North Downs - Geology

Geology

The Downland of the North Downs consists of distinct lithostratigraphic units (the types of strata, the names of which are always capitalized):

• The more level tops of the downs are often covered by acidic strata, usually including a layer of Clay-with-Flints, a sandy clay with many flints, or various sands and gravels (note: these are not shown on the geological map and section in this article).

• The Chalk Group, composed almost entirely of chalk, a kind of soft fine-grained limestone. It is formed of three parts, the Upper Chalk, which has many flints, the Middle Chalk, with fewer flints, and the Lower Chalk or Coombe Rock, greyish, with few flints. The chalk is most commonly exposed on slopes or as cliffs, where the overlying acidic strata have been quarried or washed away. The buried upper surface of the chalk beneath the acidic strata is often eroded into pipes, gulleys and pinnacles, sometimes visible in road cuttings and quarries.

• The Upper Greensand Formation, a whitish, limy sandstone, often used for building, for which it has been mined from beneath the chalk (for example from the Godstone Baby Mines). The Upper Greensand of the North Downs is a thin bed of one or two metres thickness, and it is rarely visible at the surface (it is much thicker elsewhere).

The Upper Greensand marks the southern edge of the Downs, being underlain by:

• The Gault Formation of stiff blue clay.

• The Lower Greensand Formation of Lower Cretaceous age, containing greensand, a glauconite sand or sandstone, as well as a certain amount of silts, clays, ironstone and limestone.

The topography of the North Downs mainly consists of the Chalk Group, the rock strata of the Upper Cretaceous period which in certain areas is overlain by superficial deposits of gravels or clay-with-flints.

Citing Dr D. T. Aldiss of the British Geological Survey:

The Greensand Ridge is separate from the Downs. Again, one has to be aware of the distinction between 'greensand' (typically glauconitic sand or sandstone; literally 'green sand') and 'the Greensand', a lithostratigraphic term which usually refers to the Lower Greensand Group (of Lower Cretaceous age). The Lower Greensand does contain some greensand, but also much silt, clay and limestone: most of it is neither green nor sand. It forms a distinct layer below the Gault Formation and the Upper Greensand Formation which directly underlie the Chalk Group. The 'Greensand Ridge' typically refers to one of a series of escarpments formed by the Lower Greensand. In Surrey, the Upper Greensand is thin and is not separately marked by rising ground, but elsewhere (in Sussex and Berkshire, for example) it too forms an escarpment. These groups and formations each occur in separate layers. In Surrey these dip northwards, generally at an angle of 2 degrees or less but increasing to as much as 55 degrees in the Hogs Back area, west of Guildford.