country on the western side of central southern Great Britain . [ 125 ] It is about 170 miles (270 km) north–south and 60 miles (97 km) east–west. [ 126 ] The oft-quoted ' size of Wales ' is about 20,779 km 2 (8,023 sq mi). [ 127 ] [ 128 ] Wales is bordered by England to the east and by sea in all other directions: the Irish Sea to the north and west, St George's Channel and the Celtic Sea to the southwest and the Bristol Channel to the south. [ 129 ] [ 130 ] Wales has about 1,680 miles (2,700 km) of coastline (along the mean high water mark), including the mainland, Anglesey and Holyhead. [ 131 ] Over 50 islands lie off the Welsh mainland; the largest being Anglesey , in the northwest. Much of Wales' diverse landscape is mountainous, particularly in the north and central regions. The mountains were shaped during the last ice age, the Devensian glaciation . The highest mountains in Wales are in Snowdonia ( Eryri ), of which five are over 1,000 m (3,300 ft). The highest of these is Snowdon ( Yr Wyddfa ), at 1,085 m (3,560 ft). [ 132 ] [ 133 ] The 14 Welsh mountains, or 15 if including Garnedd Uchaf – often discounted because of its low topographic prominence – over 3,000 feet (910 metres) high are known collectively as the Welsh 3000s and are located in a small area in the north-west. [