The Builth district was visited by one of us (O. T. J.) in the spring of 1939 with a view to arranging an excursion for the International Geological Congress which was to have been held in London in 1940. Amongst other places the magnificent range of quarries in the Ordovician volcanic rocks of Llanelwedd, near Builth, was examined. Beyond the western end of the quarries a striking boulder bed was observed which promised to give an insight into a remarkable episode in the history of the district. This bed is shown on the I in. Geological Survey Map, Old Series, Sheet 56, S.W.