The narrow spine of the Hog's Back between Farnham and Guildford forms the western extremity of the North Downs whilst the cliffs between Folkestone and Deal terminate the ridge in the east. There are two distinct aspects, the steep south facing escarpment and the gentle north facing dip slope. The southern boundary is defined by the foot of the escarpment which gives way to the flat, broad clay lands between the Downs and Greensand Ridge known as the Vale of Holmesdale. The northern boundary is less apparent but occurs where the chalk submerges below the more recent Paleocene deposits.
The Downs are highest near the Kent-Surrey border, often reaching heights in excess of 200 metres above sea level at the crest of the escarpment. The highest point is Botley Hill in Surrey at 269 metres (885 ft). The highest point in London, Westerham Heights, at 245 metres (804 ft), is located nearby. East of the Medway Valley the Downs become broader and flatter, extending as far as the Isle of Thanet.
The ridge is intersected by the valleys of a series of rivers: the Wey, Mole, Darent, Medway and Stour rivers. These drain much of the Weald to the south, the western ones are tributaries of the Thames; they carve steep valleys through the chalk and provide natural corridor routes. In addition to existing rivers, the Downs are crossed by a number of wind gaps – fossil river valleys no longer occupied by rivers, including those at Farnham, Betchworth, Caterham, Lyminge and Hawkinge. Except for the river valleys and wind gaps, the crest of the escarpment is almost continuous along its length. The dip slope is dissected by many small dry valleys, and in the broad eastern part in Kent, by further river valleys such as that of the Little Stour. Leith Hill is sometimes incorrectly referred to as part of the North Downs but is located on the parallel Greensand Ridge and does not consist of chalk.
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