east. This is bordered by slightly higher and older terraces often extending several miles from the floodplain, for example in Hounslow and Southwark. Other significant river valleys include those of the Colne, Crane, Brent, Lea (with a floodplain more than a mile wide in places), Wandle and Ravensbourne, which run north and south towards the Thames. There are a few notable hills in Greater London, but none of them more than a few hundred feet high, and they have not impeded the development of the city in all directions. It is therefore very roughly circular. The hills in the City of London The hills in the City of London, from west to east, Ludgate Hill, Corn Hill and Tower Hill, are presumed to have influenced the precise siting of the early city, but they are very minor, and most of central London is almost flat. These hills are developed in various gravel terrace deposits of the river Thames. North London To the north of the city a ridge capped by the mid-to-lower basin's residual sands known as the Bagshot formation forms high ground (in places around 130m) including Hampstead Heath and Highgate Hill. The ridge continues briefly eastwards over the London clay to form Crouch Hill and Queen's Wood. Immediately south, fingers of the ridge run down towards Primrose Hill and Parliament Hill. This ridge is a surviving area of Tertiary rocks younger than the London Clay, surrounded by former routes of the Thames where much younger deposits overlie the clay. Smaller outliers of such Tertiary high ground exist to the west of the main ridge especially relatively lowly Hanger Lane; the knoll Harrow Hill, and smaller Horsendon Hill where the Claygate Beds (the top of the London Clay formation) are capped by much younger gravels deposited by watercourses. North of this ridge, between the valleys of the rivers Lea and Brent, lies a second ridge (a little under 100m), formed of much younger Pleistocene deposits. Running west from Muswell Hill to Church End, Finchley and north to Whetstone, this is capped by glacial till marking the southern limit of glaciation. This overlies Dollis Hill Gravel marking the line of a former southern tributary of the Thames, when the Thames flowed further to the north than it does today, through the Vale of St Albans. At its eastern end at Alexandra Palace the gravel forms the top of the ridge,