GEOLOGY 



at Beaconsfield and Penn, between Chalfont St. Giles and Amersham, 

 and to the east of Chesham. 1 



Many of these outliers form picturesque wooded tracts, diversified 

 with commons, as at Lane End and Cadmore End Commons, where the 

 strata are evidently faulted. In some dark sands at this locality traces 

 of nickel and cobalt have been detected. 8 



The most distant outlier is that noticed by Mr. Whitaker at Ring- 

 sail to the north of Ashridge Park and not far from the Ivinghoe hills, 

 where a pit showed fine white sand. In such situations relics of Eocene 

 strata are preserved in deep pipes in the Chalk, far away from the 

 parent source, as the mass of the strata above had been removed by 



erosion. 3 



The mottled clays are dug in many places for brickmaking, the 

 sands for mortar making, and the pebble beds for road mending. Both 

 sands and pebble beds yield a fair amount of water under favourable 

 circumstances. 



LONDON CLAY 



This great clay formation forms the substratum over much of 

 south-eastern Buckinghamshire, but like the Reading Beds it is almost 

 wholly concealed by gravels. 



It has been observed at the surface at Upton and along the borders 

 of Stoke and Fulmer Commons, at Iver, and to the east and north-east 

 of Fulmer. Outliers appear at Lane End, Penn and Tyler's Hill east of 

 Chesham. In many places the clay is dug for brickmaking. 



Nowhere in the county have we the full thickness of the formation, 

 but the greatest thickness is probably on the borders of the Colne, south- 

 east of Wraysbury. In mass it is a bluish-grey clay with septaria, brown 

 at the surface. The basement bed, 6 or 8 feet thick, is a brown loam, 

 which contains flint pebbles and green sand. The sand is sometimes 

 cemented into tabular masses of rock, and these are in places crowded 

 with fossils, such as Cardium, Cytferea, Panopaea, Pectunculus, Nucu/a, 

 Natica, Rostellaria and Ditrupa plana. Many have been found at 

 Hedgerley. 



Fossils are by no means common in the mass of the London Clay. 

 We may occasionally meet with a Nautilus, but near the surface the 

 shells have been almost wholly destroyed. The fauna and flora are of 

 tropical aspect. 



There is a great gap between the London Clay and succeeding 

 deposits in Buckinghamshire. Of the interval we have no actual 

 records in the county. The Bagshot Beds were no doubt spread over 

 large areas, for they occur in Middlesex and on the Berkshire and 

 Surrey side of the Thames valley. During Oligocene, Miocene and 

 Pliocene times the area must have undergone great waste by subaerial 

 agents. The London Basin took form by the upraising of the bordering 



1 Prestwich, Quart. Journ. Geol. Sac. x. 90. 

 8 Summary of Progress of Geol. Survey for 1900, p. 123. 3 Mem. Geol. Survey, iv. 234. 



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