View on the Downs looking towards Wallington from the Icknield Way
Below the Chalk-rock we come to the Middle Chalk, or Chalk without flints, which may be so much as 350 feet in thickness, and rises in a rather steep slope or “step” from the underlying beds to be next mentioned. Flints are few and far between in the Middle Chalk, which forms the western slope of the Downs at Royston, as well as beyond the limits of the county at Luton, and so on to the Chiltern Hills. Fossils are much more numerous in the Middle than in the Upper Chalk. The lowest bed of the former is the Melbourn Rock, a hard, nodular band about 10 feet thick. Next comes the grey and white Lower Chalk, from 65 to 90 feet thick, after which we reach the Totternhoe Stone. Although only six feet in thickness, this Totternhoe Stone, which forms the escarpment of Royston Downs, is of importance as having been largely employed in the construction of churches and other buildings on the northern side of the county. It is a sandy grey limestone, which used to be largely quarried at Totternhoe, with special precautions in drying. It can be traced from Tring by way of Miswell, Marsworth, Pirton, and Radwell to Ashwell.
The Totternhoe Stone really forms the top of the Chalk-marl, which is some 80 feet thick, and consists of buff crumbling marly limestones. It forms a strip of low ground at the base of the Chalk escarpment. At the bottom of the Chalk occurs the so-called coprolite-bed, which contains large quantities of phosphate nodules. Forty years ago these beds were extensively worked between Hitchin and Cambridge for the sake of the nodules.
Only on the northern border of the county, between Hitchin and Baldock, and also near Tring and then merely to a very small extent, are any of the beds underlying the Chalk exposed. These comprise, firstly the Upper Greensand, which is either a sandy marl or a sandstone with green grains, and secondly, a dark blue impervious clay known as the Gault. These formations constitute the plain at the foot of the Chalk hills in Bedfordshire, the scenery of which is very similar to that of the London Clay plain in eastern Hertfordshire and Middlesex.
It is important to add that, at a gradually increasing depth as we proceed south, the Gault underlies the whole of the Hertfordshire Chalk, and renders the latter such an excellent water-bearing formation. If the Gault be perforated we come upon the Lower Greensand, another excellent water-bearing stratum, which comes to the surface in the neighbourhood of Silsoe, in Bedfordshire.
We may now turn to the formations overlying the Chalk in the southern half of the county. Here it should be mentioned that all the formations hitherto described overlie (or underlie) one another in what is termed conformable sequence; that is to say, there is no break between them, but a more or less nearly complete passage from one to another. Between the Chalk and the overlying Tertiary formations, there is, on the other hand, a great break or “unconformity”; the surface of the Chalk having been worn into a very irregular contour, above which we pass suddenly to the Tertiary beds, generally containing at their base a number of rolled chalk flints. This indicates that before the Tertiary beds were laid down, the Chalk had become dry land; after which a portion of it once more subsided beneath the ocean. The Tertiary beds are in fact formed for the most part from the débris, or wearing away of the old Chalk land.
The lowest Tertiary stratum of eastern and southern Hertfordshire is known as the Woolwich and Reading beds. These consist of alternations of bright-coloured plastic clays and sandy or pebble-beds; their maximum thickness in the county being about 35 feet. They form a band extending from Harefield Park to Watford, and thence to Hatfield and Hertford. Below the Woolwich and Reading beds we come on the London Clay, of which the basement bed contains a layer of flint pebbles, although the remainder of this thick formation is a stiff blue clay, turning brown when exposed to the action of the weather. Originally these Tertiary formations must have extended all over the Chalk of central Hertfordshire, as is demonstrated by the occurrence of patches, or “outliers,” of them over a zone of considerable width. Such Tertiary outliers occur at Micklefield Hall, Micklefield Green, Sarratt, Abbot’s Langley, Bedmond, Bennet’s End, and Leverstock Green, and in the northern, or St Peter’s portion of St Albans.
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.
Half way down the sides of the hills, in the district last named, the Chalk is more or less completely exposed at the surface along a narrow zone, below which we come upon deposits of gravel, sand, and clay filling the bottoms of the valleys. At Bowling Alley, Harpenden, these deposits are fully forty feet in thickness. Although they have been supposed to be the result of river action, it is more probable that they are due to rain-wash. Indeed this is practically proved in the case of the valley leading from Harpenden towards No-Man’s-Land, where the lower end is blocked by a ridge of gravel, which could not possibly have been formed by river action. The stones in these valley-gravels are of irregular shape, and thus quite different from the rounded pebbles of the gravels of the Woolwich and Reading beds, as seen at St Peter’s, St Albans. At Harpenden the uppermost layer of valley-gravel is extremely clean and sharp, generally of a golden yellow colour with blackish veins. Deposits of brick-earth occur locally throughout the Chalk area.