SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 19 
under-Edge, it is far thinner than in Dorset. It was formerly much 
used in fulling at the cloth mills of Bradford-on-Avon and elsewhere. 
Owing to its impervious character it throws out many springs and 
has caused numerous slips on the slopes of the Cottes wolds. 
The succeeding Great or Bath Oolite which caps the hills about 
Bath is well known for the excellent freestone long quarried at 
Minchinhampton, Bath, Box, Bradford-on-Avon, etc. In the Sodbury 
tunnel it has a thickness of about 100 feet and may reach a greater 
thickness near Bath, but to the south of Bradford-on-Avon it rapidly 
thins out and disappears, so that the succeeding Forest Marble rests 
directly on the Fuller’s Earth. 
The Forest Marble, which is finely exposed in the cutting eastward 
from Badminton on the South Wales direct line, consists of marly 
clays associated with irregular bands of shelly oolitic limestone 
having a thickness of about 100 feet. When followed to the north 
the Forest Marble becomes reduced in thickness, while in Dorset on 
the other hand it swells out to a thickness of 400 feet. Near 
Bradford and Bath the well-known Bradford Clay having a thickness 
of 10 feet or less underlies the Forest Marble. 
The Cornbrash, the highest member of the lower Oolites, consists 
of pale earthy and rubbly limestones. It is highly fossiliferous, and 
near Hullavington Station has a thickness of 11 feet. 
The Upper Oolitic and Cretaceous strata can scarcely be considered 
to enter the Bristol district as defined at the outset, but reference 
must be made to the oolitic ironstone which till recently was worked 
in the Corallian at Westbury. 
With the close of Cretaceous times the second prolonged period 
of depression and sedimentation forming the third chapter in the 
geological history of the Bristol district comes to an end. 
With the beginning of Cainozoic or Tertiary times the whole 
district has been elevated to form dry land, and the second period 
of prolonged denudation is initiated. This final re-emergence must 
probably itself have been accompanied by marine denudation. It 
must be remembered that the uplift of the area from beneath the 
waters of the Chalk sea was not sudden but gradual. As the oozy 
bottom was slowly raised to the surface it was played upon by the 
waves, which would readily erode the soft layers of recently 
deposited and little consolidated material. We may picture the battle 
of contending forces. Those of upheaval were striving by steady 
uplift to raise the sea-bottom into dry land. Those of marine de- 
nudation eroded the o^zy deposits as they came to the surface. 
Layer after layer was lifted to the sea level. Layer after layer 
was swept away by the breakers. What eventually decided 
the struggle and gave to upheaval the final victory, it is im- 
possible to say. It is not improbable that over the Mendips 
marine denudation, during the process of upheaval, held its own 
until the hard core of the old range was laid bare. But these 
eonclusions are largely conjectural. We know that the uplieaval, 
then or somewhat later, took place in such a way as to give to the 
