The water which disappears underground is by no means idle. It finishes its downward journey when it reaches the water-table, that is to say the surface below which all cracks and joints are already filled with water. This surface differs from that of a lake in that it is not flat and horizontal but has a general slope roughly parallel to that of the surface of the countryside above it. The water newly arrived from above now flows along this watery surface, dissolving the limestone away on either side of the crack which forms its path. The level of the water-table rises and falls according as the season is wet or dry and consequently the crack is widened into a tunnel or cave within the limits of that rise and fall. Such a cave may be seen soon after entering the dale from its upper end. In a rainy season the water-table rises above the floor of the dale and the stream is then seen issuing from the cave as a surface stream. In times of drought the water-table sinks and the stream, finding that its underground channel is enough, disappears from the surface and continues its course below ground. With this alternating rise and fall of the water the roof of the underground channel is being gradually dissolved away and ultimately collapses and thus a new and romantic addition is introduced into the floor of the dale. This type of deepening has been going on in all the dales since late Pliocene times.
The limestone varies in quality from place to place throughout the uplands. Sometimes it is rich in fossils which are slightly less soluble than the rock itself. This is particularly the case where coral or shelly reefs existed on the floor of the Carboniferous sea. At such places the rock is less rapidly dissolved than is the surrounding limestone and so it stands up as a more or less prominent hill known as a reef knoll. Thorpe Cloud, Bunster, Wetton and Gratton Hills are examples of such knolls.
One important result accruing from the solvent action of rainwater upon the limestone is that the insoluble residue remains on the surface and accumulates to form soil. This is particularly the case where the surface is almost level, for then rain-wash and soil creep have only slight effect. The soil is then said to be stationary. Where the surface has a marked slope these two factors come into action more vigorously and the soil is transported downhill. On such slopes the soil covering is thin but in the adjoining valley it is deep. These differences from place to place exert an important influence upon the agriculture of the upland.
The wet surfaces over which the escaping water flows become moss-grown. The moss, however, takes up the carbon dioxide from this water with the result that it can no longer hold much lime in solution. The latter is therefore deposited and, covering the moss, forms a spongy-looking rock called tufa. This is often used for rockeries and sometimes for building as in the case of Tufa Cottage in the Via Gellia. In some springs the water rises from great depth and as it does so the pressure upon it diminishes rapidly. Once more much of the lime held in solution is set free and covers any objects, such as toys and birds’ nests that are put into it, with a hard coating lime. As they have the appearance of having been changed to stone such springs are spoken of as petrifying wells.
In caves the water dropping from the roof or flowing down the sides also parts with its lime. In so doing it deposits this either as icicle-shaped pendants from the roof or as pinnacles rising from the floor. These features are known as stalactites and stalagmites respectively. Sheets of lime may also be laid over the walls or hang like curtains from ridges on the ceiling. When such a cave is judiciously illuminated it becomes a beautiful scene and a profitable centre of attraction.
WITHIN THE DALES
The rambler approaching Dovedale and the uplands along the Belper-Ashbourne road begins to catch visions of the promised land when he passes beyond Hulland. He looks across a succession of level-topped hills with an altitude of six or seven hundred feet. These level tops are relics of a mid-Pliocene peneplained platform. Away in the distance beyond them the uplands appear as a lofty rampart, bounding this platform on the north, and extending to the Weaver Hills.
In the centre of the rampart is Thorpe Cloud, rising like a bastion at the entry to Dovedale. Making that his first objective, he scrambles to the top and, looking northwards, finds himself on a level with the upper or early Pliocene platform and sees it as a gently undulating landscape having all the general characteristics of a peneplain (Plate VIIa, see here).
In striking contrast to that is the deep steep-sided dale looking like a cleft in that ancient landscape. Closer inspection of the cleft reveals a feature that is easily overlooked. The precipitous sides of the cleft do not rise to the level of the upland itself for their rims spread out like a shallow funnel about 200 feet deep. The sides of this funnel curve upwards from the lips of the gorge to the level of the upland platform. The funnel is in fact the profile or cross-section of a moderately broad valley in the floor of which the gorge has been carved. This floor extends southwards into the 600-700-foot platform already noticed between Hulland and Ashbourne. The sight of this valley takes the thoughts back to middle Pliocene times when the Dove flowed along this floor and debouched on to the plain of which this platform is a survival.
Leaving his eyrie, the rambler descends and sets out to explore the dale (Plate 2a, see here). At first his path lies alongside the stream. Presently it rises steeply and takes him up to the Lover’s Leap, the name given to a spur which projects towards the gorge. Standing upon the tip of the spur he looks down into the gorge and his eyes come to rest on the wooded slopes of the opposite side. Here and there amongst the greenery may be seen grey pinnacles of limestone known as the Twelve Apostles. How these came to be there will emerge later. For the present it must suffice to say that they are the degraded remnants of another spur that once projected from the other side, the counterpart of the Lover’s Leap.
Turning back from viewing the Twelve Apostles, it is seen that the flat surface of the Leap slopes upwards like a valley side and merges into the upper plain. Looking up the dale similar spurs may be seen farther on. Each of these shows the same traces of the old valley features, for all these spurs are also relics of the mid-Pliocene valley. The observer is standing where at that far distant date the Dove actually flowed.
Leaving the Lover’s Leap behind, the explorer descends past the successive levels through which the river excavated on its way down to its present bed. Once more the path lies alongside the stream through smooth grassy flats unimpeded by boulders. On either side are the rocky cliffs criss-crossed by vertical cracks or joints and horizontal or gently dipping bedding planes. In winter some of these become filled with water from melting snow. When this freezes it expands and the ice, acting like a quarryman’s wedge, gradually prises lumps of rock from the face of the cliff which fall and form a blocky scree at the base (Plate VIa, see here).
Here and there other narrow spurs once projected into the dale, but these have been partly or wholly destroyed. Frost working at both sides of the spur has worked its way in along major joints or fractures and cut it up into isolated columns such as that of the Ilam Rock (Plate III, see here). Rain falling upon such a column dissolves the corners and edges and eventually reduces it to the shape of a pinnacle such as those already seen from the Lover’s Leap.
Another feature of the dale that is easily overlooked is the fact that it follows a winding course. One result of this is that the vistas are usually not long but are closed in by a succession of rocky pictures often of great beauty. As each bend of the gorge is passed there come into view new cliffs, fresh and fantastic shapes or even a cave, such as Dove Hole and Reynard’s Cave. These last are a reminder that the excavating of the dale has not been entirely due to the direct deepening of the river channel. Disappearing streams like those of the Lathkill and the Manifold have played their part by dissolving underground passage-ways beneath the floor of the dale which became open to the