162 



T. E. LONES — GE.VVELS, SANDS, CLAYS, 



developed in the lower portions of the gravels. By washing 

 a sample of these deposits so as to remove accompanying flints 

 and pebbles, a residue may easily be obtained containing as much 

 as 30 per cent, of oxide of manganese. This residue is of a deep 

 black colour, and is exceedingly plastic. The oxide of manganese 

 I believe to have been derived from the Reading Beds, and an 

 inspection of Section 8, taken at Bernard's Heath, St. Albans, 

 will show the material, h, caught, as it were, in the act of being 

 incorporated with the gravels, B, by the action of the water from 

 which the beds were deposited. 



Another interesting feature about the beds of gravel and sand 

 is the internal evidence they present that there are, at least, two 

 distinct deposits of gravel, of different characters, and probably 

 of different ages. An ideal section of the gravel and sands would 

 be like that in Section 9, showing a lower and an upper bed of 

 gravel and sand, separated by a thinner bed of loam. Such an 

 appearance as the section represents can be made out in most 

 of the gravel-pits or cuttings in Western Hertfordshire, and the 

 upper gravel can be distinguished from the lower gravel by 

 paying attention to the mineral composition of both deposits. 



Section 9. — Ideal Section of Glacial Drift. 



A. Soil. 



B. Upper bed of gravel with numerous small fliuts and flint 



pebbles. 



7 T-~ '- C. Middle bed of loam [sometimes sand]. 



D. Lower bed of gravel with large flints, pebbles of quart/.itc, 

 quartz, and flint, and blocks of greywether sand- 

 stone, Hertfordshire pudding-stone, etc. 



The lower gravel, generally speaking, contains all, or nearly all, 

 the large quartzite-pebbles, quartz-pebbles, unworn flints, and 

 blocks of pudding - stone and greywether sandstone, while the 

 upper gravel is usually more flinty, and there are comparatively 

 few quartzite-pebbles, which are usually not more than one inch 

 long. Both upper and lower gravels contain a large number of 

 grains of quartz of a special kind. These grains are usually about 

 a twentieth of an inch in diameter and are well rounded, but 

 with this peculiarity, that each grain presents several rounded 

 protuberances, as may be seen by examining a little of the gravel- 

 matrix by means of a lens. 



