Immediately beneath the London, the Plastic clay crops out. The upper or clay-beds of this formation, as the name implies, are well suited for the manufacture of kiln-ware; it is more tenacious, and less manageable than the London clay; and usually forms a narrow band on the slopes and escarpments of the hill-sides; the lower beds are of very pure sand, sometimes perfectly white, suited for domestic, horticultural, and other purposes, though some of the beds are interspersed with rolled pebbles. To the breaking up of this stratum much of the soil that covers the chalk is due, and from hence the hard conglomerate known as Hertfordshire pudding-stone is derived. The most fertile spots in this district are found at the outcrop of these strata, where the clay and sand are amalgamated so as to form a friable and kindly soil. The geological condition here described extends more or less in a band across the county from Moor Park, near Rickmansworth, on the west, to its eastern limit bounded by the river Stort.
The neighbourhood of Bishops Stortford furnishes a good example of farming under geological conditions not found elsewhere in this county, but resembling those which subsist in some parts of Essex. The river Stort runs through a trough in the chalk, over which the Plastic clay-formation crops out on the side of the valley. Its beds of clay and sand here amalgamate with the flint-gravel, with which the chalk is covered on the lower levels, to form a light, friable, and fertile soil, suited to four-course husbandry. On the higher levels the tertiary clay forms rather wide-spread “plateaus,” covered very generally with a drift consisting of water-worn chalk, with some chalk-flints. This drift, for such it appears to be, is not found in the western parts of the clay-districts of Hertfordshire under the same geological conditions of subsoil, though it is very extensively diffused in Essex, where it presents some of the best cornland in that county. It would be very interesting to trace the extent of this deposit in both counties, and, if possible, account for its unusual presence as a covering to the tertiary beds resting on the chalk.
The Water-level in the Chalk.
As in the case of the clays, the chalk-district may be subdivided; it has been so treated by Arthur Young, who, in his maps, lays down the principal part as loam, distinguishing as chalk only that small space which is drained by rivers running to the north with a fall anticlinal to the natural dip of the stratum. Adopting this division, of which it would be difficult to define the exact limits, we find that the southern slope from the northern limits of the county to the point where the river
