Chalk and Clay
Chalk and Clay
Formed 70 to 100 million years ago, chalk is also called cretaceous limestone.It is a soft, white rock.
Chalk is an example of a porous rock, as it has pore spaces, which can store water. It does not have joints and bedding planes like carboniferous limestone.
Is a product of chemical weathering and river erosion.
Clay is porous, but becomes impermeable when wet, as the particles expand and fill the pore spaces.
The main areas of chalk and clay in this country are in the South and East of the country. Places like the North and South Downs are good examples.
- Bournes are streams that occasionally flow down the dry valleys in times of prolonged wet weather, when the ground may have become saturated.
- Clay vales are the valleys between the chalk escarpments. The clay, when drained is a fertile soil suitable for a range of farming methods. Clay vales are flat, and have a number of streams meandering through them.
- Dry valleys, such as Devil's Dyke, were formed in periglacial times, when the ground froze, so melt water rivers ran over the surface of the chalk rather than flowing down through it. These rivers carved out steep sided valleys. Once the climate had warmed again a dry valley was left behind. These also are common features of limestone landscapes.
- Escarpments or cuesta's are the main landform of chalk and clay areas. Initially the layers of chalk and clay were tilted by the collision of the African and Eurasion plates. The soft clay was then eroded faster than the more resistant chalk, leaving escarpments (chalk hills) behind. Because of the angle of the tilt, these escarpments have two distinctive sides. The steeper side is called the scarp slope, whilst the gently sloping side is called the dip slope.
- Springs form at the bottom of the escarpment, where the chalk meets the clay. This is why many settlements can be found along spring lines in chalk and clay areas.
Clay is very fertile, but must be drained first. Once that has been done farming includes dairying, sheep grazing, and some arable farming. On the chalk escarpments the main agriculture is sheep grazing.
Many settlements were built at the bottom of the scarp or dip slope, as the land was less likely to flood, there was a good water supply, and there was good farming land nearby. Very early settlements would have been higher up the chalk escarpment for defensive purposes.
Chalk is a main ingredient in cement making, and is quarried for that purpose. Clay can be used in pottery.
Underground aquifers act as a store for water within the chalk and are used as a natural water supply for London.