I no longer flinch and go into shutdown when I hear the phrase ‘Ordovician mudstone’ because Andrew Lingham (geologist and recent Tegfan visitor) has taught me a little and encouraged me to read further: http://www.pembrokeshireonline.co.uk/geology2.htm. This is obviously a simplified historical overview of the rocks in our area – which I, shamefully, did not find a particularly easy read. However, it’s just what was needed to upgrade my knowledge from nil to superficial (where I am happy to leave it for the time being).
But I do love slate – on the beach – on roofs or floors. I love the colours and the feel and the shapes it makes along the coast where the vertical lines hint at earth forces of unimaginable power. So, don’t drift off because I’m going to give you a condensed version of the abbreviated history so that you too can add Ordovician mudstone to your vocabulary (if it’s not there already).
These vertical sheets of slate originated as fine mud on the sea floor in deep oceans further south than the tip of S America. And the mud, laid down in horizontal layers 450(ish) million years ago, has been turned into slates by pressure of earth movements. Movement of the crustal plates then relocated the rocks thousands of miles to the north and introduced the different orientations of the joints in the slates; the jagged rocks seen today at our local Pwllgwaelod and Cwm yr Eglwys beaches are the result.