Yol. 68.'] GLACIAL OKIGIN OF THE CLAT-WITH-FLimS. 201 



brickyard there are a Dumber of small deep pits, usually from 20 

 to 30 feet deep. The bulk of the deposit is a mass of reddish-browu 

 clay, containing abundant angular flints of various sizes. Irregular 

 masses of bright red clay, frequently containing well-rounded flint 

 pebbles, are found incorporated in the clay and Hints, and in these 

 pockets are found sarseu-stones of all sizes up to about 600 cubic 

 feet, which would weigh about 40 tons. Mr. Spicer ^ mentions one 

 which was reported to have weighed over 200 tons. The clay and 

 flints extends under the pockets of red clay with sarsens, and as it 

 is the stones that are of most economic importance, the pits do not 

 reach to the Chalk beneath. The workmen say that the deposit is 

 at least 50 feet thick here. 



In the midst of a ring of sections of the kind just described, one 

 pit showed material of a totally different character. Here was 

 -I feet of grey and brown clay overlying about 20 feet of laminated 

 sandy brick-earth. The laminae show little V-shaped loops, 

 probably due to ' creep,' and the brick-earth breaks away in vertical 

 sheets. Minute flint flakes are present, but very few flints ; and 

 these, of moderate size, are arranged along one small band near 

 the bottom. The deposit bears a close resemblance to ' kettle drift ' 

 (PI. XII). 



The sarsens consist of a hard white sandstone, with occasional 

 cavities containing loose white sand. In some cases one end 

 of the stone is a flint conglomerate, and there is little doubt 

 that the sandstone and the Hertfordshire Pudding-stone are 

 parts of the same deposit. The red clay in which they are 

 embedded is clearly derived from the Eeadnig Beds, and the sarsens 

 and well-rounded flint pebbles that are found in it have the same 

 derivation.^ 



The deposit, therefore, is composed of insoluble matter from the 

 Chalk and Heading-Bed material variously mixed together. 



Mr. Spicer's ^ theory is that the sarsens and red clay have sunk 

 down into swallow-holes in the Chalk-with-Flints. By under- 

 ground solution the Chalk was dissolved, aud left Clay- with- Flints 

 surrounding the foundered mass of Eeading Beds. He states that 

 the sarsens slope on all sides towards the centre of each pit, and at 

 the bottom are horizontal. This is essentially the ' Chalk residue ' 

 theory of Mr. Whitaker, who admits the addition of Eocene 

 material in cases such as this. 



That there has been a certain amount of sinking is probable, but 

 this is insufficient to account for the facts. We have seen that, on 

 the 'Chalk residue' theory, the Clay-with-Flints is formed practi- 

 cally in situ ; it has merely sunk vertically, as also on the swallow- 

 hole theory — in fact, the unworn character of the flints is regarded 



^1 Q. J. a. S. vol. Ixi (1905) p. 40. 



2 The origin of the sarsens in the Reading Beds was suspected by Prestwich 

 (Q. J. G-. S. vol. X, 1854, p. 127) ; and Mr, Whitaker (' Geology of Middlesex, &c.' 

 Mem. Geol. Surv. 1864, p. 66) and others have agreed that this is the true source 

 of the sarsens. 



3 Q. J. a. S. vol. Ixi (1905) pp. 40-41. 



