London and the Thames Valley Region - Southern England - page 214

FIG 200. Main river pathways, flow rates and coastal flooding zone for Area 11.
South of the Lower Greensand ridge, earlier Cretaceous bedrock has again influ-
enced the topography, particularly towards the centre of the Weald Uplift. Here the
tough, sandstone-rich Hastings Beds create more hilly topography, surrounded by the
more readily eroded Weald Clay. As described more fully in Area 6, the hills here are
prominent, but lack the linear pattern of the Lower Greensand ridge to the north.
Landscape C: The Chiltern Hills
The Chilterns are the northeasterly continuation of the Marlborough and Berkshire
Downs (Landscape C of Area 10). The Chalk bedrock that has produced the Chilterns
slopes gently downwards to the southeast beneath the London Basin and has been
eroded to produce distinctive features in the Chiltern scenery. The first of these is the
Chiltern Edge ( c1 ), which faces to the northwest and results directly from the greater
strength of the chalk material, in the face of river erosion, compared with the older and
weaker underlying mudrocks. Southeast of the scarp, the Chiltern plateau ( c2 and c3 )
dips very gently to the southeast and is divided into distinct sectors by a remarkable
series of branching, largely dry valleys, picked out beautifully in Figures 201 and 208.
These dry valleys were formed during Ice Age times when freezing of the ground mo-
bilised the near-surface Chalk and prevented water from soaking away, creating sur-
face streams that cut downwards into the plateau. Railways and roads heading northw-
est from London make good use of these valleys to take travellers across the Chilterns,
avoiding the steepest slopes and highest ground.