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Rh district is partially covered by Boulder Clay, which occupies hollows and thus modifies the extent and flow of the underground water.

Important beds of fuller's earth occur in the Lower Greensand at Brickhill and Wavendon. So long ago as 1723 the working of fuller's earth at Wavendon Heath was described by the Rev. B. Holloway, who noted the succession of strata as follows :

The section at Woburn, afterwards published by Fitton, closely agrees with the above, except that the thickness of the sands overlying the fuller's earth is there estimated at 130 feet. He notes that the fuller's earth is of a very light olive green colour, in which particular it agrees with the fuller's earth in the Lower Greensand at Nutfield, and with that in the Oolitic series at Midford near Bath.

At Great Brickhill, to the north-east of Brickhill Manor, resting on the upper beds of the Oxford Clay there was discovered in 1873 by Mr. J. J. H. Teall a bed with phosphatic nodules of somewhat similar character to that which was formerly worked at Potton in Bedfordshire. The nodules or so-called ' coprolites ' were scattered through about 30 feet of sands, but more abundantly in the lower part.

For some time the beds at Brickhill were worked, the coprolites being separated by sifting, and the quartz, chert, lydites and other stones being picked out. Among the coprolites were phosphatized remains of saurians and fishes, worn casts of ammonites and other mollusca, also brachiopoda and other fossils from the Portland Beds and Kimeridge Clay, together with a few that may have been derived from the Corallian. Some fossils from the Oxford Clay occurred, the ammonites in this case ' being preserved in oxide of iron (Limonite) and never phosphatized.' This is noteworthy, but many of the Oxford Clay fossils are preserved in pyrites, and on this account they may have been proof against phos- phatization. Evidently during the overspread of the Lower Greensand fossils were derived from many of the underlying formations.

Such beds of phosphatic nodules usually indicate a pause in deposi- tion, and may sometimes represent one or more zones. The minerali- zation of the fossils was due to decomposing animal matter, the carbonate of lime being replaced by phosphate of lime ; and even wood was thus mineralized, as well as bones and nodules of limestone. The nodules, which are dark brown or yellow, yield from 30 to 50 per cent of phosphate of lime. In the Gault other bands of phosphatic nodules occur, generally black in appearance but usually pale grey or buff in the interior.