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Rh origin ; there are plant remains, abundant cyprides (ostracoda), insect remains, mollusca such as Cyrena, Paludina and Mytilus, also fish remains and bones of turtles. Evidence of the changing conditions which fol- lowed the deposition of the marine Portland Beds is shown by an admixture of marine and freshwater ostracods in the Lower Purbeck Beds, and likewise in the uppermost Portland Beds. No doubt during the Purbeck period there were occasional irruptions of the sea over the area in which freshwater beds were for the most part accumulated.

In the building up of what is now Buckinghamshire the Jurassic strata form the immediate foundation, but the Lias and Inferior Oolite Series so far as we know occur only in the northern part of the county, while the Great Oolite Series probably extends from north to south of the county being connected underground with beds of this age proved in deep borings in Middlesex and Surrey.

The succeeding Jurassic strata occupy a lesser area in the central portion of the county, owing to disturbance and erosion. They were spread over the entire area ; at any rate such was the case with Oxford Clay, Corallian and Kimeridge Clay, and possibly with the Portland Beds, which initiate changes that ultimately led to the estuarine and freshwater Purbeck Beds. The land must then have been to some extent upraised and the strata bent into broad folds, and during the closely connected Wealden epoch there may have been much erosion and possibly deposition of freshwater strata.

The exposed areas of Oxford Clay and newer Jurassic strata evidently form part of a broad synclinal structure, the anticlinal portions north and south having been worn away, and this erosion took place to some extent prior to, and to some extent during, the deposition of the Lower Greensand. Thus the Lower Greensand rests indifferently on any of the Jurassic formations from the Purbeck Beds at Stone to the Oxford Clay at Brickhill. During these periods of erosion the Portland and Purbeck Beds were to some extent separated into outlying masses, the shapes of which have been modified during later epochs.

LOWER GREENSAND The Lower Greensand comprises a variable group of sands and sandstones, with ochre, clays and fuller's earth, and it forms the charm- ing and salubrious region of Woburn, on the borders of which in Buckinghamshire are the heaths of Wavendon and Bow Brickhill, the pleasant uplands of Little and Great Brickhill, and the wooded valley at Linslade. The Lower Greensand appears again at Bishopstone, and near Towersey from beneath the main mass of Gault, and it occurs in outliers near Bierton, Hartwell and Stone, at Brill (603 feet) and Muswell Hill (649 feet), and in other eminences resting directly on Purbeck or Portland Beds, and overlapping their margins in places.