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Rh 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.

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 Belemnitella 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 bellovuacina 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,