The Rocks of Portsdown Hill

Portsdown Hill is a ridge of pure white chalk, which is surrounded by the clays and gravels of younger strata. This chalk is almost pure calcium carbonate, and is biogenic - it is composed of the calcite shells and skeletons of organisms, most of which are microscopic. Calcium carbonate is one of the major rock-forming minerals of the world; the other being silicates of various compositions.
Chalk is widely used in manufacturing, as well as forming much of the basal layers of the local roads, such as the M27 which runs in front of the Hill.

The Chalk of the Hill is frequently studded with flint, in either sheets or layers of nodules. This flint is again biogenic, being the result of dissolved silica from sponges and the skeletons of silica-based micro-organisms.
Flint has been used since Neolithic man made tools and weapons, and more recently as a hard facing on buildings.

Portsdown Hill is thus composed of the two major rock-forming minerals of the world. However, it is unusual and particularly interesting because both are almost entirely biogenic in origin.

The following section covers:

• An overview of the chalk - what it is made of and its various properties, and the conditions in which it was deposited.
• A description of the microscopic coccoliths which are the principle component of chalk.
• An overview of the flint - what it is made of and its various properties, and the conditions in which it formed.
• A description of the sponges which produced most of the silica that led to the formation of flints.

Please begin by reading about the chalk, return Home, or use the navigation links on your left.

For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk