The British Geological Survey map of the valley shows the rock layers from the top of the hill in the sequence:
forest marble limestone
Bath oolite limestone
Combe Down oolite limestone
Fuller’s Earth calcareous mudstone
This group is described as a set of sedimentary bedrocks laid down in Jurassic period (165 million years ago) under shallow carbonate seas.
The mini cliff that runs through the woods seems to correspond to the transition between the oolite limestones and the Twinhoe seam. Any geologists out there, please correct me!