Geology
Geologically Portsdown Hill is an anticline in Upper Cretaceous chalk. The chalk that makes up Portsdown was laid down between 100-65 million years ago. The chalk is part of a continuous strata that appears to the north were it forms the South Downs and to the south where it forms the chalk outcrops on the Isle of Wight. It is overlain by more recent deposits such as Reading beds and London clay. After the last ice age soil developed as a result of forest that once covered the area. This layer of soil gradually slipped down slope to form the coombe deposits after the trees were cleared by early man.
