File:The Quarterly journal of the Geological Society of London (12684253905).jpg

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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .Apr. 21,
General considerations respecting the distribution of land and
water in the English Tertiary area at the commencement of the
Eocene period. — In no part of the tertiary area of the south of
England is there any indication of a passage either in mineral struc-
ture or in organic remains between the Chalk and the Tertiary seizes.
With regard to the physical conditions, the change is most marked,
and abrupt. Extensive and deep wear of the chalk evidently took
place before the commencement of the lowest Eocene deposits. In the
neighbourhood of Calais, at St. Vallery-sur-Somme, Pegwell Bay,
Upnor, Woolwich, Stortford, and thence to Reading, Newbury, Salis-
bury, Newhaven, and Alum Bay, the chalk invariably presents a worn
though not very irregular surface, and is strewed over, as before men-
tioned, with those peculiar green-coated flints. This mass of flints,
although generally not above 1 to 2 feet thick, in itself indicates a
wide destruction of the chalk. But independently of this, another
denudation had probably previously worn down a very large mass of
the chalk along the southern boundary of the London tertiary area.
In this direction all the upper beds, and a large portion of the middle
beds of the chalk, have been removed ; and it is probably owing to
this cause that the chalk, which has been ascertained to be above
1000 feet thick at Saffron Walden, and is apparently about 800 to
900 feet thick at Luton and along its northern line of escarpment,
becomes apparently gradually thinner as it ranges towards London,
and eventually becomes reduced at the edge of the escarpment over-
looking the Weald to a thickness not exceeding on the average 400
feet *. The following diagram (fig. 6), which gives the general re-
presentative section, independently of the exact conformation of the
surface, from Saffron Walden to the chalk-escarpment above God-
stone, exhibits the structure of the chalk here referred to : —
Fig. 6. — Diagram to illustrate the thinning out of the Chalk in the
direction of the axis of the Weald.
A. Relative position of Saffron Walden.
B. Chalk escarpment above Godstone, sur-
mounted with a patch of the Lower Ter-
tiary beds.
a. London Clay.
b. Lower Tertiaries.
e. Chalk,
d. Upper Greensand.
e. Gault.
/. Lower Greensand and Wealden.
X. Point at which the present upper and under sur-
faces of the Chafk, if they were prolonged,
would converge. « 

• At Bushey near Watford the chalk-marl was reached at a depth of less than
  

400 feet beneath the tertiaries, and at London the lower chalk without flints com-