The Kent Downs AONB covers 878 sq. km from the White Cliffs at Dover and Folkestone to the Surrey Border. The geology of the area defines the topography and landscape features that form the basis of the natural beauty for which it is nationally recognised. A large arch of chalk, shaped like an eyelid, sweeps across Kent. This creates a central ridge through the county, and forms the most important geological feature of the AONB. This chalk ridge is part of the same formation found in Sussex, Hampshire and Surrey, and it also reappears across the Strait of Dover in northern France. In addition, the highest areas of the Greensand ridge in Kent, together with a spit of ragstone escarpment above the Romney Marsh, are also within the Kent Downs AONB.

Some of the most important physical features of the AONB include:

• One of south east England's highest points at 250 metres on the Sevenoaks Greensand ridge;

• Three main river catchments, which cut through the AONB, draining in a south to north direction, namely the Rivers Darent, Medway and Great Stour (including the Nailbourne or Little Stour with its source deep in the east Kent Downs);

• The dramatic chalk cliffs where the chalk reaches the English Channel. The whole of the Dover and Folkestone White Cliffs are defined as Heritage Coasts;

• The folded and undulating chalk, Greensand and ragstone escarpments rising high above the low Weald and Romney Marsh below.