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Sunday, 4 June 2023

Dolerite erratics in the Nevern Estuary


This is a nice rounded dolerite erratic on the foreshore at Parrog, Newport.  It rests on a seaweed-covered wave-cut platform of dark Ordovician shales and mudstones. Dimensions approx 1m x 50 cm x 45 cm.  There are scores of others on the foreshore not far from this one, and large dolerite erratics are also used in the foundations for the sea walls, on the quays near the Boat Club, and on the other side of the river.  

The foreshore is littered with erratics of all shapes and sizes, sometimes associated with Devensian till exposures

I'm intrigued, because there are no known exposures of dolerite or gabbro anywhere near Newport.  The closest outcrops are to the south, a couple of km away, on the northern flank of Carningli. How did they get to the coast, and why in such abundance?  There is no reason to invoke human transport, even for building work or for ship's ballast, and it must be assumed that they are GLACIAL erratics.

Could these erratics have been carried northwards by ice from a Carningli or Preseli ice cap?  It's possible, but I suspect that when these small upland ice masses existed, they were small, cold-based and sluggish -- that means there was not much erosion, transport, and deposition going on.

Another possibility is that the erratics have come from unknown dolerite outcrops on the floor of Cardigan Bay.

A third possibility is that they have come from the dolerite outcrops on the high interior of Pen Caer, between 10 and 12 km away, carried by ice that was travelling pretty well directly from west to east........  Well, we know from crossing striations that there were big swings in ice movement directions during the Pembrokeshire glaciations.  Are these boulders, like many of those on the coast near St Davids, relics of an Anglian Glaciation?  The jury is still out.

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