A very erratic Essex erratic

The "Welsh" erratic found in ancient River Thames gravels near Colchester 

on the North Sea coast.

A 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
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.

This is quite a puzzle.  the authors have had detailed analyses of the erratic done, and they seem pretty sure of the suggested provenance, although there are a few other possibilities -- mostly also in Wales.  What was a striated boulder like this doing in river gravels?  The only thing they can think of is that the boulder was carried from North Wales and incorporated into ancient glacial deposits (now destroyed) in the Cotswolds, in the headwaters of the early Thames river.  After the passage of an unknown amount of time, the glacial deposits were eroded away at a time of very cold climate, and the boulder was picked up (maybe many times) from the bed of the river and incorporated into seasonal ice -- then swept along at the time of the spring flood, then dropped, and then picked up again later, and carried, and dropped, and so on and so on, maybe thousands of times before reaching its final destination.  That's a very clunky explanation, as appreciated by the authors themselves -- but nobody, for the time being, can come up with anything better........

My own problem with the "multi-trip ice rafting explanation" is that in a turbulent shallow water environment such as that proposed, dominated by coarse river gravels, there must have been a huge amount of abrasion on all the materials being gradually moved downstream -- and in that sort of environment I do not see how the glacial striations could have survived.