The decay of flint itself, by the removal of part of its substance in solution, is the cause of the white surface on specimens from the Chalk, and of the crumbling white residues found in certain gravels. This process has been fully discussed by J. W. Judd, who believes that the material removed is silica in the opaline condition[27].
LIMESTONE AND SCENERY
Limestones in the field are characterised by joints which traverse considerable thicknesses of strata, until some shaly bed is met with, in which earth-stresses cannot set up such continuous planes of fracture. Since the conditions of deposition may remain constant for a long time in open seas, and since stratification cannot be obvious until these conditions change, limestones may have a massive character that is exceptional among sedimentary rocks. In some cases, however, where muddy rivers in times of flood have brought in detritus from the land, rapid and no doubt seasonal alternations of shale and limestone may be observed.
The Chalk of north-western Europe remains typically soft, lending itself to cliff-formation along the coast, where landslides are frequent through undercutting from below. Were it not for the development of flints along stratification-planes, it would be impossible at a distance to detect any bedded structure in the rock. Its representatives in eastern France, in the north zone of the Alps, or in the central Apennines, are compressed into far more resisting masses, and rear themselves as terraced crags and sheer rock-walls, in which the structure due to vertical joints is paramount. The English Chalk weathers into round-backed downs, clothed with thin grass, and hollowed into combes by streams that have long ago run dry. The soil owes hardly anything but its abundant flints to the white limestone rock on which it lies. Residual clays and sands derived from the breaking up of later beds allow of cultivation here and there, and beechwoods flourish even on the crests of the high downs. But water sinks freely into the ground, and may so far saturate the mass as to appear again in wet seasons in hollows of the surface as temporary springs or "bournes." When deep wells are sunk and pumping is begun, it is found that the supply varies greatly in different spots under seemingly uniform conditions. Even in so permeable a mass, there are waterways where maximum flow occurs. Channels where water soaks in from above, or weak places in the roofs of underground watercourses, become marked at the surface by sinkings known as swallow-holes. These increase in size with time, and are abandoned to the growth of scrub and trees.
Among more consolidated limestones, as we have hinted, the joints are effective in promoting bold rock-scenery. The absorptive power of the rock, rather than its hardness, prevents it from being washed away. Water that might round the edges of escarpments and send down taluses to modify the slopes sinks into the ground and works out passages by solution. On level surfaces, the solubility of limestone in water charged with carbon dioxide from the atmosphere is apparent by the formation of pitted hollows, with edges between them that grow sharper until they are worn through. Where a rain-drop first secures a resting-place, its successors deepen the little hollow. Water lies in this after every shower, working its way gently downwards. In time the rock may seem bored into as if prepared for blasting; the holes unite to form vertical grooves, and the surface is cut deeply into fantastic forms.
The face of the rock, formed by weathering on a valley-side or towards the sea, or occurring on any mass that is being cut back and reduced by denudation, is likely to be vertical, or at any rate perpendicular to the bedding. The form of the surfaces of the beds is perpetuated by their fairly uniform lowering through solution. The result is that stratification surfaces and planes perpendicular to them control in a very marked degree the scenery of limestone lands ([Fig. 1]).
Fig. 1. Surface of Limestone Plateau. Causse du Larzac, Aveyron, France.
Where the beds are level, with occasional partings of a slightly different composition, the country will develop terraces, like those of the Burren in northern Clare. Where they are folded, as in the Juras, scarps and dip-slopes follow one another picturesquely, the weathered edge of the bed, the true escarpment, being sometimes at an angle as steep as that of the dip. Hence a false effect of sharp peaks is produced, when these "edges" are seen end on at a distance.
The terrace-structure may be seen in miniature forms upon a rocky shore, where the blocks loosened from the escarpments of the successive beds are carried away by the waves. Frost-action is powerful in larger instances, and sends down huge blocks upon the lower terraces. A combination of shale bands and massive limestones, especially with a dip outward from the highland, leads to destructive landslips, since the sloping surface of shale is lubricated by water that passes through the limestone (see [Fig. 9]). Outward slips of the coast are thus common in Antrim, and have been extensive near Axmouth, two regions where chalk rests upon Liassic clays.