The oldest rocks proved through boreholes to exist below London are the old, hard rocks of the Palaeozoic and the Palaeozoic rocks dip southwards and are more than 1,000 metres below the English Channel also, above this is a 60-metre thick layer of impermeable Gault clays.
The oldest rocks proved through boreholes to exist below London are the old, hard rocks of the Palaeozoic. The Palaeozoic rocks dip southwards and are more than 1,000 metres below the English Channel. Above this is a 60-metre thick layer of impermeable Gault clays. 
