Map showing the <a href="//commons.wikimedia.org/wiki/Geology" title="Geology">geology</a> of Bryn Eglwys slate quarry
<a href="//commons.wikimedia.org/wiki/User:Gwernol" title="User:Gwernol">Gwernol</a>
self-made - information from J.I.C. Boyd's "The Talyllyn Railway"