By Geology we mean the study of the rocks, and we must at the outset explain that the term rock is used by the geologist without any reference to the hardness or compactness of the material to which the name is applied; thus he speaks of loose sand as a rock equally with a hard substance like granite.

Rocks are of two kinds, (1) those laid down mostly under water, (2) those due to the action of fire.

The first kind may be compared to sheets of paper one over the other. These sheets are called beds, and such beds are usually formed of sand (often containing pebbles), mud or clay, and limestone or mixtures of these materials. They are laid down as flat or nearly flat sheets, but may afterwards be tilted as the result of movement of the earth’s crust, just as you may tilt sheets of paper, folding them into arches and troughs, by pressing them at either end. Again, we may find the tops of the folds so produced worn away as the result of the wearing action of rivers, glaciers, and sea-waves upon them, as you might cut off the tops of the folds of the paper with a pair of shears. This has happened with the ancient beds forming parts of the earth’s crust, and we therefore often find them tilted, with the upper parts removed.

The other kinds of rocks are known as igneous rocks, which have been molten under the action of heat and become solid on cooling. When in the molten state they have been poured out at the surface as the lava of volcanoes, or have been forced into other rocks and cooled in the cracks and other places of weakness. Much material is also thrown out of volcanoes as volcanic ash and dust, and is piled up on the sides of the volcano. Such ashy material may be arranged in beds, so that it partakes to some extent of the qualities of the two great rock groups.

The relations of such beds are of great importance to geologists, for by means of these beds we can classify the rocks according to age. If we take two sheets of paper, and lay one on the top of the other on a table, the upper one has been laid down after the other. Similarly with two beds, the upper is also the newer, and the newer will remain on the top after earth-movements, save in very exceptional cases which need not be regarded here, and for general purposes we may look upon any bed or set of beds resting on any other in our own country as being the newer bed or set.

The movements which affect beds may occur at different times. One set of beds may be laid down flat, then thrown into folds by movement, the tops of the beds worn off, and another set of beds laid down upon the worn surface of the older beds, the edges of which will abut against the oldest of the new set of flatly deposited beds, which latter may in turn undergo disturbance and renewal of their upper portions.

Again, after the formation of the beds many changes may occur in them. They may become hardened, pebble-beds being changed into conglomerates, sands into sandstones, muds and clays into mudstones and shales, soft deposits of lime into limestone, and loose volcanic ashes into exceedingly hard rocks. They may also become cracked, and the cracks are often very regular, running in two directions at right angles one to the other. Such cracks are known as joints, and the joints are very important in affecting the physical geography of a district. Then, as the result of great pressure applied sideways, the rocks may be so changed that they can be split into thin slabs, which usually, though not necessarily, split along planes standing at high angles to the horizontal. Rocks affected in this way are known as slates.

If we could flatten out all the beds of England, and arrange them one over the other and bore a shaft through them, we should see them on the sides of the shaft, the newest appearing at the top and the oldest at the bottom, as shown in the figure. Such a shaft would have a depth of between 10,000 and 20,000 feet. The strata are divided into three great groups called Primary or Palaeozoic, Secondary or Mesozoic, and Tertiary or Cainozoic, and the lowest of the Primary rocks are the oldest rocks of Britain, which form as it were the foundation stones on which the other rocks rest. These may be spoken of as the Pre-Cambrian rocks. The three great groups are divided into minor divisions known as systems. The names of these systems are arranged in order in the table with a very rough indication of their relative importance, though the divisions above the Eocene have their thickness exaggerated, as otherwise they would hardly show in the figure. On the right hand side, the general characters of the rocks of each system are stated.

With these preliminary remarks we may now proceed to a brief account of the geology of the county.

In Hertfordshire, apart from the soil and the superficial accumulations of gravel, sand, and clay, only the lower beds or strata of the Tertiary and the uppermost formations of the Secondary period are represented.

As the greater portion of the subjacent rocks of the county is formed by the Chalk, it will be convenient to commence with this formation. The Chalk extends, or “strikes,” across all but the south-eastern portion of the county in a broad belt, with a general south-westerly or north-easterly direction, reaching on the northern side, with a few exceptions, to the border of the county and beyond, while to the southward its boundary runs approximately through Bushey, South Mims, Hertford, and Bishop’s Stortford. At Dunstable the Chalk forms what is called an “escarpment,” that is to say a high and somewhat precipitous (although rounded) cliff overlooking the great plain formed by the marls and clays of the underlying strata. As in all true escarpments, the beds, or strata of the Chalk, which are somewhat tilted by earth-movements out of their originally horizontal plane, incline, or “dip” away from the main face of the cliff, that is to say, towards the south-east; and this south-easterly dip of the Chalk, apart from local interruptions and folds, continues to its southerly boundary. Now since the Chalk is a porous formation admirably fitted to collect and retain the rain-water falling upon it, while it is underlain, as we shall see shortly, by the impervious Gault Clay of the Bedfordshire plain, and overlain along its southern boundary by the equally impervious London Clay, it is obvious that it will hold all the water thus collected, and that this water will tend to run deep down in the rock in a south-easterly direction. Hence the northern part of the Chalk zone forms an almost perfect water-collecting area, which can be tapped along the southern side of the county by boring through the overlying London Clay.

The Chalk comprises several main divisions, of which the highest is known as the Upper Chalk, or the Chalk with flints; this when fully developed being about 300 feet thick. It is a soft white limestone traversed by nearly horizontal layers of black, white-coated flint, which have originated by a process of “segregation” in the rock subsequent to its deposition as ooze on the old sea-bed. Usually these layers consist of irregular nodular masses; but there is sometimes a continuous thin layer of scarcely more than half-an-inch in thickness, locally known as “chimney-flint.” The south-easterly dip of the Chalk is shown by the layers of flint to be not more, as a rule, than three or four degrees. The Upper Chalk extends from the summits of the hills as far down as Rickmansworth, Watford, Hatfield, and Hertford, thus forming the bed-rock of the greater portion of the county. By the wearing away of the overlying Tertiary strata, a small cone, or “inlier,” of Chalk is exposed at Northaw.

Next comes a bed of about four feet thick known as the Chalk-rock. It is a hard cream-coloured rock, containing layers of green-coated nodules, is traversed by numerous vertical joints, and rings to the stroke of the hammer. Owing to its hardness, it resists the action of the weather, and is therefore in evidence at or near the summits of the hills, where it can be traced from close to Berkhampstead Castle by Boxmoor and Apsley, and thence to the south-west of Dunstable, Kensworth, the south of Baldock, and so in a north-easterly direction to Lannock Farm.

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.

Over the greater part of the county the soil is the result of the decomposition of the foregoing superficial formations; and is consequently in most cases of a stiffer and more clayey character on the hill-tops than in the valleys, where it frequently forms only a bed of a foot, or even less, in thickness above the sharp, running gravel. Everywhere in the Chalk districts the soil contains a vast number of flints; but it is, nevertheless, admirably adapted for corn-growing, and especially for malting-grain; Hertford being one of the four English counties best suited to crops of the latter nature. On many of the unenclosed commons the soil is, however, of a poor and hungry nature, producing various kinds of inferior grass, together with spring-flowering gorse, as on Harpenden Common, or heather, as at Kingsbourn Green, between Harpenden and Luton, and at Gustard Wood, near Wheathampstead. On the higher Chalk Downs near Dunstable and Royston there is little or no soil properly so-called; the short, but sweet and nourishing grass growing on the chalk itself. A very different type of soil obtains in the London Clay area in the south and east of the county; this being heavy and clayey, and thus better suited for grass than for corn; in fact in the old days the Middlesex portion of this district was known to the country people as the “Hay-country.” Along many of the river-valleys peaty soils of a marshy and swampy nature prevail.