Estuary ending at (but not including) the islands of Flat Holm (in Cardiff, Wales) and Steep Holm. This "seaward extension" can be traced back to the original boundary of the County of Bristol laid out in the charter granted to the city by Edward III in 1373. The Office for National Statistics (ONS) has defined a Bristol Urban Area, which includes developed areas adjoining Bristol but outside the city-council boundary, such as Kingswood, Mangotsfield, Stoke Gifford, Winterbourne, Almondsbury, Easton in Gordano, Whitchurch village, Filton, Patchway and Bradley Stoke, but excludes undeveloped areas within that boundary. Geography Bristol lies within a limestone area running from the Mendip Hills in the south to the Cotswolds in the northeast. The rivers Avon and Frome cut through the limestone to the underlying clay, creating Bristol's characteristically hilly landscape. The Avon flows from Bath in the east, through flood plains and areas which were marshes before the city's growth. To the west the Avon cuts through the limestone to form the Avon Gorge, formed largely by glacial meltwater after the last ice age. The gorge, which helped protect Bristol Harbour, has been quarried for stone to build the city, and its surrounding land has been protected from development as The Downs and Leigh Woods. The Avon estuary and the gorge form the county boundary with North Somerset, and the river flows into the Severn Estuary at Avonmouth. A smaller gorge, cut by the Hazel Brook which flows into the River Trym, crosses the Blaise Castle estate in northern Bristol. Bristol is sometimes described, by its inhabitants, as being built on seven hills. From 18th century guidebooks, these 7 hills were known as simply Bristol (the Old Town), Castle Hill, College Green, Kingsdown, St Michaels Hill, Brandon Hill and Redcliffe Hill. Other local hills include Red Lion Hill, Barton Hill, Lawrence Hill, Black Boy Hill, Constitution Hill, Staple Hill, Windmill Hill, Malborough Hill, Nine Tree Hill, Talbot, Brook Hill and Granby Hill. Bristol is west of London, south-southwest of Birmingham and east of the Welsh capital Cardiff. Areas adjoining the city fall within a loosely defined area known as Greater Bristol. Bath is located south east of the city centre, Weston-super-Mare is to the south west, and the Welsh city of Newport is to the north west. Climate The climate is oceanic (Köppen: Cfb), milder than most places in England and United Kingdom. Located in southern