The Pamphlet Collection of Sir Robert Stout: Volume 33
The Geological Action of Water.*
The Geological Action of Water.*
On the last evening that I had the pleasure of addressing you on elementary geology, the subject I endeavoured to bring under your notice was that of the "Geological Action of Fire," and of the many ways in which geologists recognised its agency in forming the great mass of the earth—the various rock masses which under certain changes of conditions constitute not only the interior or central parts, but the whole of the earth with which geologists have to deal at the present time; and we saw how all these variously modified rocks, am all these accidents of surface, hills and valleys, depended mainly on various effects, both chemical and physical, clearly and exclusively due to igneous action.
I showed you, that if you supposed the whole earth to be originally a molten mass, composed of mineral materials comparatively few and simple, that according to degrees of fusibility, density, and other difference of circumstances, certain of these materials, or igneous rocks, would cool first, the result would be the actual mineral composition you find in nature, for the oldest and most inferior in position of the rocks you have to deal with.
I then showed you that all the investigations that had been made by geologists into the mineral composition of the rock of the lower portion of the earth, and experiments made a to the various temperatures now perceptible at different depth in the earth's interior, all coincided in pointing out the fact of the original intense heat by which all the rocky materials c the earth had in the earliest times been fused, existing probably page 16 at first as mere gaseous vapour, then condensing into a gigantic liquid drop, revolving on its axis as it now does. I then pointed out that if you had this liquid mass twirling round on such axis at the speed that we know the earth does so revolve, then in virtue of the molten condition of every part of the mass, it would, as a physical necessity of such rotation, assume the exact shape it now has—that is, somewhat like an orange, a little flattened at the poles, and bulging out at the equator. As philosophers are in the habit of referring effects to causes proved capable of producing them, all philosophers are now pretty well agreed on this elementary part of the subject to which I directed your attention in my last lecture.
I need not now refer again to the chemical and mineral nature of all the older rocks found in such conditions and places in the crust of the earth as would necessarily exhibit, if this theory of intense central heat were true, characters exclusively such as those produced by the liquefaction of the various minerals from heat, and not from solution in water; conditions in which all experimental observation agrees in showing that these igneous rocks do actually present.
Assuming then the theory correct of the original great heat and gradual cooling of the earth's crust, I proceeded to show you how it would necessarily contract in cooling and break up in various ways, and that in consequence of these ruptures various lines of mountains would be thrown up, the violence of the fractures displacing some portions of the crust upwards and others downwards, thus giving rise to the great physical features of the earth—the mountains of the land and the depths of the sea.
Having thus occupied your time on a former evening in considering the geological action of fire, I have now to invite your attention to the important geological actions of water.
To commence pretty much where I left off in the last lecture, you must consider that in the earliest time at which the earth assumed definite form and shape in space, it was still so hot that no water could exist on it, and that while you had a spherical form with a comparatively thin crust solid from cooling, there was another portion or envelope within that, in a liquid condition owing to the intense heat of the whole mass below the superficial crust; and then inside this liquid envelope you had the great central mass, increasing in its intense heat at a definite ascertained rate towards the centre, but nevertheless solid from the immense superincumbent pressure. When things were in this condition, you had granite forming page 17 the first crust, composed of quartz and mica, the two least fusible substances of the general melted mass, and therefore the first to solidify; and bearing in mind the necessity of the theory we have thus far elucidated, it would follow that the whole of the water we now know of in the ocean and elsewhere must have existed at this early period in a great atmosphere surrounding the earth, and formed of thin diffused clouds. The same cooling process which solidified the crust of the earth would extend further, space being intensely cold (much below zero of Fahrenheit), and such cooling going on with great rapidity, a portion of this atmospheric envelope would at last become so condensed as to be precipitated, and then down it would come in a great shower of rain. As soon as this first shower came down, it would boil from contact with the still extremely heated surface, and ascend again in vapour, but having had the effect of causing a still more rapid cooling of the surfaces on which it first fell, and then down would continue to come the water in still greater and increasing deluges.
Now, you must bear in mind that the first form of the earth would be mathematically correct in the regular uniform shaping of the surface, affording no dips or hollows for the water to lie in, being what is in natural philosophy called a "spheroid of rotation;" and the consequence would be that, when these deluges came down, the water would at first evenly cover the whole surface, moistening it to a trifling depth. While the cooling process became by this means exceedingly accelerated on the surface, the inner fluid envelope we have before referred to would also in cooling shrink away from the superincumbent crust, which latter, being thus deprived of support, would at last break in with meridional lines of fracture running from pole to pole, and appearing sub-parallel to each other when viewed on the small scale. Thus as some portions of the crust fell in, forming great hollows in some places, while other portions being tilted up formed great hills in others, the waters till then uniformly distributed would rush into the deeps and hollows, and in the old words, "the waters would be gathered together, and the dry land would appear."
Thus far, then, the necessary preliminary for the simplest consideration of the geological action of water—by far the most important agent with which we are acquainted in altering the surface of the earth; which it does principally by degrading or wearing down the high lands, and distributing page 18 the worn portion in horizontal layers in the deep hollows of the ocean. These two actions may be, separately considered, viz.:—1st, as to the degration; 2nd, the transport of the debris, or material, worn away.
The first, or most simple, mode of the geological action of water, is that of clouds condensing on the mountain tops. You have first to consider the action of the sun heating the sea, and thus raising the water by evaporation from the surface of the sea; and this action of the sun in raising water from the surface of the earth is the simple foundation upon which must rest all we have to consider with regard to the geological action of water.
You will have then the sun raising the water by evaporation, and bringing the aqueous vapour to a greater height. But directly this vapour is made to rise much into space, the capacity for heat of the higher regions is so great that the vapour is cooled very quickly, and returned into the lower atmosphere in the shape of mist or watery rain; or if carried still higher into still colder regions, it is brought down again in the shape of hailstones. Now, take the case of this aqueous vapour impelled by air-currents, and passing along a low-lying region where the heat is sufficient to maintain in a vapoury condition a large quantity of water until it reaches a mountain side; the current forces it to turn upwards to get over this mountain top; and the air still appearing perfectly clear below, suddenly shows on the mountain top in the shape of cloud. The most common of appearances in every mountainous country, due to the greater capacity of the air in the higher region for heat, is this appearance of clouds resting on the mountain tops. For this reason it is that we have the simplest commencement of the geological action of water as a general rule on the tops of mountains, although it must not be forgotten that many mountain peaks pierce so high above the cloud-level, as it is termed, that they reach a region which is perfectly dry, owing to the intense cold permitting no vapour at all to mount there; but, taking the ordinary level of cloud condensation, you have the common condition known as Scotch mist, or more potent showers occurring almost perpetually on these elevated parts of the country.
These ridges or lines of mountains, sometimes called the backbone, and also the water shed of a country, take the water from the atmosphere condensed into the ordinary liquid condition, which then by the force of gravity descends the sides of the water-sheds, and commences its geological work, page 19 which is in the first instance to decompose the superficial rocks at the top of the mountain, and this it does in different ways, partly chemical, and in all cases by physical or mechanical means, but whether by one or both operations the result is the same; invariably a tendency to disintegrate—in homely term, to rot away—that is, to produce a falling to pieces of all rock masses on which the water acts. You find that the first action even on the summits of the mountains is to denude them of all surface soil; and the waters then finding some particular channel, become more potent for grinding down material than before, forming beds of streams in the upper portions, gradually increasing to rivers below. As they go they continually wear away their channels, and still as they progress they add to their wearing powers enormously by carrying with them sand, pebbles, and other small portions of the rock masses worn away above, until the torrents are often of prodigious power, striking even the least observant eye. But geologists care little for these local instances of grandeur, and generally fix their attention more on the quiet, universal, and in reality much more powerful continuous action of this kind in every part of the world; a power having a definite relation to the velocity of the current, although an ordinary observer would scarcely be prepared to recognise the moving power of water in rivers at the ordinary rates; but striking examples are furnished at the mouths of some of the large rivers of the world, where the materials carried down form great triangular masses called deltas, from their form, resembling the Greek letter A—manifestly the result of the mud, sand, and shingle brought down to a level where the waters lose their transporting power from losing their velocity. These deltas have attracted the attention of all geological observers from early time, and I may refer here to some definite experiments made by Mr. Horner some thirty or forty years ago on the Rhine, at Bonn, on the exact transporting power of the river, which at that spot is some 1200 feet wide, 15 feet deep, and with a mean velocity of about 2½ miles an hour. His calculations gave as result the transport of 400 tons of material every hour, or about 8,000,000,000 of tons every year, nearly the whole of which is deposited in Holland, where the current loses its velocity, from the level nature of the country. This prepares you to consider the nature and extent of deltas formed at the mouths of greater rivers, such as the Ganges in India, the delta there being 200 miles long and 200 wide at the base, and covering 20,000 page 20 square miles of country, every atom of which has been carried down as mud in the water from the distant mountains in the interior.
I might multiply extraordinary examples of the definitely ascertained mechanical results produced by the greater rivers in various parts of the world, but general principles are all I have time to direct your attention to, so that I will pass on to fix your attention on the fact that the transporting power of a current of water increases enormously with its velocity; and this becomes a very interesting geological study.
When a geologist looks at any aqueous rock material which before solidifying must have merely presented the appearance of mud suspended in water, he knows that such fine material must have been deposited where no currents ran, probably at a depth in the ocean where no currents exist; and so, in looking at sandstone, or conglomerate containing sand, pebbles, &c., he knows that such a material must have been deposited where forcible currents existed, capable of moving such coarse portions; and carrying out this train of thought, the force of the current exhibits itself to the mind of the geologist in the size and density of particles in the deposits.
To take a few experimental examples of the actual transporting powers of different velocities, I may mention that common clay would be torn up easily by fresh water running at the rate of three inches per second, while it requires a velocity of six inches per second to carry fine sand such as that met with on the beach; eight inches per second will carry the coarser sand of the sandstone rocks, but it requires twelve inches per second to carry along even ordinarily fine gravel, twenty-four inches to carry round stones or pebbles two inches in diameter, and a swiftness of stream of thirty-six inches per second would be required for angular stones of the size of common road-metal.
The ordinary streams in hilly countries in flood-time frequently exceed this velocity, but when extraordinary floods occur masses of rock of enormously greater proportions, weighing even tons, are moved by currents of fresh water. To give a precise generalisation, the transporting power of moving water so augments with the quickness of the current, that in the language of the physicists it is said to increase "as the square of the velocity," a fact which will the more readily enable you to comprehend the great apparent increase of result in the case of deluges as compared to the comparatively small increase in the rate of velocity.page 21
This remarkably potent action of fresh water in carrying along masses of stone arises from the fact of its great density as compared with air; and such being the case, it can be readily understood that sea-water, being from its saline constituents much denser than fresh, marine currents are consequently much the more powerful transports of rocky materials.
It becomes therefore necessary to apply a correction to the calculations I have mentioned as to transporting power, when considering materials deposited in sea-water; and in this way you find that very feeble currents indeed in the ocean are sufficient to account for the coarse texture of many sandstones and conglomerate rocks with which geologists have to deal.
The currents in the sea are of great importance to the geologist, not only (as in the case of what is known as the Gulf Stream, in the Atlantic) modifying the character of the climate, but even modifying the character of the animal and vegetable life to kinds which, were it not for such currents, could not exist in many definite localities. Thus, the immense current of the great Gulf Stream which I have just alluded to is the great transporter of the water and temperature of the tropics towards the north. Being nearly 1000 miles wide near St. Helena, and carrying on its vast surface a heated atmosphere of its own, it is reflected from the Gulf of Mexico so as to bring a portion of the temperature of the air and waters, as well as many of the floating seeds and fruits of the West India Islands, to the western coast of Ireland and Great Britain; and even as far north as to the Hebrides, four thousand miles off, portions of vessels wrecked in the West Indies have been carried by the current. The climatic effect of this on the British Isles is that, whereas by geographical position they should be surrounded with icebergs and icebound coasts; that the mountains should be covered with perpetual snow, and the valleys filled with glaciers; that only in the summer heats should the coasts be approachable; that scarcely any of the vegetables, and very few of the kinds of animals that now inhabit the isles, should be able to exist there—as is the case in the Island of South Georgias, of similar geographical position in the southern hemisphere: instead of all this there is a mild climate—snow only in the winter, no glaciers and no icebergs, and an abundance of the plants and animals of warmer latitudes; and this wonderful fact is due to the raising of the whole temperature of the country by the warm waters of the Gulf Stream. But yet glaciers and other evidences of ice have left page 22 their traces in various parts of the British Isles, indicating probably a time when the temperature was much lower, or the climate colder, than at present.
Another mode of the action of sea-water is in undermining and breaking down rocky coasts. Every shore presenting a rocky front to the sea is crumbling away, the materials being carried off and deposited on beaches at the angle of rest, or that slope at which grains of loose sand will arrange themselves for quiet rest—an angle offering a powerful barrier to the sea. The sand and broken detritus taken from every rocky coast is carried out and laid at that same angle on some other low sandy shore or beach; and so every rocky coast is going back and is encroached on by the sea, and every humble sandy shore is advancing, and driving the sea back from before it.
This action of the sea, combined with that of the streams from the tops of the mountains, bringing down mud, sand, and other gravelly detritus, tends to counteract all that igneous agencies have effected in raising the mountain ranges of the earth; the rocky cliffs will be laid low, the mountain ranges worn down, and both be deposited on the low-lying shores; and the consequence will be that in no long time—speaking geologically, of course—we shall have no more land; it will all go into the depths of the ocean, have a uniform covering of sea to a uniform depth over it, and we shall arrive at pretty much the condition in which we began, and there will be an end to our geology.
Few things are more interesting than the power which a little knowledge of scientific matters gives us of seeing wonders in the commonest objects before us, and if what I have said be true on a grand scale, it is also true on a small scale, if we cultivate the faculty of observing nature truthfully, taking care not to be deceived by thinking we understand the things themselves because we have learned to make use of the terms applied to them. Always make it a point in attempting to investigate a subject, to know definitely what is meant by every word you use and hear, and do not be deceived by mere terms or theories; but go plainly to ascertain the laws of nature for yourself, apply your reasoning powers to them; and every one of you may add something important to the stores of human knowledge, if you will only find the occasion to examine common things more accurately than others have had time or opportunity to do before you, and record your observations in perfectly simple and truthful language.page 23
Nothing can be more marvellous than to look at the surface of the earth, and bearing in mind all this wearing away of that surface which we have been describing, to realise the fact that the present appearance of this our earth cannot have been, or be, of very long duration.
Go to Westmoreland, Cumberland, or any other lake district, and the idea will rest in your mind that the streams from these lakes are wearing away their channels, and that when these channels become more and more worn all the water must inevitably be let out from the lakes, and there will be an end of the great geographical features of the district.
To impress this point on your mind on a grand scale, let me invite your attention to this diagram of the Niagara River and Falls.
From Lake Erie you have the river coming down to another great lake below, Lake Ontario. On leaving Lake Erie it is a broad shallow stream, varying in width from one to three miles, and running a sluggish course through a great flat table-land—Canada on the one side, New York on the other—on a bed of silurian limestone, known as the Niagara Limestone, common also in Cumberland, Westmoreland, and in some parts of this country as well.
This table-land of nearly dead level ends abruptly at Queenstown, dropping to a lower level; and it is absolutely certain, from examination of the sides of the ravine, that originally the falls were at the point I now indicate to you on the diagram, where, as you see, an escarpment takes place, and formed a cascade such as we now see at the present actual falls.
In this escarpment you see that the upper half of the cliff is composed of hard dark grey marble, having under it a layer of shale, a soft muddy material, very evidently not so fit to bear the wear and tear of the river as the upper layer. Now the fall of water over the edge of the limestone at the escarpment wears away the soft rock below, and in doing so undermines a portion of the overhanging limestone; down that comes, and the consequence is that the Falls of Niagara have in this manner worked their way backwards for miles to their present position, where the retrograde movement is still acting in the same way. During the comparatively short period of observation of those living in the country, calculations have been carefully made as to the rate of this retrograde movement, a foot a year being the lowest, although many believe that three feet a-year would be a truer average. At this rate page 24 it would have taken only 35,000 years for the falls to have travelled from Queenstown to their present position; but lengthened period as that appears, you can well imagine the water still continuing to work back and back until the waters of Lake Erie are let out, and the whole face of the country changed by this incessant backward action and removal of the upper portions of the rock-bed to the lower levels, until these beautiful physical features of the landscapes, accidents of nature, destroy themselves by their own action.
Time is so short that I can scarcely touch on the more important matters I wished to draw your attention to, as showing the connection between the action of water and the grander geological features that geologists found much of their science upon; but I must draw your attention specially to the fact that, great as are the effects of ordinary currents at ordinary temperatures, both in the decomposition of land and in transporting it to lower levels, yet these effects are insignificant when compared with the increased powers developed by a reduction of temperature to a little below 32° of Fahrenheit, the freezing point of water. In all the cooler latitudes in which water freezes in the winter, you have this enormous and new power superadded. Water expands at the moment of freezing, a most curious and unusual property, which it possesses in common with but very few other bodies, the opposite being the rule. The heat of water may be reduced to forty degrees, and the water agrees with everything else by increasing in density as its temperature lowers to that point; but continuing to cool below forty degrees you suddenly find that it has a tendency to become lighter, and at 32°, or a little below, it suddenly expands, and increases by about 1-14th in bulk, and this it does with a power practically irresistable; so that you can well realise how wonderfully the degrading power of water is increased by the exercise in certain climates of this new natural property.
The ordinary slates covering our houses are not as a rule split by human means, for such an amount of labour would render them too costly. Take the instance of a slate-quarry in Wales. The masses of slate-rock are quarried, and left exposed with the edges upwards to the summer rains; the water is soaked up by them; and then, when the frost of winter comes, the water which has percolated through and amongst the laminœ of the slaty mass freezes, expands, and the whole mass opens out into so many leaves, like a book or fan; these laminœ being afterwards shaped into their due pro- page 25 portions for roofing and other purposes. What water does in this practically useful case it does invariably and everywhere in the cooler latitudes; so that in every valley the rains fall and drain into the crevices of the stony banks, and with the arrival of winter the rocks are torn asunder, and great sharp-edged and angular masses hurled around, as all who have travelled in such countries may have observed repeatedly. It is this expansion of the freezing water, wedging out great masses of rock from the mountain sides, which forms the main power in nature by which such masses are subjected the more easily to the comparatively weak action of the river currents that I referred to previously. The action of ice is enormous, not only for degrading but also for transporting; and this increase of power is also due to the expansion and consequent lightness of ice. The diminution of density is so great, that not only can enormous masses of frozen water float on the surface, but these can also support immense blocks of rock, and carry them thousands of miles over the ocean, moved by its currents from both polar regions to the warmer seas nearer the equator. You find in both hemispheres, towards the poles, these immense ice-islands, as depicted on the diagram now before you, coming regularly in spring and summer to the warmer seas, as far as the Azores in one hemisphere and the Cape of Good Hope in the other, and laden with these enormous rock-masses, of many tons in weight, having the peculiar character which the geologist recognises in them when found in all gravel and detritus of the "northern drift" deposits, as they are called, the peculiar character by which he at once recognises all these ice-borne rocks—viz., the edges and corners remaining sharp and unworn. As the icebergs melt in the warmer seas they frequently topple over, and then these immense blocks of stone are dropped to the bottom of the sea in this our present day, just as they were in the older geological times of the drift formations. The diagram before you illustrates some examples of these "erratic blocks" as they are termed, in the midst of large beds of fine sand and mud, deposited in all their angular character in those deep-sea beds in which no current could have travelled to roll them along; and also in rounded shingle-beds of boulders, or rounded water-worn stones, transported along the bottom by currents giving them a rounded, worn character, unlike that of the erratic blocks dropped amongst them.
A very curious circumstance connected with these "erratic blocks" is the fact that in Alpine countries you find them page 26 more commonly on the tops of the mountains than in the valleys or low plains. Let me instance to you the familiar example of what is known as the Three-rock Mountain, near Dublin, and which I draw roughly on the board before us. As you look from the city you distinctly see on the rounded top of the granite mountain, seven or eight miles off, three immense sharp-angled stones, which, on close inspection, prove to be of different material from the mountain itself on which they rest, and obviously come from some great distance. It becomes evident that at one time the surface of the sea must have been as high as the top of the mountain, or rather that the mountain must have been sunk below the level of the sea, so that the icebergs floating with these stones upon them there became stranded; and so with other submarine mountains on the shallows, over the upper points of which such stones have been deposited; and then when a whole country or continent was raised, there would remain the blocks on the tops of the mountains, and not in the valleys or plains, these latter in the submarine period being of course the deepest hollows of the ocean, which the icebergs would pass over unarrested.
The examination of the drift deposits in England and Scotland shows the geologist that at the latest of the geological periods immediately preceding human time, or even in human time itself, icebergs passed over those two countries in great number.
You doubtless know the configuration of the mountainous North of England tract of country joining England and Scotland, with its great mountains of limestone, and having towards the centre a low break, or north and south valley or depressed plain, with evidence of a great current having come from the north in the drift period, and rushed through this gap into the middle of England, and undoubtedly bringing great quantities of icebergs with it; for the northern sides of these limestone hills are so thickly covered over with unworn granite blocks that Dr. Buckland supposed they were in situ, and actually mapped out these hills as mountains of granite in his older plans; but subsequent research has shown them to be really mountains of limestone merely covered over with these granite and syenite masses, stranded on their higher parts; and what is more extraordinary still, every one of these masses shows traces in its mineral character of having come from mountains in Scotland, Sweden, or Norway, from which they were broken and to which they may be traced; while as you travel southward into England you find on the tops of the page 27 mountains the familiar North of England rocks; and so they all carry out the same story of ice-transporting action covering the land nearly to London itself with these obvious indications of its work.
In the same way you find geological evidence of the existence of ice and its mechanical action on this grand scale in many localities in Europe and America where such action no longer takes place, furnishing very conclusive evidence in its way of the existence during the formation of the drift deposits of what is known to geologists as "the glacial period," presenting the singular apparent anomaly of the particular localities in which these glacial formations make their appearance having been formerly visited with a very much colder climate than they now enjoy, in contradistinction to the general rule of evidence in all parts of the world in all geological times of the gradual cooling of the earth and climate as time progressed
An extraordinary evidence of the mechanical action of ice, besides that of transportation, is found in these regions. For example, nearly over the whole of Great Britain under the drift you find that the rocks coming to the surface appear as if planed by some vast planing machine, and not only so, but grooved, scratched, and scored in a nearly uniform north and south direction, and running for miles and miles in the same way as you would find at present produced on the beds of rivers or shallow seas by ice in which rocky masses had become entangled, marked with parallel lines not producible by any other means, and affording the most extraordinary evidence to the young geologist of the passing of ice with all these mineral masses entangled in it over the face of every familiar country of Europe, and the milder latitudes of North America.
From the failure of our time, I regret to find that I must leave untouched the subject of glaciers themselves, and the discussion of the "Glacial Period" of geologists.
* This lecture is printed from the reporters notes, as revised by Professor M'Coy.