The North Wales coalfield extends in a shallow arc from the Point of Ayr, at the north-western tip of the Dee estuary, southward through Clwyd (formerly Flintshire and Denbighshire), and ends a mile or so inside the boundary of Shropshire, near Oswestry. It is about 45 miles long and 9 miles across at its widest point. The geological strata in this corner of Wales dip steeply eastward, producing a sharp and dramatic landscape, as limestone outcrops north of Llangollen give way to the rounded sandstone mountains which present their long eastern slopes to the Dee Valley and the rich pasture-lands of Cheshire. The coal-bearing measures, reaching the surface on the sides of Esclusham and Ruabon mountains, the hills of Flint and the Dee estuary, disappear to a great depth beneath the Triassic sandstone of the Cheshire Plain and re-emerge at a workable level in Lancashire and north Staffordshire. The seams are much broken by faulting and discontinuities. The Great Bala Fault, following a line running roughly north-east to southwest, effectively cuts off the Flintshire coalfield (to use the old names) from that of Denbighshire, which itself is divided into several large sections by faults, some running transversely, others from north to south. This pattern largely determined the development of the mining industry in the coalfield.