A HISTORY OF BEDFORDSHIRE 
and the plant, Exdogenites erosa, and remains of Iguanodon from Potton, 
have been derived from the Wealden as suggested by Mr. Walter Keeping.’ 
In all probability however the deposition of the Wealden strata over the 
south-east of England, which must have occupied a great length of time, 
took place whilst the whole of Bedfordshire was above sea-level, and 
therefore whilst the Kimeridge Clay was subjected to sub-aerial denu- 
dation, by which its higher beds, with the overlying Portlandian, were 
removed. 
During the long period occupied in the accumulation of the Creta- 
ceous strata there was a nearly continuous and usually gradual change 
from comparatively shallow seas to those which were much deeper if not 
truly oceanic, occasionally interrupted by temporary elevations of the 
sea-bed. These alterations in the depths of the marine areas were 
accompanied by important changes in the character of the fauna. Many 
of the earlier Cretaceous forms became extinct and were superseded by 
others of newer types, noteworthy amongst which are the echinoderms 
Holaster, Galerites, and Maicraster ; the lamellibranch Hippurites; the 
cephalopods Hamutes and Scaphites ; the fishes Lamna, Otodus, and Pty- 
chodus; and the reptile Po/yptychodon; species of all of these genera 
having been recorded for the county. 
LOWER CRETACEOUS 
Vectian or Aptian. Lower Greensand. Zone of Terebratula sella 
Owing to the absence of beds of Wealden age the Lower Creta- 
ceous rocks are in this district represented only by the Lower Green- 
SAND, which is the equivalent of the higher division of the Vectian of the 
south of England and the Aptian of western Europe; the lower division 
consisting in the Wealden area and the Isle of Wight of the Atherfield 
Clay, a very fossiliferous marine deposit having no representative in 
Bedfordshire. 
The Lower Greensand extends across the county from Leighton 
Buzzard to Potton, forming a very picturesque range of hills running 
generally from south-west to north-east. For some portion of its extent 
it is concealed by the boulder clay and alluvium of the valley of the 
Ivel, and on reappearing it attains a considerable elevation in the vicin- 
ity of Sandy, having there a rather steep and very prominent escarp- 
ment. Its maximum thickness is about 220 feet and its exposure is 
several miles in width, the area it covers being more extensive than 
in any other of the midland counties. In passing under the newer 
strata in a south-easterly direction it thins out rapidly, terminating in 
the adjoining county of Hertford somewhere between Hitchin and Ware. 
It was the absence of the knowledge of this fact which led the late Sir 
Joseph Prestwich to believe that it might form a valuable water-bearing 
stratum for the supply of London ; but although he was mistaken on this 
point, owing to no boring having at that time (18 51) been carried to a 
* In Geol. Mag. dec. 2, vol. ii. p. 375. 
Io 
