The principal areas were centred on the upland areas of the River Ystwyth and River Rheidol with some outliers to the east in the catchment of the River Severn and some to the south in the headwaters of the River Teifi. The largest of these mines were the Cwmystwyth and Rheidol United mines in Cwm Rheidol. The ore extracted was galena which in many cases had a high silver content, especially at Cwm Ystwyth. It also occurred alongside large quantities of sphalerite, the principal ore of zinc. However, the zinc was only occasionally processed and much remains on the very extensive discard heaps around the mines.