106 PROF. MORBIS — GEOLOGY OF THE LONDON BASIN 



are supposed to be pre-glaciul, and are known as the pebble-gravel 

 in this district, and similar gravels are found at Ilemel Hemp- 

 stead and other places.* Secondly, of later date than these pebble- 

 gravels are still more instructive beds, and which afford far better 

 indications of great physical and climatic changes. Hertfordshire 

 presents us with some marked features of the Glacial Period ; for 

 in certain districts occurs a deposit of sand and gravel, with foreign 

 rocks not belonging to the present County of Hertford, as in the 

 gravel-pits west of "Watford, the chalk-pit near to Bushey Station, 

 and the pit on the hill east of the railway, at Radlett near Watford. 



Overlying that again, as at Bricket Wood, is a more or less 

 thick accumulation of clay, known as Boulder-clay. f The lower 

 sands are of middle glacial age ; where they are wanting, the 

 Boulder-clay rests directly on the older Tertiary beds. This clay 

 presents some remarkable features, and the study of its origin may 

 assist in explaining the causes by which the present contour of 

 the country has been partly produced. Some of you have pro- 

 bably read of the " Great Ice Period," and geologists refer this 

 superficial covering spread over some portion of Hertford.shire, and 

 other parts of England, to that earlier period of the earth's 

 history when higher mountains existed in Wales and Scotland, 

 down which large glaciers moved, and probably extended as a 

 great ice-sheet, like that of Greenland, over the lower land to the 

 border of the old icy sea. From the margin of this old shore, 

 masses of ice loaded with their debris of earth and stones protruded 

 into the sea, and becoming detached as icebergs, di-ifted away, and 

 when melted deposited their earthy burden over the old sea-bed 

 in the more temperate region to the south. 



For in the north-east of Hertfordshire are found boulders of 

 limestone, which cannot have come nearer than from the Derby- 

 shire hills, and also other rocks from still further north. It is 

 evident, therefore, that these superficial coverings indicate a very 

 different period from that of the London Clay, — one of intense cold, 

 when there existed to the north large glaciers and snowy moun- 

 tains, from which were derived the materials of the wide-spread 

 deposits and far-brought boulders found over parts of the present 

 area of Hertfordshire. | 



* " On the whole, it seems safer to conchide that this trravel is the oldest 

 Drift of the district, and is a bed of somewhat local occurrence. The chief 

 localities are Stanmore Heath, from Shenlcy south-eastward, west and north of 

 IJarnet, and at Totteridge, in Middlesex ; at IIi,2:hbeach, Jack's Hill, and 

 Gayne's Park, east of Eppinj?, in Essex; and at Shooter's Hill, in Kent." — 

 Wliitaker, 'Geology of London,' p. 51. 



t I'restwich, The Geologist, vol. i. p. 241. 



J "The chief tracts of Boulder-clay in our area are on the north-east of 

 "Watford, our most westerly patch ; at Finchley, its most southerly point, where 

 it ends off at the northern slope of the ridge that bounds the valley of the 

 Thames ; and in Essex on the north and north-west of Urentwood, wiience it 

 spreads over a very large district, beyond our bounds." — "NVhitaker, I.e. p. 64. 

 — " From an examination of tlie sections tliat have been exposed during tlie last 

 seven years, in making the double line of the Loudon and A' orth- Western 



