SKETCH OF THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 17 
to the Lias of Lyrae Regis or Whitby, till a Pecten pollux or other 
characteristic fossil reveals their true age. To the north-east of 
Durdham Downs, to the north of Chipping Sodbury, on Broadtield 
Down to the west of Dundry, near Shepton Mallet, and above the 
Harptree ridge on Mendip, to mention but a few localities, Liassic 
strata rests unconformably upon Mountain Limestone without any 
intervening Trias or Rhsetic. Thus the archipelago sank, and only 
the upper part of the Mendip Isle remained above the water, not to 
be completely submerged till the Inferior Oolite times, even if then 
it had wholly disappeared beneath the Jurassic waters. 
Owing to the diversities of the physical features of the sea in which 
they were deposited, the Liassic ^ strata vary much in volume and 
distribution. The finest sections of Liassic strata in the Bristol 
district are those of the Sodbury railway cutting, now unfortunately 
earthed in and invisible, of the Keynsham quarries and others near 
Stinchcombe, around Bath, and in the Radstock district. Through- 
out the central and southern parts of the area the Lower Lias beds 
are both thicker and better developed than the Middle or Upper 
Lias ; and owing to the fact that they contain hard bands they are 
far more frequently quarried than are the higher beds. But towards 
the northern part of our area in the Wootton-under-Edge and Dursley 
districts the Middle Lias includes the hard beds of earthy limestone 
or calcareous sandstone known as the Marlstone, and this is frequently 
quarried. 
Below the normal Lias strata come the beds known as the White 
Lias series, consisting of an upper relatively persistent band of cream 
coloured limestone, the “ Sun Bed,” and a lower equally persistent 
band, the well known Gotham or Landscape marble. Between these 
two is a series of thin-bedded limestones, which while having a thick- 
ness of 5 feet in the Radstock district dies out as one passes 
northward from Bristol. 
The most noteworthy point about the Lower Lias of our area is 
the decrease in thickness and increase in calcareous character as one 
passes southwards. At Leckhampton hill, Cheltenham, the Lower 
Lias is from 600 to 700 feet thick ; in the Sodbury section it is about 
200 feet thick to the top of the Capricornus zone, and at both 
localities it is predominately shaly. Near Keynsham only part of 
the Lower Lias can be seen, but such zones as are exposed have 
only half the thickness of the corresponding strata at Sodbury, while 
Tawney’s well-known descriptions show that the thickness at Radstock 
is about half that at Sodbury, the whole series up to the Jamesoni 
beds being only about 10 feet thick. The Keynsham development 
is far more calcareous than that at Sodbury, while that at Radstock 
^For the Lias see H. B. Woodward, “Mem. of Geol. Survey”, ‘The 
Jurassic Rocks of Britain, vol. S, the Lias of England and Wales (York- 
shire excepted)’; C. Moore, ‘ Q. Journ. Geol. Soc.’, XXIII. (1867), p. 450; 
E. B. Tawney, ‘ Proc, Bristol Nat. Soc.’, Series 2, i. (1874-6), p. 167; and 
more recently papers by A. Vaughan, J. W. Tutcher, and others in the 
‘ Q. Journ. Geol. Soc.’, LV^III. (1902), p. 719; ‘Proc. Bristol Nat. Soc.’, New 
Series X., pt. 1 (1903 issued for 1901), p. 3. 
B 
