If the authors are right, this boulder (maybe larger at the time) must have been transported eastwards of south-eastwards by the Irish Sea Glacier and / or Welsh ice, then southwards by ice into the Thames catchment, and then broadly eastwards by mechanisms unknown.
And I am still troubled by the clear striations on the flanks of the boulder -- could they really have survived if the boulder had been transported hundred of kilometres in turbulent meltwater torrents from destroyed glacial deposits in the Welsh Borders? I'm not all that happy with the idea that the clast was carried along all the way from its distant glacial source in a convenient block of frozen material.........
Is it possible that this stone has not come from NW Wales at all, but from N Pembrokeshire? I am intrigued because the three BGS geologists involved in the writing of this paper considered the Pebidian (Pre-Cambrian) volcanic rocks of the St Davids area as a possible source, but not the rhyolitic tuffs of the Fishguard Volcanic Group (Ordovician) of Mynydd Preseli. Yet they DID consider the Ordovician rhyolitic pyroclastic rocks of North Wales and the Lake District as possible source areas. Of course, fifteen years ago (when this paper was written) there was no great focus in the literature on the FVG rocks, and Ixer, Bevins and Parker Pearson had not begun their serious (some might say obsessive) hunt for bluestone provenances in Mynydd Preseli.
Confused? Yes, so am I. I am trying to get to the bottom of it....... and I'll do another post on the riddle of the Thames terraces.
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