The Avon Gorge (grid reference ST560743) 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 1.5 miles (2.4 km) 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.