The Cambrian Mts. cover most of Wales, with high points at Snowdon (3,560 ft/1,085 m), Plynlimon (2,468 ft/752 m), and Cadair Idris (2,970 ft/905 m). The eastern rivers—the Dee, Severn, and Wye—drain into England. The Usk flows through Monmouthshire and Newport into the Bristol Channel. The Tywi (Towy), Taff, Teifi, Dovey (Dyfi), and Conwy (Conway) rivers lie completely in Wales. The eastern boundary, drawn in 1536, united England and Wales politically but disregarded cultural and linguistic distribution. Welsh-speaking areas were added to England's Herefordshire, Shropshire, and Gloucestershire; the language survived in Herefordshire until the 18th cent. and survives to a small extent in Shropshire today. Wales has maintained a distinctive culture despite its long union with England, though English has become the main language. In the 1990s about 25% of the population spoke Welsh, although in certain regions the percentage was much higher. Wales comprises 22 administrative divisions (unitary authorities): Flintshire, Wrexham, Denbighshire, Conwy, the Isle of Anglesey, Gwynedd, Powys, Ceredigion, Pembrokeshire, Carmarthenshire, Swansea, Neath Port Talbot, Bridgend, the Vale of Glamorgan, Rhondda Cynon Taf, Merthyr Tydfil, Cardiff, Caerphilly, Blaenau Gwent, Torfaen, Newport, and Monmouthshire.