Geoscience Reference
In-Depth Information
Landscape D: Early Cretaceous bedrock
The tops of some of the distinct hills in the southeast corner of Area 9 contain small
areas with thin layers of Early Cretaceous material. This situation is discussed more
fully in the treatment of Areas 10, 11 and 13; in Area 9 these are very small features of
the landscape.
Younger drainage and erosion patterns
The discussion of this Area concludes with a review of the present drainage systems
(Fig. 174) and their Tertiary and Quaternary evolution, because it is largely the rivers
that have been responsible for the shape of the present landscape.
FIG
174.
Main river pathways and coastal flooding zone of Area 9.
The National River Flow Archive gives us insights into the present drainage pat-
tern by providing estimates of mean flow rates (in m
3
/s) for the main rivers, together
with figures for the upstream areas (in km
2
) being drained by the rivers in question.
Here I have selected some of these flow data to demonstrate the present-day import-
ant role of the River Severn and its major tributary, the Avon. The very largest mean
river flow rate is 106 m
3
/s at the downstream end of the Severn-Avon system. This
station has a catchment area of 9,900 km
2
, which reflects its drainage of large parts of
central Wales and the Midlands. For comparison, the Thames at Kingston has a mean
river flow rate of 66 m
3
/s from a catchment of very similar size (9,948 km
2
), reflect-
