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.
