Chap03

[31]

CHAPTER III.

DENUDATION, SYNCLINAL AND ANTICLINAL CURVES, UNCONFORMABLE STRATIFICATION, AND WASTE PRODUCED BY CHEMICAL ACTION.

I MUST now more precisely explain the meaning of a few terms which I have already employed, and shall have occasion to use very frequently.

Denudation, in the geological sense of the word, means the stripping away of rocks from the surface, so as to expose other rocks that lay concealed beneath them.

Running water wears away the ground over which it passes, and carries away detrital matter, such as pebbles, sand, and mud; and if this goes on long enough over large areas, there is no reason why any amount of matter should not in time be removed. For instance, we have a notable case in North America of a considerable result from denudation, now being effected by the river Niagara, where, below the Falls, the river has cut a deep channel through the rocks, about seven miles in length. The proofs are perfect that the Falls originally began at the great, escarpment at the lower end of what is now this gorge; that the river, falling over this ancient cliff, by degrees wore for itself a channel backwards, from two hundred to a hundred and sixty feet deep, through strata that on either side of the gorge once formed a continuous plateau.

[32 Denudation.]

I merely give this instance to show what I mean by denudation produced by running water. At one time the channel did not exist. The river has cut it out, and in doing so, strata—some of them formerly one hundred and sixty feet beneath the surface—have been exposed by denudation. Possible, but very uncertain calculations, show that to form this gorge a period at the least of something like thirty-five thousand years has been required. This is an important instance, and it is similar to many other cases constantly before our eyes, on a smaller scale, which rarely strike the ordinary observer.

Refer to fig. 6, and suppose that we have different strata, 1, 2, 3, and 4, lying horizontally one above the other, together forming a mass several hundreds of feet in thickness. The running water of a brook

FIG. 6.

or river by degrees wore away the rocks more in one place than another, so that the strata 3, 2, and 1, were successively cut into and exposed at the surface, and a valley in time is formed. This is the result of denudation.

Or to take a much larger instance. The strata that form the outer part of the crust of the Earth have, in many places, by the contraction of that crust due to cooling of the mass, been thrown into anticlinal and synclinal curves. A synclinal curve means that the curved strata are bent downwards as in 1, Fig. 7, an anticlinal curve that they bend upwards as in 2. The whole were originally deposited horizontally,

[Denudation. 33]

consolidated into rock, and afterwards bent and contorted. The strata marked x may perfectly correspond in all respects in their structure and fossils, and in hundreds

FIG. 7.

1. Synclinal curves. 2. Anticlinal curve,

of similar cases it is certain that they were once joined as horizontal strata, and afterwards thrown into anticlinal and synclinal curves. The strata indicated by dotted lines (and all above) have been. removed by denudation, and the present surface is the result.

Chemical action is another agent that promotes waste or denudation. Thus rain water, always charged with carbonic acid, falling on limestone rocks such as the Carboniferous Limestone, or the Chalk, not only wears away part of these rocks by mechanical action, but also dissolves the carbonate of lime and carries it off in solution as a bicarbonate. This fact is often proved by numbers of unworn flints sometimes several feet in thickness scattered on the surface of the table-land of chalk in Wilts and Dorsetshire. The flints now lying loose on the surface once formed interrupted beds often separated by many feet of chalk. The chalk has been dissolved and carried away in solution chiefly by moving water, and the insoluble flints remain.

Degradation of the rocks of many regions is also powerfully affected by occasional landslips. The waste thus produced is seen on a large scale in many of the Yorkshire valleys, where Carboniferous sandstones and shales are jnterstratified, and vast shattered ruins of

[34 Denudation.]

sandstones cumber the sides of the hills and the bottoms of the valleys in wild confusion (fig. 66, p. 329). In Switzerland the relics of old landslips are often seen on a magnificent scale; and some of these, such as those of the Rossberg, and St. Nicholas in the valley of Zermatt, have taken place in the memory of living men.

The constant atmospheric disintegration of cliffs, and the beating of the waves on the shore, often aided by landslips, is another mode by which watery action denudes and cuts back rocks. This has been already mentioned. Caverns, bays, and other indentations of the coast, needle-shaped rocks standing out in the sea from the main mass of a cliff, are all caused or aided by the long-continued wasting power of the sea, which first helps to destroy the land and then spreads the ruins in new strata over its bottom.

It requires a long process of geological education to enable anyone thoroughly to realise the conception of the vast amount of old denudations; but when we consider that, over and over again, strata thousands of square miles in extent, and thousands of feet in thickness, have been formed by the waste of older rocks, equal in extent and bulk to the strata formed by their waste, we begin to get an idea of the greatness of this power. The mind is then more likely to realise the vast amount of matter that has been swept away from the surface of any country, in times comparatively quite recent, before it has assumed its present form. Without much forestalling the subject of a subsequent chapter, I may now state that a notable example on a grand scale may be seen in the coal-fields of South Wales, of Bristol, and of the Forest of Dean. These three coalfields were once united, but those of South Wales and Dean Forest are now about twenty-five miles

[Unconformity. 35]

apart, while the Bristol and Somersetshire coal-field is separated from both by the estuary of the Severn. These separations have been brought about by the agency of long-continued denudations, which have swept away thousands of feet of strata bent into anticlinal and synclinal curves in the manner shown at x in fig. 7, p. 33, and fig. 115, p. 601. The coal-field of the Forest of Dean has thus become an outlier of the great South Wales coal-field; and the Bristol or Somersetshire coal-field forms another outlier of a great area, of which even the South Wales coal-field is a mere fragment. Such denudations have been common over large areas in Wales and the adjacent counties, and in many another county besides.

Observation and argument alike tell us that we need have no hesitation in applying this reasoning to all hilly regions, whether formed of stratified rocks alone or intercalated with igneous rocks, and thus we come to the conclusion that the greater portion of the rocky masses of our island have been arranged and re-arranged under slow processes of the denudation of old, and the reconstruction of newer strata, extending over periods that seem to our finite minds almost to stretch into infinity.

Unconformable stratification, when its significance has been realised by the student, cannot fail at once to impress on the mind a sense of the degradation of strata in some old epoch similar to that which is now going on, and I know of few objects that speak more eloquently of geological time.

In the following diagram No. 1 represents an old land surface, in which perhaps beds of sandstone and slate or shale have been upheaved at a high angle. Let us then suppose that, by the wasting power of weather and

[36 Unconformity.]

the sea, the strata No. 2 have been won from that old land and deposited on the upturned and denuded edges of the strata No. 1. This constitutes a case of unconformable stratification, and this alone marks the lapse

FIG. 8.

1. Old disturbed strata.
2. Later beds lying unconformably upon them.

of immense periods of geological time, first by the deposition, consolidation, and upheaval of the strata No. 1, and secondly in the deposition of the strata No. 2, which were made from the waste of No. 1.

There are many cases of this kind of unconformity extending through all geological time from the Laurentian epochs onwards. If, in addition to this, we consider the meaning of the progressive changes of genera and species of animals and plants, as we proceed from the older to the newer formations (as expressed in Chapter II.), it soon becomes obvious, that as yet we have no means of even attempting to form any clear idea of the time that has elapsed since life first appeared on the surface of the world, whether we adopt the original view of a distinct creative act for each individual species, or prefer the later one of evolution and progressive development.

To explain in some detail the anatomical structure or existing Physical Geography of our island, as dependent on the nature of its strata and the alterations and denudations they have undergone is the main object of this book. In making this attempt I shall

[Object of Book. 37]

first describe in some detail and in chronological order all the rocky formations that constitute Great Britain, with reference, where it can be done, to the Physical Geology and Geography of each large special epoch, for only in this way may we hope to get an idea of how our island at length got into that phase of its history in which we happen to live. If the reader has been able to follow me in what I have already written, I think he will understand what I shall have to say in the remaining chapters.