GEOLOGY than the age of that formation, for it is also present when sands of the Reading Beds rest upon the Chalk, as in the Bushey chalk-pit near Watford. When fully developed, as in Kent, the Thanet Sands are 50 or 60 feet thick, but they thin out under London to 20 feet, and are only known to occur in Hertfordshire from their presence in the Cheshunt boring, where their thickness is reduced to about 10 feet and they consist of grey and black sand. 1 They are of marine origin. South-east of a line preserving a general north-east and south-west trend, but very irregular, crossing the Lea and Colne districts from a point about half a mile south of Stocking Pelham near Bishop Stortford to Woodcock Hill near Rickmansworth, the Chalk is overlaid by the Reading Beds and London Clay, the escarpment of which follows, at a distance varying from less than a quarter of a mile to a mile and a half, the river Ash downwards from Furneaux Pelham to Amwell Magna, the river Lea upwards from Hoddesdon to Hatfield, and the river Colne downwards from North Mimms to Harefield. Of the Reading Beds there is normally a narrow outcrop along this line, wider in the east than in the west owing to the difference in the slope of the ground ; and the London Clay reposes upon them, forming a range of hills along its escarpment generally from about 300 to 400 feet in height, and, at its highest point, Stanmore Common, between Watford and Elstree, rising to 500 feet. The Reading Beds are represented in Hertfordshire by a very variable series of sands, mottled clays, and pebble-beds, there usually being at their junction with the Chalk the layer of green-coated flints already mentioned. They are here of estuarine origin, thus differing from all the formations already considered, which are of marine origin. From their small thickness, which varies from about 25 to 40 feet, and the usually rather steep slope of the ground at their outcrop, they do not occupy any great extent of country, and in most places have but little effect upon the surface-soil. Where their sands and clays predominate and get mixed with the London Clay, the soil is usually fertile, but where their beds of rounded flint-pebbles are much developed, as in the neighbourhood of Hatfield and North Mimms, the soil is particularly sterile. At Radlett and near North Mimms the principal pebble-bed is consolidated by a silicious cement into a conglomerate, well known as the Hertfordshire conglomerate or ' plum-pudding stone.' Although it is only known to occur with certainty in situ in this part of Hertfordshire at the present time, it has probably at some former period had a much greater extent, for masses of the conglomerate are strewn here and there nearly all over the county, and are also found beyond it. In a gravel-pit north of St. Albans there are large unwaterworn masses of it apparently but little disturbed from their original position, for they seem to form part of too extensive a bed to have been shifted horizontally ; indeed, in 1 Whitaker and Jukes-Browne, ' On Deep Borings,' etc., Quart. Journ. Geol. Soc., vol. 1. (50), p. 508 (1894). 13
