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Monday, 17 November 2014

Vyrnwy excavations 1882

This fantastic 1882 photo (from the Hubbard Collection) has just been released by Heritage of Wales News. It shows the sheer scale of the excavation work that went in to the creation of the dam which flooded a whole community and created a huge new water supply lake. But it also shows in amazing detail the glacial geomorphology of the area, with the superficial deposits stripped off and the jagged bedrock surface beneath exposed so as to create a firm foundation for the masonry dam. This is apparently the SW end of the vast excavation trench. The deposits are here about 16m thick and seem to consist of relatively fine-grained materials at the base (fine-grained till?) overlain by a more pebble layer and then by something that looks like a major unconformity, and some flat-lying sediments about 2 m thick above that. That's all a bit speculative. I wonder how these sediments were described by the engineers of the day? And the bedrock surface.. In places it looks distinctly moulded or streamlined -- but there is clear structural control here, so it would not be very wise to speculate too much......

This photo shows the completed dam before the lake was filled.

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