GEOLOGY 



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. 1 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 evi- 

 dently 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. 



1 T. R. Jones, Quart. Jourrt. Geol. Soc. xli. 328 ; and H. B. Woodward, ' Middle and Upper Oolitic 

 Rocks of England,' Geol. Survey, p. 280. 



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