Geoscience Reference
In-Depth Information
FIG
7.
Bedrock timescale for Southern England.
The rocks at, or just below, the surface of Southern England range in age over
hundreds of millions of years, and most of them were formed long before the present
scenery began to appear. At the time of their origin, these rocks were deposited in a
variety of different environments, mostly when mud or sand materials were transported
into and/or around the seas that existed where England is now. Most of the bedrock of
Southern England was formed in this way and is said to be of
sedimentary
origin. The
depositional conditions varied from time to time: the climate varied, the geographical
pattern of rising and sinking land movements changed, and the supply of mud and sand
brought downstream by rivers changed also. Despite these fluctuations, it is possible
to generalise the way that sediment has accumulated over an area the size of Southern
England, and to offer a succession of layers of different composition, age and average
thickness that can provide a general guide. This is shown in Figure 8.
For each of the Regions (and some of the Areas) discussed in Chapters 4 to 8, a
rock column, generalised for that particular area, will show the main bedrock layers.
Each column will be coloured using the standard colour codes of this topic to represent
the ages of the layers.
As an example, we will consider another particularly distinctive layer of bedrock,
the Chalk, which ranges between 200 and 400 m in thickness. Chalk is visible quite
widely at or just below the surface over perhaps a quarter of the area of Southern Eng-
land (Fig. 9). Chalk is an easily recognised rock because it is made of very small frag-
ments of lime (calcium carbonate) and is usually brilliant white. It formed from fine-
grained limey mud deposited on the sea bed, but through many millions of years of
burial below other sediments it has been compressed and altered into the hard rock we
recognise today. The Chalk is a result of a unique combination of environmental con-
ditions and the presence of particular algal organisms in the history of evolution. It is
only found in northwest Europe, and was only formed in Late Cretaceous times.
