A HISTORY OF BEDFORDSHIRE 
abundant. Right across Bedfordshire, from Leighton to Potton, the 
Lower Greensand covers much of the upper portion of the Oxford Clay 
as well as the succeeding Ampthill and Kimeridge Clays, overlapping 
these formations, and more to the north the lower portion is overlaid by 
boulder clay. This makes its precise limits difficult to ascertain in some 
districts, but it is known to extend over the greater part of North Beds. 
There are many brickfields in it, and where boulder clay overlies it an 
admixture of the two clays is found to be advantageous in brick-making. 
Boulder clay over Oxford Clay makes a very tenacious soil, retentive of 
moisture and well suited to the growth of corn. 
The AmpTuiLt CLay was formed under much the same conditions 
as the Oxford Clay below it, and the Kimeridge Clay above, and consti- 
tutes a passage-bed between them, having an admixture of their fossils. 
Being a comparatively deep-sea representative of the Corallian beds, 
which owe their origin to the destructive action of the sea on coral-reefs, 
it is frequently called Corallian, but the term is scarcely applicable. The 
two life-zones of the Corallian stone-beds are very distinct, but although 
both are probably present in the Ampthill Clay, they cannot be dis- 
tinguished in either Beds or Cambs owing to the Oxfordian and Kime- 
rigian fossils being mixed. The Ampthill resembles the Oxford Clay in 
not being a bed of clay only. In the cutting north of Ampthill railway 
station it consists of a thin septarian band with Ostrea discoidea in and 
beneath it ; 50 feet of marly shale and stiff clay, with selenite, a layer 
of calcareous nodules with Ischyodus, and a rusty band with Ammonites 
cordatus and other fossils ; a thin band of pale earthy limestone ; 6 feet 
of grey and yellow marly shale; anda rubbly rock-bed at the base, 4 ft. 
6 in. thick, and containing numerous fossils ; the whole (given in de- 
scending order) resting upon Oxford Clay and being surmounted by 
Kimeridge Clay.’ The outcrop of the Ampthill Clay is concealed for 
the most part by the overlapping of the Lower Greensand. 
In this section at Ampthill there appears above the Ampthill Clay 
a bed of clay and dark-blue shale, 10 feet thick, with Ammonites biplex, 
Ostrea deltoidea, and other fossils, which is of KimeripGr CLay age, and 
has boulder clay resting unconformably upon it. The Kimeridge Clay 
must here have suffered very considerable denudation. Elsewhere it 
usually varies from 400 to 1,000 feet in thiekness, but near Aylesbury it 
is not more than 100 feet thick. Here only its base is seen, the higher 
beds having been washed away, together probably with the succeeding 
Portland Beds, of which the only trace in the county consists of fossils in 
the Lower Greensand which have been derived from Portlandian strata. 
It was probably the presence of these fossils which led Professor A. C. 
Ramsay to say that outliers of the Portland Stone occur in Bedfordshire 
rightly adding that ‘the whole has evidently been exposed to denudation 
before the deposition of the Cretaceous rocks.’? 
+ Jurassic Rocks of Britain, v. 135. 
* Physical Geolgy and Geography of Great Britain, ed. 5, p. 191. 
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