Geology
Three parallel veins of Ordovician slate run through mid Wales, from the region north of Dinas Mawddwy through Corris and south west towards Tywyn. These veins are the southern edge of the Harlech Dome anticline which surfaces in the north at Blaenau Ffestiniog. Where the veins cross the site of Bryn Eglwys, they are inclined at an angle of about 30 degrees from the horizontal, sloping downwards to the south-west. The widest of these veins is the 600-ft-thick "Broad Vein" that lies to the north of the site and consists of layers of hard, grey shale with patches of slate. The Broad Vein slate is hard and durable, but does not split into thin sections, so is generally unsuitable for use as roofing slates.
The "Middle Vein" (sometimes known as the "Red Vein") lies about 100 yards south of the Broad Vein. It is about 60 ft thick but contains low-quality, friable slate that contains a large number of fossils, predominately graptolites. This vein was not worked commercially at Bryn Eglwys.
The third vein is the "Narrow Vein" which lies about 100 yards south of the Middle Vein and is also about 60 ft thick. It contains the highest quality slate of the three veins and the most commercially valuable, being easy to split into roofing slates and slabs and both durable and strong. The vein is mostly a continuous bed of slate, containing only the occasional seam of quartz. However the quality of the rock varies over the depth of the vein, with the best material found nearest the surface.
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