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 Palaeozoic 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 Plesiosaurus 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, 1 9 2
