GEOLOGY 
arctic current set in froma northerly direction, from its coldness tra- 
versing the lower part of the sea and effecting the extinction, at least 
locally, of many of the Mollusca, as only a few species survived the 
interval which elapsed between the deposition of the Lower and Middle 
Chalk. Mr. Jukes-Browne has identified the following fossils found in 
this bed between Luton and Leagrave: Rhynchonella cuvieri, R. plica- 
tilis, Ostrea vesicularis var. baylet, Ptychodus decurrens, Lamna gracilis, and 
a crustacean of the genus Lepas. 
The escarpment of the Lower Chalk forms the most striking 
physical feature of South Beds as well as of the adjoining counties 
through which it passes. In most of its course it rises abruptly from 
the plain lying at its base, often to a height of 200 feet above it, 
clothed only with closely-cropped herbage. Occasionally the monotonous 
though not unpleasing rounded contour ot the hills is relieved by groves 
of trees, mostly beech, the best illustrations of which are in the parishes 
of Sundon and Streatley. In several places the escarpment is intersected 
by the remarkable ‘ coombes’ which are a striking feature of this range 
of hills. These have been formed mainly by the erosive action of springs 
which in former times were much more copious than they are now, and 
also issued at higher levels. ‘This process of erosion still continues, but 
on a greatly reduced scale, the Barton and Streatley valleys furnishing 
examples. On the eastern side of Barton there originates a narrow pre- 
cipitous valley which describes a semicircle in the heart of the hills. 
It starts from the base of the north-west angle of Ravensbury Castle (a 
Roman camp), takes a southerly curve, then turns south-easterly, and 
finally debouches in a north-easterly direction in the parish of Hexton, 
Herts. ‘The spring which has been the prime factor in the formation 
of the coombe is known as ‘Burwell,’ and now originates at the angle 
at which the valley is deflected from an easterly to a north-easterly 
course. At Pegsdon, on the eastern confines of the county, there is a 
peculiar coombe designated ‘Pegsdon Barns,’ the sides of which are as 
smooth and sharply defined as though they had been cut by human 
agency. When this valley is approached from the northern end of 
Lilley Hoo it opens out to the observer with startling suddenness. 
Many springs formerly existed at the junction of the Lower with 
the Middle Chalk at Houghton Regis, Leagrave, Limbury, and Biscot, 
but they are now greatly reduced in number and volume. 
The Mexzourn Rock, which forms the base of the Middle Chalk, 
has been traced in the neighbourhood of Dunstable, Houghton Regis, 
Sundon, and Barton. ‘There are outliers of it on Totternhoe Knoll. 
Sections are exposed in chalk-pits about a mile south-east of Leagrave, 
and between that village and Sundon. Its thickness in the county is 
about 10 feet. It is an impure limestone having in its lower part many 
well-defined nodules which are often greenish-grey in colour, especially 
after weathering. Some of these nodules have on their surface shells of 
immature Ostree, which suggests that they were exposed on the soft 
ooze of the sea-bed for a considerable period before they were covered 
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