A HISTORY OF BUCKINGHAMSHIRE 



and middle divisions are practically undivided by any impervious layers. 

 The marly chalk at the base of the Melbourn Rock sometimes throws 

 out springs, but the more copious outflows of water are at the base of 

 the Totternhoe Stone which rests on marl. Here strong springs are 

 thrown out as at Wendover. As the strata dip towards the south-east 

 the underground flow follows that course, and the streams which have cut 

 down to the plane of saturation carry away the overflow in that direc- 

 tion, and they issue at higher places in the valleys when the plane of 

 saturation rises after long continued rain. The Colne, which cuts into 

 the saturated Chalk carries away much water and receives springs along 

 its bed. 



Water is naturally scarce on the higher Chalk uplands, and on 

 some of the downs ' dew ponds ' have been constructed. These are 

 made from 30 to 40 feet in diameter and from 4 to 6 feet deep in the 

 chalk, and they are clayed at the base. A high and exposed situation 

 is selected, and in the first instance water or snow is introduced, and 

 the supply is then maintained by the condensation of moisture from 

 the atmosphere as well as by rain, the condensation exceeding the 

 evaporation. 1 



READING BEDS 



There is evidence of a considerable change in conditions between 

 the Chalk, a deep sea deposit, and the next succeeding deposits of Eocene 

 age which exhibit shallow-water conditions, both estuarine and fresh- 

 water. There are no traces of Thanet Sands which occur directly 

 above the Chalk under parts of London and eastwards. During these 

 early Eocene times the Chalk was upraised over large areas and much 

 eroded. Evidence of this is furnished by the pebble beds made of rolled 

 flints, which occur in the Reading Beds and other Eocene strata, also 

 by the fact that higher stages of the Chalk characterized by Eelemnitella 

 mucronata occur elsewhere in England, and are not known in Bucking- 

 hamshire owing to the erosion to which the Chalk has been subjected. 



The Reading Beds comprise dark grey and mottled clays with 

 crimson, green and other tints and white and coloured sands, together 

 with pebble beds made up of black flints. Occasional seams of iron- 

 stone occur, and there is often a layer containing unworn green-coated 

 flints at the base. The strata rest on the Chalk and are overlain by the 

 London Clay to the south-east, but their outcrop is largely concealed by 

 gravel both in the Thames valley and on the uplands to the north of it. 



Usually Ostrea bello'uacina is found in the lower beds, and higher 

 up remains of leaves of plants have been observed. The flora indicates 

 conditions approaching to tropical in character. 



Portions of the main outcrop of the Reading Beds may be seen 

 here and there at East Burnham, Cliefden, Hedsor, Hedgerley and Ful- 

 mer, while outlying masses occur at Taplow, between Little Marlow 

 and Loudwater, at Lane End and Cadmore End, Turville and Ibstone, 



1 Rev. J. C. Clutterbuck, Journ. Roy. Agtic. Sac. ser. z, i. 273. 



18 



