The Avon Gorge () is a 1.5-mile (2.5-kilometre) long gorge on the River Avon in Bristol, England. The gorge runs south to north through a limestone ridge west of Bristol city centre, and about 3 miles (5 km) from the mouth of the river at Avonmouth. The gorge forms the boundary between the unitary authorities of North Somerset and Bristol, with the boundary running along the south bank. As Bristol was an important port, the gorge formed a defensive gateway to the city. On the east of the gorge is the Bristol suburb of Clifton, and The Downs, a large public park. To the west of the gorge is Leigh Woods, the name of both a village and the National Trust forest it is situated in. There are three Iron Age hill forts overlooking the gorge, as well as an observatory. The Clifton Suspension Bridge, an icon of Bristol, crosses the gorge. Geology and formation The gorge cuts through a ridge mainly of limestone, with some sandstone. This particular ridge runs from Clifton to Clevedon, 10 miles (16 km) away on the Bristol Channel coast, although limestone is found throughout the Bristol area. The fossil shells and corals indicate that the limestone formed in shallow tropical seas in the Carboniferous, 350 million years ago. For a long time it was unclear what caused the Avon to cut through the limestone ridge, rather than run southwest through the Ashton Vale towards Weston-super-Mare. However, Bristol was at the southern edge of glaciation during the Anglian ice age, and it has been suggested that ice blocked the river's natural route through Ashton Vale to the west. At the Clifton Suspension Bridge the Gorge is more than wide and deep. In the 18th century the gorge was quarried to produce building stone for the city. Stone was taken by boat into the floating harbour. In the 19th century celestine was discovered in Leigh Court estate and the Miles family authorised quarrying. Between 1880 and 1920 Bristol was producing 90% of the world's Celestine, but the enterprise did not last long into the 20th century. Bristol Diamonds, brilliant quartz crystals found in geodes in dolomitic conglomerate in the gorge, were popular souvenirs for visitors to the Hotwells spa in the eighteenth and nineteenth centuries. Quarries on the Bristol side of the gorge are now popular with climbers and are a habitat for Peregrine falcons and