A Manual of Elementary Geology Part 15

[Ill.u.s.tration: Fig. 96. Cross section.]

This appearance of a range of marble seats circling round the head of a valley, or of great flights of steps descending from the top to the bottom, on the opposite sides of a gorge, may be accounted for, as already hinted, by supposing the sea to have stood successively at many different levels, as at _a a_, _b b_, _c c_, in the accompanying fig. 96. But the causes of the gradual contraction of the valley from above downwards may still be matter of speculation. Such contraction may be due to the greater force exerted by the waves when the land at its first emergence was smaller in quant.i.ty, and more exposed to denudation in an open sea; whereas the wear and tear of the rocks might diminish in proportion as this action became confined within bays or channels closed in on two or three sides. Or, secondly, the separate movements of elevation may have followed each other more rapidly as the land continued to rise, so that the times of those pauses, during which the greatest denudation was accomplished at certain levels, were always growing shorter. It should be remarked, that the cliffs and small terraces are rarely found on the opposite sides of the Sicilian valleys at heights so precisely answering to each other as those given in fig. 96., and this might have been expected, to whichever of the two hypotheses above explained we incline; for, according to the direction of the prevailing winds and currents, the waves may beat with unequal force on different parts of the sh.o.r.e, so that while no impression is made on one side of a bay, the sea may encroach so far on the other as to unite several smaller cliffs into one.

Before quitting the subject of ancient sea-cliffs, carved out of limestone, I shall mention the range of precipitous rocks, composed of a white marble of the Oolitic period, which I have seen near the northern gate of St. Mihiel in France. They are situated on the right bank of the Meuse, at a distance of 200 miles from the nearest sea, and they present on the precipice facing the river three or four horizontal grooves, one above the other, precisely resembling those which are scooped out by the undermining waves. The summits of several of these ma.s.ses are detached from the adjoining hill, in which case the grooves pa.s.s all round them, facing towards all points of the compa.s.s, as if they had once formed rocky islets near the sh.o.r.e.[78-A]

Captain Bayfield, in his survey of the Gulf of St. Lawrence, discovered in several places, especially in the Mingan islands, a counterpart of the inland cliffs of St. Mihiel, and traced a succession of shingle beaches, one above the other, which agreed in their level with some of the princ.i.p.al grooves scooped out of the limestone pillars. These beaches consisted of calcareous shingle, with sh.e.l.ls of recent species, the farthest from the sh.o.r.e being 60 feet above the level of the highest tides. In addition to the drawings of the pillars called the flower-pots, which he has published[78-B], I have been favoured with other views of rocks on the same coast, drawn by Lieut. A. Bowen, R. N. (See fig. 97.)

[Ill.u.s.tration: Fig. 97. Limestone columns in Niapisca Island, in the Gulf of St. Lawrence. Height of the second column on the left, 60 feet.]

In the North-American beaches above mentioned rounded fragments of limestone have been found perforated by _lithodomi_; and holes drilled by the same mollusks have been detected in the columnar rocks or "flower-pots," showing that there has been no great amount of atmospheric decomposition on the surface, or the cavities alluded to would have disappeared.

[Ill.u.s.tration: Fig. 98. The North Rocks, Bermuda, lying outside the great coral reef. A. 16 feet high, and B. 12 feet. _c._ _c._ Hollows worn by the sea.]

We have an opportunity of seeing in the Bermuda islands the manner in which the waves of the Atlantic have worn, and are now wearing out, deep smooth hollows on every side of projecting ma.s.ses of hard limestone. In the annexed drawing, communicated to me by Lieut. Nelson, the excavations _c_, _c_, _c_, have been scooped out by the waves in a stone of very modern date, which, although extremely hard, is full of recent corals and sh.e.l.ls, some of which retain their colour.

When the forms of these horizontal grooves, of which the surface is sometimes smooth and almost polished, and the roofs of which often overhang to the extent of 5 feet or more, have been carefully studied by geologists, they will serve to testify the former action of the waves at innumerable points far in the interior of the continents. But we must learn to distinguish the indentations due to the original action of the sea, and those caused by subsequent chemical decomposition of calcareous rocks, to which they are liable in the atmosphere.

Notwithstanding the enduring nature of the marks left by littoral action on calcareous rocks, we can by no means detect sea-beaches and inland cliffs everywhere, even in Sicily and the Morea. On the contrary, they are, upon the whole, extremely partial, and are often entirely wanting in districts composed of argillaceous and sandy formations, which must, nevertheless, have been upheaved at the same time, and by the same intermittent movements, as the adjoining calcareous rocks.

FOOTNOTES:

[67-A] Western Islands, vol. ii. p. 93. pl. 31. fig. 4.

[69-A] See Mammat's Geological Facts, &c. p. 90. and plate.

[69-B] Conybeare's Report to Brit. a.s.soc. 1842, p. 381.

[70-A] Prestwich, Geol. Trans. second series, vol. v. pp. 452. 473.

[75-A] Section given by Dr. Christie, Edin. New Phil. Journ. No.

xxiii., called by mistake the Cave of Mardolce, by the late M. Hoffmann.

See account by Mr. S. P. Pratt, F. G. S. Proceedings of Geol. Soc.

No. 32. 1833.

[78-A] I was directed by M. Deshayes to this spot, which I visited in June, 1833.

[78-B] See Trans. of Geol. Soc., second series, vol. v. plate v.

CHAPTER VII.

ALLUVIUM.

Alluvium described--Due to complicated causes--Of various ages, as shown in Auvergne--How distinguished from rocks in situ--River-terraces--Parallel roads of Glen Roy--Various theories respecting their origin.

Between the superficial covering of vegetable mould and the subjacent rock there usually intervenes in every district a deposit of loose gravel, sand, and mud, to which the name of alluvium has been applied. The term is derived from _alluvio_, an inundation, or _alluo_, to wash, because the pebbles and sand commonly resemble those of a river's bed or the mud and gravel spread over low lands by a flood.

A partial covering of such alluvium is found alike in all climates, from the equatorial to the polar regions; but in the higher lat.i.tudes of Europe and North America it a.s.sumes a distinct character, being very frequently devoid of stratification, and containing huge fragments of rock, some angular and others rounded, which have been transported to great distances from their parent mountains. When it presents itself in this form, it has been called "diluvium," "drift," or the "boulder formation;" and its probable connexion with the agency of floating ice and glaciers will be treated of more particularly in the eleventh and twelfth chapters.

[Ill.u.s.tration: Fig. 99. Lavas of Auvergne resting on alluviums of different ages.]

The student will be prepared, by what I have said in the last chapter on denudation, to hear that loose gravel and sand are often met with, not only on the low grounds bordering rivers, but also at various points on the sides or even summits of mountains. For, in the course of those changes in physical geography which may take place during the gradual emergence of the bottom of the sea and its conversion into dry land, any spot may either have been a sunken reef, or a bay, or estuary, or sea-sh.o.r.e, or the bed of a river. For this reason it would be unreasonable to hope that we should ever be able to account for all the alluvial phenomena of each particular country, seeing that the causes of their origin are so complicated. Moreover, the last operations of water have a tendency to disturb and confound together all pre-existing alluviums. Hence we are always in danger of regarding as the work of a single era, and the effect of one cause, what has in reality been the result of a variety of distinct agents, during a long succession of geological epochs. Much useful instruction may therefore be gained from the exploration of a country like Auvergne, where the superficial gravel of very different eras happens to have been preserved by sheets of lava, which were poured out one after the other at periods when the denudation, and probably the upheaval, of rocks were in progress. That region had already acquired in some degree its present configuration before any volcanos were in activity, and before any igneous matter was superimposed upon the granitic and fossiliferous formations. The pebbles therefore in the older gravels are exclusively const.i.tuted of granite and other aboriginal rocks; and afterwards, when volcanic vents burst forth into eruption, those earlier alluviums were covered by streams of lava, which protected them from intermixture with gravel of subsequent date. In the course of ages, a new system of valleys was excavated, so that the rivers ran at lower levels than those at which the first alluviums and sheets of lava were formed. When, therefore, fresh eruptions gave rise to new lava, the melted matter was poured out over lower grounds; and the gravel of these plains differed from the first or upland alluvium, by containing in it rounded fragments of various volcanic rocks, and often bones belonging to distinct groups of land animals which flourished in the country in succession.

The annexed drawing will explain the different heights at which beds of lava and gravel, each distinct from the other in composition and age, are observed, some on the flat tops of hills, 700 or 800 feet high, others on the slope of the same hills, and the newest of all in the channel of the existing river where there is usually gravel alone, but in some cases a narrow stripe of solid lava sharing the bottom of the valley with the river. In all these acc.u.mulations of transported matter of different ages the bones of extinct quadrupeds have been found belonging to a.s.semblages of land mammalia which flourished in the country in succession, and which vary specifically, the one from the other, in a greater or less degree, in proportion as the time which separated their entombment has been more or less protracted. The streams in the same district are still undermining their banks and grinding down into pebbles or sand, columns of basalt and fragments of granite and gneiss; but the older alluviums, with the fossil remains belonging to them, are prevented from being mingled with the gravel of recent date by the cappings of lava before mentioned. But for the accidental interference, therefore, of this peculiar cause, all the alluviums might have pa.s.sed so insensibly the one into the other, that those formed at the remotest era might have appeared of the same date as the newest, and the whole formation might have been regarded by some geologists as the result of one sudden and violent catastrophe.

In almost every country, the alluvium consists in its upper part of transported materials, but it often pa.s.ses downwards into a ma.s.s of broken and angular fragments derived from the subjacent rock. To this ma.s.s the provincial name of "rubble," or "brash," is given in many parts of England. It may be referred to the weathering or disintegration of stone on the spot, the effects of air and water, sun and frost, and chemical decomposition.

[Ill.u.s.tration: Fig. 100. Cross section.

_a._ Vegetable soil.

_b._ Alluvium.

_c._ Ma.s.s of same, apparently detached.]

The inferior surface of alluvial deposits is often very irregular, conforming to all the inequalities of the fundamental rocks (fig. 100.).

Occasionally, a small ma.s.s, as at _c_, appears detached, and as if included in the subjacent formation. Such isolated portions are usually sections of winding subterranean hollows filled up with alluvium. They may have been the courses of springs or subterranean streamlets, which have flowed through and enlarged natural rents; or, when on a small scale and in soft strata, they may be s.p.a.ces which the roots of large trees have once occupied, gravel and sand having been introduced after their decay.

[Ill.u.s.tration: Fig. 101. Sand-pipes in the chalk at Eaton, near Norwich.]

But there are other deep hollows of a cylindrical form found in England, France, and elsewhere, penetrating the white chalk, and filled with sand and gravel, which are not so readily explained. They are sometimes called "sand-pipes," or "sand-galls," and "puits naturels," in France. Those represented in the annexed cut were observed by me in 1839, laid open in a large chalk-pit near Norwich. They were of very symmetrical form, the largest more than 12 feet in diameter, and some of them had been traced, by boring, to the depth of more than 60 feet. The smaller ones varied from a few inches to a foot in diameter, and seldom descended more than 12 feet below the surface. Even where three of them occurred, as at _a_, fig. 101., very close together, the parting walls of soft white chalk were not broken through. They all taper downwards and end in a point. As a general rule, sand and pebbles occupy the central parts of each pipe, while the sides and bottom are lined with clay.

Mr. Trimmer, in speaking of appearances of the same kind in the Kentish chalk, attributes the origin of such "sand-galls" to the action of the sea on a beach or shoal, where the waves, charged with shingle and sand, not only wear out longitudinal furrows, such as may be observed on the surface of the chalk near Norwich when the inc.u.mbent gravel is removed, but also drill deep circular hollows by the rotatory motion imparted to sand and pebbles. Such furrows, as well as vertical cavities, are now formed, he observes, on the coast where the sh.o.r.es are composed of chalk.[82-A]

That the commencement of many of the tubular cavities now under consideration has been due to the cause here a.s.signed, I have little doubt.

But such mechanical action could not have hollowed out the whole of the sand-pipes _c_ and _d_, fig. 101., because several large chalk-flints seen protruding from the walls of the pipes have not been eroded, while sand and gravel have penetrated many feet below them. In other cases, as at _b b_, similar unrounded nodules of flint, still preserving their irregular form and white coating, are found at various depths in the midst of the loose materials filling the pipe. These have evidently been detached from regular layers of flints occurring above. It is also to be remarked that the course of the same sand-pipe, _b b_, is traceable above the level of the chalk for some distance upwards, through the inc.u.mbent gravel and sand, by the obliteration of all signs of stratification. Occasionally, also, as in the pipe _d_, the overlying beds of gravel bend downwards into the mouth of the pipe, so as to become in part vertical, as would happen if horizontal layers had sunk gradually in consequence of a failure of support. All these phenomena may be accounted for by attributing the enlargement and deepening of the sand-pipes to the chemical action of water charged with carbonic acid, derived from the vegetable soil and the decaying roots of trees. Such acid might corrode the chalk, and deepen indefinitely any previously existing hollow, but could not dissolve the flints. The water, after it had become saturated with carbonate of lime, might freely percolate the surrounding porous walls of chalk, and escape through them and from the bottom of the tube, so as to carry away in the course of time large ma.s.ses of dissolved calcareous rock[83-A], and leave behind it on the edges of each tubular hollow a coating of fine clay, which the white chalk contains.

I have seen tubes precisely similar and from 1 to 5 feet in diameter traversing vertically the upper half of the soft calcareous building stone, or chalk without flints, const.i.tuting St. Peter's Mount, Maestricht. These hollows are filled with pebbles and clay, derived from overlying beds of gravel, and all terminate downwards like those of Norfolk. I was informed that, 6 miles from Maestricht, one of these pipes, 2 feet in diameter, was traced downwards to a bed of flattened flints, forming an almost continuous layer in the chalk. Here it terminated abruptly, but a few small root-like prolongations of it were detected immediately below, probably where the dissolving substance had penetrated at some points through openings in the siliceous ma.s.s.

It is not so easy as may at first appear to draw a clear line of distinction between the _fixed_ rocks, or regular strata (rocks _in situ_ or _in place_), and _alluvium_. If the bed of a torrent or river be dried up, we call the gravel, sand, and mud left in their channels, or whatever, during floods, they may have scattered over the neighbouring plains, alluvium. The very same materials carried into a lake, where they become sorted by water and arranged in more distinct layers, especially if they inclose the remains of plants, sh.e.l.ls, or other fossils, are termed regular strata.

In like manner we may sometimes compare the gravel, sand, and broken sh.e.l.ls, strewed along the path of a rapid marine current, with a deposit formed contemporaneously by the discharge of similar materials, year after year, into a deeper and more tranquil part of the sea. In such cases, when we detect marine sh.e.l.ls or other organic remains entombed in the strata, which enable us to determine their age and mode of origin, we regard them as part of the regular series of fossiliferous formations, whereas, if there are no fossils, we have frequently no power of separating them from the general ma.s.s of superficial alluvium.

The usual rarity of organic remains in beds of loose gravel and sand is partly owing to the rapid and turbid water in which they were formed having been in a condition unfavourable to the habitation of aquatic beings, and partly to their porous nature, which, by allowing the free percolation of rain-water, has promoted the decomposition and removal of organic matter.

It has long been a matter of common observation that most rivers are now cutting their channels through alluvial deposits of greater depth and extent than could ever have been formed by the present streams. From this fact a rash inference has sometimes been drawn, that rivers in general have grown smaller, or become less liable to be flooded than formerly. But such phenomena would be a natural result of any considerable oscillations in the level of the land experienced since the existing valleys originated.

Suppose part of a continent, comprising within it a large hydrographical basin like that of the Mississippi, to subside several inches or feet in a century, as the west coast of Greenland, extending 600 miles north and south, has been sinking for three or four centuries, between the lat.i.tudes 60 and 69 N.[84-A] There might be no encroachment of the sea at the river's mouth in consequence of this change of level, but the fall of the waters flowing from the interior being lessened, the main river and its tributaries would have less power to carry down to its delta, and to discharge into the ocean, the sedimentary matter with which they are annually loaded. They would all begin to raise their channels and alluvial plains by depositing in them the heavier sand and pebbles washed down from the upland country, and this operation would take place most effectively if the amount of subsidence in the interior was unequal, and especially if, on the whole, it exceeded that of the region near the sea. If then the same area of land be again upheaved to its former height, the fall, and consequently the velocity, of every river would begin to augment. Each of them would be less given to overflow its alluvial plain; and their power of carrying earthy matter seaward, and of scouring out and deepening their channels, would continue till, after a lapse of many thousand years, each of them would have eroded a new channel or valley through a fluviatile formation of modern date. The surface of what was once the river-plain at the period of greatest depression, would remain fringing the valley sides in the form of a terrace apparently flat, but in reality sloping down with the general inclination of the river. Everywhere this terrace would present cliffs of gravel and sand, facing the river. That such a series of movements has actually taken place in the main valley of the Mississippi and in its tributary valleys during oscillations of level, I have endeavoured to show in my description of that country[85-A]; and the freshwater sh.e.l.ls of existing species and bones of land quadrupeds, partly of extinct races preserved in the terraces of fluviatile origin, attest the exclusion of the sea during the whole process of filling up and partial re-excavation.

In many cases, the alluvium in which rivers are now cutting their channels, originated when the land first rose out of the sea. If, for example, the emergence was caused by a gradual and uniform motion, every bay and estuary, or the straits between islands, would dry up slowly, and during their conversion into valleys, every part of the upheaved area would in its turn be a sea-sh.o.r.e, and might be strewed over with littoral sand and pebbles, or each spot might be the point where a delta acc.u.mulated during the retreat and exclusion of the sea. Materials so acc.u.mulated would conform to the general slope of a valley from its head to the sea-coast.

_River terraces._--We often observe at a short distance from the present bed of a river a steep cliff a few feet or yards high, and on a level with the top of it a flat terrace corresponding in appearance to the alluvial plain which immediately borders the river. This terrace is again bounded by another cliff, above which a second terrace sometimes occurs: and in this manner two or three ranges of cliffs and terraces are occasionally seen on one or both sides of the stream, the number varying, but those on the opposite sides often corresponding in height.

[Ill.u.s.tration: Fig. 102. River Terraces and Parallel Roads.]

These terraces are seldom continuous for great distances, and their surface slopes downwards, with an inclination similar to that of the river. They are readily explained if we adopt the hypothesis before suggested, of a gradual rise of the land; especially if, while rivers are shaping out their beds, the upheaving movement be intermittent, so that long pauses shall occur, during which the stream will have time to encroach upon one of its banks, so as to clear away and flatten a large s.p.a.ce. This operation being afterwards repeated at lower levels, there will be several successive cliffs and terraces.