Closely connected with these Tertiary formations is the well-known Hertfordshire pudding-stone; a conglomerate formed of stained flint-pebbles cemented together by a flinty matrix as hard as the pebbles themselves, so that a fracture forms a clean surface traversing both pebbles and cement. This pudding-stone is usually found in the gravels (or washed out of them) in irregular masses, weighing from a few pounds to as many tons. It is stated, however, to occur in its original bedding between Aldenham and Shenley; and the rock evidently represents a hardened zone of the Woolwich and Reading beds. Pudding-stone is found in special abundance at St Albans and again in the neighbourhood of Great Gaddesden. In some St Albans specimens the pebbles are stained black for a considerable thickness by the oxides of iron, while the central core is bright red or orange. Such specimens, when cut and polished, form ornamental stones of great beauty; but, on account of their hardness, the expense of cutting is very heavy. Except on the higher part of the Chalk Downs, and very generally along a narrow band half way up the sides of the valleys, the aforesaid formations are but rarely exposed in the county at the surface, on account of being overlaid with superficial deposits of gravel, clay, etc., which are of post-Tertiary age, and were deposited for the most part during the time that man has been an inhabitant of the world. These superficial beds are very frequently termed “drift,” on account of a large portion being formed by ice, at the time that northern Europe was under the influence of the great glacial period. Over most of the chalk area the denuded surface of the Chalk is covered with a thick layer of stiff clay full of flints, this layer being formed by the disintegration of the Chalk itself, the soluble calcareous portion being dissolved and carried away, while the insoluble flints and clay remain. Above this layer in the neighbourhood of Hertford, Barnet, and elsewhere, is a series of gravelly beds assigned to the middle division of the glacial period; while these in turn are overlaid locally, as at Bricket Wood, between St Albans and Watford, by the chalky Boulder-clay, of upper glacial age, which is there some twenty feet in thickness. In other places, as at Harpenden, the hills are capped by a still greater thickness of clayey deposits, mingled with flints, resting upon a very irregular surface of Chalk, which appears to be for the most part of glacial origin. Speaking generally, Boulder-clay is characteristic of the east, and clay with flints and gravel of the western side of the county.