LOWER CHALK—OXFORD AND BECKS. 
173 
CHAPTER XIV. 
THE LOWER CHALK IN OXFORDSHIRE AND 
BUCKINGHAMSHIRE. 
General Description. 
The Lower Chalk emerges from beneath the Melbourn Rock 
near Spring Farm, north of Goring, and borders the alluvium of 
the Thames, though partly covered by gravel, for a distance of 
nearly five miles, as far as Crowmarsh, opposite Wallingford. Thence 
its outcrop trends north-east at the foot of the main chalk escarp¬ 
ment, and it occupies a tract which varies from one to two miles in 
width by Ewelme, Watlington, Aston, Chinnor, Risborough, Wend- 
over, and Tring, which last place we shall include in the district, 
although it is actually in Hertfordshire. 
The composition of the Lower Chalk in these counties is the same 
as in Berkshire, and the total thickness in Oxfordshire is about 
200 feet; in Bucks, however, it is probably not more than 180 feet, 
and in some places it seems to be even less than this. 
The zone of Ammonites varians (or Chalk Marl) ma}^ be 120 feet 
thick in Oxfordshire, but is not more than 100 feet in Bucks. It 
is based on a soft glauconitic marl, which appears to form a complete 
passage between the Chalk Marl and the underlying Greensand 
(see vol. 1, p. 283). A special feature of the zone in this area is the 
occurrence of a bed of very compact limestone, about 50 or 60 feet 
above its base ; for this bed the name “ mail-rock ” has been sug¬ 
gested, * and it is traceable through the greater part of the district 
owing to the fact that strong springs are thrown out along its course 
because it rests on a tenacious argillaceous marl. 
The zone of Holaster subglobosus has a thin but persistent 
bed of Totternlioe Stone at its base. In Oxfordshire this is only 
from 2 to 4 feet thick, but it becomes thicker toward the north¬ 
east, and near Tring there are sometimes two beds with 5 or 6 feet 
of sandy marl between them. 
The chalk which forms the rest of the zone is from 70 to 80 feet 
thick, consisting of grey and greyish chalk in the lower part, chang¬ 
ing in the higher part to nearly white chalk. 
In Bucks a peculiar bed of rough grey chalk occurs about 30 or 
35 feet above the Totternhoe Stone, which stone it much resembles 
in general appearance. It is locally known as “ rag,” is about 2\ feet 
* The Geology of London, Vol. i. p. 59, Mem. Geol. Survey} 1889. 
