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SECTIONAL TRANSACTIONS.—C. 367 
SECTION C.—GEOLOGY. 
Thursday, September 24. 
PRESIDENTIAL ApprReEss by Prof. J. W. Gregory, F.R.S., on Problems of 
Geology contemporary with the British Association (see page 51). 
Mr. H. Dewey and Dr. 8. W. Wootpriper.—The Geology of the London 
District. 
The London Basin is nearly coincident with the Thames drainage system, and 
extends eastwards from near Cirencester to the North Sea. On the south its boundary 
may be taken as the base of the Lower Greensand escarpment of N. Hants, Surrey 
and Kent, while the northern margin extends from north of Devizes to the Wash. 
This extensive area is underlain for the greater part by rocks of Cretaceous age, the 
centre of the basin being lined with the overlying Tertiary Beds. The solid rocks 
have been folded into anelongated syncline with its greater limbs puckered into smaller 
folds, and in part faulted. Across the solid rocks the Thames and its tributaries have 
cut a series of steps or terraces, and have laid down on them beds of gravel, brickearth 
and alluvium. 
Many years ago the synclinal structure of the Cretaceous rocks led to the inference 
that a great reservoir lay under London formed of Lower Greensand beds; to tap 
the supposed reserves of water a borehole was put down at Meux’s Brewery in 
Tottenham Court Road. It failed in its object but attained an important result in 
proving the presence at no great depth of Paleozoic rocks immediately beneath the 
Gault. These rocks are Devonian or Old Red Sandstone; the boring therefore 
proved that Coal Measures are absent under that part of London. 
Subsequently about a dozen other boreholes have been made in search of the 
Lower Greensand, and have found it absent, but have proved the extension of the 
Palzozoic rocks from Essex to Bucks. At Richmond, Streatham and near Chatham 
the several divisions of the Jurassic thin out on the Paleozoic rocks. Further south 
the Palzozoic rocks have not been reached west of Bobbing, near Canterbury, the over- 
lying Cretaceous and Jurassic rocks being of great thickness. This series of boreholes 
has therefore demonstrated the long history of the London area as one of a gradually 
sinking landmass that ultimately was overwhelmed by the Gault sea and continued 
to sink beneath its load of sediment for a prolonged period. A movement of elevation, 
however, reversed the process, and the Chalk became a land surface with a south- 
easterly tilt. Before subsidence had resumed its sway great thicknesses of the chalk 
had been removed, especially in the London area. This fact is proved by the relation 
of the fossil zones of the Chalk to the overlying Tertiary Beds. 
The subsequent earth movements were not uniformly downwards but oscillating 
with the gain on the side of subsidence. Eastwards the depression was greater than 
on the west throughout Tertiary times, for the Thanet Sands die out west of Central 
London, the estuarine and freshwater Reading Beds are contemporary with the marine 
Woolwich series, the shingle banks of the Blackheath Beds do not extend far west, 
and the London Clay dies out completely and is overlapped by the Bagshot Sands 
at Great Bedwyn, Savernake, but after the Eocene the record is too incomplete to 
continue the history. Exposure of successively lower beds accompanied the depres- 
sion, so that the Blackheath Beds recline on Chalk of the Cor-testudinarium zone, 
while the Thanet Sand rests only on the higher Marsupites zone. After the long 
period of clay deposition of the London Clay sea, when presumably clay beds of 
earlier periods contributed to its formation, earlier sand formations were tapped to 
supply the Bagshot Sands, and that these sands were in part at least of the Lower 
Greensand beds is proved by the occurrence of Hythe Bed chert detritus at the base 
of the Barton Sands of Surrey. Denudation had therefore locally at least exposed 
the base of the Lower Greensand before the Alpine movements had commenced with 
vigour. The Oligocene period had long continued before the Alpine storm sent waves 
outwards that affected the London district and folded the Cretaceous and Tertiary 
Beds into the London Basin. Planation occurred in Miocene times, and possibly 
some upland plains may be remnants from this period. 
