Part 37 (1/2)

5. Weald clay.

6. Hastings sand.

7. Purbeck beds.

8. Oolite.]

[Ill.u.s.tration: Fig. 253. Section from the London to the Hamps.h.i.+re basin across the valley of the Weald.

1. Tertiary strata.

2. Chalk and firestone.

3. Gault.

4. Lower greensand.

5. Weald clay.

6. Hastings sands.]

[Ill.u.s.tration: Fig. 254. Highest point of South Downs, 858 feet.

Anticlinal axis of the Weald. Crowborough Hill, 804 feet.

Highest point of North Downs, 880 feet.[243-A]

Section of the country from the confines of the basin of London to that of Hants, with the princ.i.p.al heights above the level of the sea on a true scale.[243-B]]

The s.p.a.ce here inclosed within the escarpment of the chalk affords an example of what has been sometimes called a ”valley of elevation” (more properly ”of denudation”); where the strata, partially removed by aqueous excavation, dip away on all sides from a central axis. Thus, it is supposed that the area now occupied by the Hastings sand (No. 6.) was once covered by the Weald clay (No. 5.), and this again by the Greensand (No. 4.), and this by the Gault (No. 3.); and, lastly, that the Chalk (No. 2.) extended originally over the whole s.p.a.ce between the North and the South Downs. This theory will be better understood by consulting the annexed diagram (fig. 253.), where the dark lines represent what now remains, and the fainter ones those portions of rock which are believed to have been carried away.

At each end of the diagram the tertiary strata (No. 1.) are exhibited reposing on the chalk. In the middle are seen the Hastings sands (No. 6.) forming an anticlinal axis, on each side of which the other formations are arranged with an opposite dip. It has been necessary, however, in order to give a clear view of the different formations, to exaggerate the proportional height of each in comparison to its horizontal extent; and a true scale is therefore subjoined in another diagram (fig. 254.), in order to correct the erroneous impression which might otherwise be made on the reader's mind. In this section the distance between the North and South Downs is represented to exceed forty miles; for the Valley of the Weald is here intersected in its longest diameter, in the direction of a line between Lewes and Maidstone.

Through the central portion, then, of the district supposed to be denuded runs a great anticlinal line, having a direction nearly east and west, on both sides of which the beds 5, 4, 3, and 2, crop out in succession. But, although, for the sake of rendering the physical structure of this region more intelligible, the central line of elevation has alone been introduced, as in the diagrams of Smith, Mantell, Conybeare, and others, geologists have always been well aware that numerous minor lines of dislocation and flexure run parallel to the great central axis.

In the central area of the Hastings sand the strata have undergone the greatest displacement; one fault being known, where the vertical s.h.i.+ft of a bed of calcareous grit is no less than 60 fathoms.[244-A] Much of the picturesque scenery of this district arises from the depth of the narrow valleys and ridges to which the sharp bends and fractures of the strata have given rise; but it is also in part to be attributed to the excavating power exerted by water, especially on the interstratified argillaceous beds.

Besides the series of longitudinal valleys and ridges in the Weald, there are valleys which run in a transverse direction, pa.s.sing through the chalk to the basin of the Thames on the one side, and to the English Channel on the other. In this manner the chain of the North Downs is broken by the rivers Wey, Mole, Darent, Medway, and Stour; the South Downs by the Arun, Adur, Ouse, and Cuckmere.[244-B] If these transverse hollows could be filled up, all the rivers, observes Mr. Conybeare, would be forced to take an easterly course, and to empty themselves into the sea by Romney Marsh and Pevensey Levels.[245-A]

Mr. Martin has suggested that the great cross fractures of the chalk, which have become river channels, have a remarkable correspondence on each side of the valley of the Weald; in several instances the gorges in the North and South Downs appearing to be directly opposed to each other. Thus, for example, the defiles of the Wey in the North Downs, and of the Arun in the South, seemed to coincide in direction; and, in like manner, the Ouse corresponds to the Darent, and the Cuckmere to the Medway.[245-B]

[Ill.u.s.tration: Fig. 255. View of the chalk escarpment of the South Downs.

Taken from the Devil's Dike, looking towards the west and south-west.

_a._ The town of Steyning is hidden by this point.

_b._ Edburton church.

_c._ Road.

_d._ River Adur.]

Although these coincidences may, perhaps, be accidental, it is by no means improbable, as hinted by the author above mentioned, that great amount of elevation towards the centre of the Weald district gave rise to transverse fissures. And as the longitudinal valleys were connected with that linear movement which caused the anticlinal lines running east and west, so the cross fissures might have been occasioned by the intensity of the upheaving force towards the centre of the line.

But before treating of the manner in which the upheaving movement may have acted, I shall endeavour to make the reader more intimately acquainted with the leading geographical features of the district, so far as they are of geological interest.

In whatever direction we travel from the tertiary strata of the basins of London and Hamps.h.i.+re towards the valley of the Weald, we first ascend a slope of white chalk, with flints, and then find ourselves on the summit of a declivity consisting, for the most part, of different members of the chalk formation; below which the upper greensand, and sometimes, also, the gault, crop out. This steep declivity is the great escarpment of the chalk before mentioned, which overhangs a valley excavated chiefly out of the argillaceous or marly bed, termed Gault (No. 3.). The escarpment is continuous along the southern termination of the North Downs, and may be traced from the sea, at Folkestone, westward to Guildford and the neighbourhood of Petersfield, and from thence to the termination of the South Downs at Beachy Head. In this precipice or steep slope the strata are cut off abruptly, and it is evident that they must originally have extended farther. In the woodcut (fig. 255. p. 245.), part of the escarpment of the South Downs is faithfully represented, where the denudation at the base of the declivity has been somewhat more extensive than usual, in consequence of the upper and lower greensand being formed of very incoherent materials, the upper, indeed, being extremely thin and almost wanting.

[Ill.u.s.tration: Fig. 256. Chalk escarpment, as seen from the hill above Steyning, Suss.e.x. The castle and village of Bramber in the foreground.]

The geologist cannot fail to recognize in this view the exact likeness of a sea cliff; and if he turns and looks in an opposite direction, or eastward, towards Beachy Head (see fig. 256.), he will see the same line of heights prolonged. Even those who are not accustomed to speculate on the former changes which the surface has undergone may fancy the broad and level plain to resemble the flat sands which were laid dry by the receding tide, and the different projecting ma.s.ses of chalk to be the headlands of a coast which separated the different bays from each other.