A photo from the 190s showing evidence of slate quarrying in North Wales in the distance.
Slate originated as mudstones about 450 million years ago during the Ordovician geological era. Mudstones were formed by silt filtered by marine deposition, often at the mouth of large rivers. The flow of river water carrying silt in suspension is checked at the mouth of the river with the heavier silt particles then falling to the sea bed while the finer particles are still impacted by the remaining force of the river water are carried further out before sinking to the sea bed. Such deposited particles tend to form very thin strata, and when compressed by later deposits form a rock which retains very thin strata. For these strata to form slate over geological time not only compression but also great heat is needed, this heat coming from volcanic activity in the Earth’s crust forming a metamorphic rock. This explains why slate rock has proved to be a valuable roofing agent, it is impervious and, because of the filtered strata, can be split into sheets. However, it is not a freestone, so cutting across the plain of the rock has to be done carefully to avoid fracturing.
So here we are two and a half millennia into historic time using a material that first saw the Planet nearly half a billion years ago. What was the Planet like then?
(to be continued)