GEOLOGY 
THE BREAK IN THE SEQUENCE OF THE JURASSIC AND CRETACEOUS 
STRATA 
It will be seen from the foregoing that although the Kimeridge Clay 
in this district is in immediate contact with the Lower Greensand, the 
two formations are in point of time widely separated; but the slight 
unconformability between them gives very little indication of the great 
changes which occurred in the distribution of land and sea, and of the 
vast amount of sediment which was elsewhere deposited between the 
close of the Jurassic period in this district and the commencement of 
the Cretaceous. 
With the increasing depth of the Jurassic sea, the western margin 
of the land of the south-east of England was gradually encroached upon ; 
but it was not until Cretaceous times that the whole of this area was 
completely submerged, and it is doubtful how far until then the sea 
covered that portion of it which is now known as Bedfordshire. To- 
wards the close of the Jurassic period the sea became shallower; but the 
Portland Beds, consisting mainly of sands and limestone, appear to have 
been laid down on the whole in clear water at some distance from the 
estuaries of rivers. The Portland sea probably extended over part of the 
county, and on the upheaval of the sea-bed its sediment would form the 
surface of the land, constituting a plain flanking the Paleozoic hills of 
Middlesex, Hertfordshire, and South Bedfordshire. That this plain 
was subjected to denudation we know, for not only have the Portland 
Beds been washed away on the north of these hills, leaving a few of 
their fossils as witnesses of their former presence, but a great part of 
the underlying Kimeridge Clay has also been removed. 
About this period considerable earth-movements took place here and 
elsewhere, the result in Bedfordshire being an elevation of the strata 
towards the west or a depression towards the east, and in Hertfordshire 
a depression by which the Paleozoic range of hills was ultimately sub- 
merged some 1,000 or 2,000 feet. This was probably between the 
close of the Portland and the commencement of the Purbeck period, or 
mainly in this interval, for not a single species is known to pass from the 
one formation to the other, which indicates a great lapse of time. It is 
true that some forms, such as Ammonites and Belemnites amongst the Mol- 
lusca, and Ichthyosaurus and Plestosaurus amongst the Vertebrata, main- 
tained their existence as genera and are well represented in the local 
strata of both the Jurassic and the Cretaceous period, but no species ap- 
pears to have survived the changes which took place between the depo- 
sition of the Kimeridge Clay and that of the Lower Greensand. 
The Portland Beds are marine; the Purbecks are partly marine 
and partly freshwater, and they seem to have been deposited chiefly in 
an extensive lagoon on the eroded surface of the Portlands. In the south 
of England they are succeeded by the freshwater Wealden strata, 2,000 
feet in thickness, of which no trace is known to exist beyond the Vale 
of Wardour ; unless the silicified wood found at Brickhill near Woburn, 
I 9 2 
