192 NOTES ON THE GLACIAL FORMATION NEAR CHELMSFORD. 



or under the ice, and to floods caused by the melting of greater or 

 lesser portions of the ice. 



The materials of which the gravel is composed are — 



(i) Flint pebbles more abundant at Rolstons than at Rainsford 

 End, probably derived for the most part from pre-glacial 

 pebble gravel, such as that which still caps the hill at- 

 Writtle Park. 



(2) Sub-angular flints, the brown colour of which shows that they 



have not been derived directly from the Chalk, but from 

 older gravels. 



(3) Black flints, not much worn or rolled, derived from the Chalk. 



The nearest point at which the Chalk crops out is twenty- 

 five miles distant. These flints have, therefore, travelled 

 at least twenty-five miles, and must have been carried 

 embedded in ice, for they would have been rolled and 

 water-worn had they been brought all the way by water. 



(4) Pebbles of quartz of a white or pink colour, derived either 



from older pebble gravels or from the Triassic beds. 



(5) Pebbles of red and white quartzite from the Triassic beds of 



the north. The nearest point at which these beds now 

 reach the surface is near Leicester, ninety miles N.W., so 

 that these pebbles have been carried at least ninety miles, 

 no doubt by ice. The presence of these pebbles in a 

 gravel proves it to be either Glacial or formed of debris 

 from Glacial Gravel. 



(6) Blocks of white quartz and of various old rocks, and frag- 



ments of Lias, Oolite, &c., probably nearly all brought by 

 ice from the north. 



(7) Small pieces of chert, originally from the Lower Greensand, 



but derived at secondhand from older gravels. 



The Chalky Boulder Clay is here mainly composed of Chalk, and 

 contains pebbles of chalk, chalk flints, pebbles, and fossils from 

 Oolitic or Liassic strata, the whole having come from the north. 



The thickness of the Glacial beds is very variable, and they rest 

 on a very uneven surface of London Clay, Chalk, &c., often filling 

 deep Pre-glacial valleys, as at Littlebury, near Saffron Walden, for 

 instance (see Whitaker on " A Deep Channel of Drift in the Valley 

 of the Cam, Essex," Quart. Journ. Geol. Soc, 1890, p. 333, and 

 Essex Nat., vol. iii., pp. 140-142, and vol. iv., p. T17), or banked 

 up against Pre-glacial hills which in places rise above them, as at 

 Writtle Park. There are some patches of gravel and brick-earth in 

 this neighbourhood which are believed to be newer than the Glacial 

 beds, but in none of them, nor, indeed, in any beds newer than the 



