Town Geology 4

Town Geology 4

And thus I end this paper. I trust it has not been intolerably dull.

But I wanted at starting to show my readers something of the right

way of finding out truth on this and perhaps on all subjects; to make

some simple appeals to your common sense; and to get you to accept

some plain rules founded on common sense, which will be of infinite

use to both you and me in my future papers.

 

I hope, meanwhile, that you will agree with me, that there is plenty

of geological matter to be seen and thought over in the neighbourhood

of any town.

 

Be sure, that wherever there is a river, even a drain; and a stone

quarry, or even a roadside bank; much more where there is a sea, or a

tidal aestuary, there is geology enough to be learnt, to explain the

greater part of the making of all the continents on the globe.

 

 

 

II. THE PEBBLES IN THE STREET

 

 

 

If you, dear reader, dwell in any northern town, you will almost

certainly see paving courts and alleys, and sometimes--to the

discomfort of your feet--whole streets, or set up as bournestones at

corners, or laid in heaps to be broken up for road-metal, certain

round pebbles, usually dark brown or speckled gray, and exceedingly

tough and hard. Some of them will be very large--boulders of several

feet in diameter. If you move from town to town, from the north of

Scotland as far down as Essex on the east, or as far down as

Shrewsbury and Wolverhampton (at least) on the west, you will still

find these pebbles, but fewer and smaller as you go south. It

matters not what the rocks and soils of the country round may be.

However much they may differ, these pebbles will be, on the whole,

the same everywhere.

 

But if your town be south of the valley of the Thames, you will find,

as far as I am aware, no such pebbles there. The gravels round you

will be made up entirely of rolled chalk flints, and bits of beds

immediately above or below the chalk. The blocks of "Sarsden"

sandstone--those of which Stonehenge is built--and the "plum-pudding

stones" which are sometimes found with them, have no kindred with the

northern pebbles. They belong to beds above the chalk.

 

Now if, seeing such pebbles about your town, you inquire, like a

sensible person who wishes to understand something of the spot on

which he lives, whence they come, you will be shown either a gravel-

pit or a clay-pit. In the gravel the pebbles and boulders lie mixed

with sand, as they do in the railway cutting just south of

Shrewsbury; or in huge mounds of fine sweet earth, as they do in the

gorge of the Tay about Dunkeld, and all the way up Strathmore, where

they form long grassy mounds--tomauns as they call them in some parts

of Scotland--askers as they call them in Ireland. These mounds, with

their sweet fresh turf rising out of heather and bog, were tenanted--

so Scottish children used to believe--by fairies. He that was lucky

might hear inside them fairy music, and, the jingling of the fairy

horses' trappings. But woe to him if he fell asleep upon the mound,

for he would be spirited away into fairyland for seven years, which

would seem to him but one day. A strange fancy; yet not so strange

as the actual truth as to what these mounds are, and how they came

into their places.

 

Or again, you might find that your town's pebbles and boulders came

out of a pit of clay, in which they were stuck, without any order or

bedding, like plums and raisins in a pudding. This clay goes usually

by the name of boulder-clay. You would see such near any town in

Cheshire and Lancashire; or along Leith shore, near Edinburgh; or, to

give one more instance out of hundreds, along the coast at

Scarborough. If you walk along the shore southward of that town, you

will see, in the gullies of the cliff, great beds of sticky clay,

stuffed full of bits of every rock between the Lake mountains and

Scarborough, from rounded pebbles of most ancient rock down to great

angular fragments of ironstone and coal. There, as elsewhere, the

great majority of the pebbles have nothing to do with the rock on

which the clay happens to lie, but have come, some of them, from

places many miles away.

 

Now if we find spread over a low land pebbles composed of rocks which

are only found in certain high lands, is it not an act of common

sense to say--These pebbles have come from the highlands? And if the

pebbles are rounded, while the rocks like them in the highlands

always break off in angular shapes, is it not, again, an act of mere

common sense to say--These pebbles were once angular, and have been

rubbed round, either in getting hither or before they started hither?

 

Does all this seem to you mere truism, my dear reader? If so, I am

sincerely glad to hear it. It was not so very long ago that such

arguments would have been considered not only no truisms, but not

even common sense.

 

But to return, let us take, as an example, a sample of these boulder

clay pebbles from the neighbourhood of Liverpool and Birkenhead, made

by Mr. De Rance, the government geological surveyor:

 

Granite, greenstone, felspar porphyry, felstone, quartz rock (all

igneous rocks, that is, either formed by, or altered by volcanic

heat, and almost all found in the Lake mountains), 37 per cent.

 

Silurian grits (the common stones of the Lake mountains deposited by

water), 43 per cent.

 

Ironstone, 1 per cent.

 

Carboniferous limestone, 5 per cent.

 

Permian or Triassic sandstones, i.e. rocks immediately round

Liverpool, 12 per cent.

 

Now, does not this sample show, as far as human common sense can be

depended on, that the great majority of these stones come from the

Lake mountains, sixty or seventy miles north of Liverpool? I think

your common sense will tell you that these pebbles are not mere

concretions; that is, formed out of the substance of the clay after

it was deposited. The least knowledge of mineralogy would prove

that. But, even if you are no mineralogist, common sense will tell

you, that if they were all concreted out of the same clay, it is most

likely that they would be all of the same kind, and not of a dozen or

more different kinds. Common sense will tell you, also, that if they

were all concreted out of the same clay, it is a most extraordinary

coincidence, indeed one too strange to be believed, if any less

strange explanation can be found--that they should have taken the

composition of different rocks which are found all together in one

group of mountains to the northward. You will surely say--If this be

granite, it has most probably come from a granite mountain; if this

be grit, from a grit-stone mountain, and so on with the whole list.

Why--are we to go out of our way to seek improbable explanations,

when there is a probable one staring us in the face?

 

Next--and this is well worth your notice--if you will examine the

pebbles carefully, especially the larger ones, you will find that

they are not only more or less rounded, but often scratched; and

often, too, in more than one direction, two or even three sets of

scratches crossing each other; marked, as a cat marks an elder stem

when she sharpens her claws upon it; and that these scratches have

not been made by the quarrymen's tools, but are old marks which

exist--as you may easily prove for yourself--while the stone is still

lying in its bed of clay. Would it not be an act of mere common

sense to say--These scratches have been made by the sharp points of

other stones which have rubbed against the pebbles somewhere, and

somewhen, with great force?

 

So far so good. The next question is--How did these stones get into

the clay? If we can discover that, we may also discover how they

wore rounded and scratched. We must find a theory which will answer

our question; and one which, as Professor Huxley would say, "will go

on all-fours," that is, will explain all the facts of the case, and

not only a few of them.

 

What, then, brought the stones?

 

We cannot, I think, answer that question, as some have tried to

answer it, by saying that they were brought by Noah's flood. For it

is clear, that very violent currents of water would be needed to

carry boulders, some of them weighing many tons, for many miles. Now

scripture says nothing of any such violent currents; and we have no

right to put currents, or any other imagined facts, into scripture

out of our own heads, and then argue from them as if not we, but the

text of scripture had asserted their existence.

 

But still, they may have been rolled hither by water. That theory

certainly would explain their being rounded; though not their being

scratched. But it will not explain their being found in the clay.

 

Recollect what I said in my first paper: that water drops its

pebbles and coarser particles first, while it carries the fine clayey

mud onward in solution, and only drops it when the water becomes

still. Now currents of such tremendous violence as to carry these

boulder stones onward, would have carried the mud for many miles

farther still; and we should find the boulders, not in clay, but

lying loose together, probably on a hard rock bottom, scoured clean

by the current. That is what we find in the beds of streams; that is

just what we do not find in this case.

 

But the boulders may have been brought by a current, and then the

water may have become still, and the clay settled quietly round them.

What? Under them as well as over them? On that theory also we

should find them only at the bottom of the clay. As it is, we find

them scattered anywhere and everywhere through it, from top to

bottom. So that theory will not do. Indeed, no theory will do which

supposes them to have been brought by water alone.

 

Try yourself, dear reader, and make experiments, with running water,

pebbles, and mud. If you try for seven years, I believe, you will

never contrive to make your pebbles lie about in your mud, as they

lie about in every pit in the boulder clay.

 

Well then, there we are at fault, it seems. We have no explanation

drawn from known facts which will do--unless we are to suppose, which

I don't think you will do, that stones, clay, and all were blown

hither along the surface of the ground, by primeval hurricanes, ten

times worse than those of the West Indies, which certainly will roll

a cannon a few yards, but cannot, surely, roll a boulder stone a

hundred miles.

 

Now, suppose that there was a force, an agent, known--luckily for

you, not to you--but known too well to sailors and travellers; a

force which is at work over the vast sheets of land at both the north

and south poles; at work, too, on every high mountain range in the

world, and therefore a very common natural force; and suppose that

this force would explain all the facts, namely--

 

How the stones got here;

 

How they were scratched and rounded;

 

How they were imbedded in clay;

 

because it is notoriously, and before men's eyes now, carrying great

stones hundreds of miles, and scratching and rounding them also;

carrying vast deposits of mud, too, and mixing up mud and stones just

as we see them in the brick-pits,--Would not our common sense have a

right to try that explanation?--to suspect that this force, which we

do not see at work in Britain now, may have been at work here ages

since? That would at least be reasoning from the known to the

unknown. What state of things, then, do we find among the highest

mountains; and over whole countries which, though not lofty, lie far

enough north or south to be permanently covered with ice?

 

We find, first, an ice-cap or ice-sheet, fed by the winter's snows,

stretching over the higher land, and crawling downward and outward by

its own weight, along the valleys, as glaciers.

 

We find underneath the glaciers, first a moraine profonde, consisting

of the boulders and gravel, and earth, which the glacier has ground

off the hillsides, and is carrying down with it.

 

These stones, of course, grind, scratch, and polish each other; and

in like wise grind, scratch, and polish the rock over which they

pass, under the enormous weight of the superincumbent ice.

 

We find also, issuing from under each glacier a stream, carrying the

finest mud, the result of the grinding of the boulders against each

other and the glacier.

 

We find, moreover, on the surface of the glaciers, moraines

superieures--long lines of stones and dirt which had fallen from

neighbouring cliffs, and are now travelling downward with the

glaciers.

 

Their fate, if the glacier ends on land, is what was to be expected.

The stones from above the glacier fall over the ice-cliff at its end,

to mingle with those thrown out from underneath the glacier, and form

huge banks of boulders, called terminal moraines, while the mud runs

off, as all who have seen glaciers know, in a turbid torrent.

 

Their fate, again, is what was to be expected if the glacier ends, as

it commonly does in Arctic regions, in the sea. The ice grows out to

sea-ward for more than a mile sometimes, about one-eighth of it being

above water, and seven-eighths below, so that an ice-cliff one

hundred feet high may project into water eight hundred feet deep. At

last, when it gets out of its depth, the buoyancy of the water breaks

it off in icebergs, which float away, at the mercy of tides and

currents, often grounding again in shallower water, and ploughing the

sea-bottom as they drag along it. These bergs carry stones and dirt,

often in large quantities; so that, whenever a berg melts or

capsizes, it strews its burden confusedly about the sea-floor.

 

Meanwhile the fine mud which is flowing out from under the ice goes

out to sea likewise, colouring the water far out, and then subsiding

as a soft tenacious ooze, in which the stones brought out by the ice

are imbedded. And this ooze--so those who have examined it assert--

cannot be distinguished from the brick-clay, or fossiliferous

boulder-clay, so common in the North. A very illustrious

Scandinavian explorer, visiting Edinburgh, declared, as soon as he

saw the sections of boulder-clay exhibited near that city, that this

was the very substance which he saw forming in the Spitzbergen ice-

fiords. {3}

 

I have put these facts as simply and baldly as I can, in order that

the reader may look steadily at them, without having his attention

drawn off, or his fancy excited, by their real poetry and grandeur.

Indeed, it would have been an impertinence to have done otherwise;

for I have never seen a live glacier, by land or sea, though I have

seen many a dead one. And the public has had the opportunity,

lately, of reading so many delightful books about "peaks, passes, and

glaciers," that I am bound to suppose that many of my readers know as

much, or more, about them than I do.

 

But let us go a step farther; and, bearing in our minds what live

glaciers are like, let us imagine what a dead glacier would be like;

a glacier, that is, which had melted, and left nothing but its

skeleton of stones and dirt.

 

We should find the faces of the rock scored and polished, generally

in lines pointing down the valleys, or at least outward from the

centre of the highlands, and polished and scored most in their upland

or weather sides. We should find blocks of rock left behind, and

perched about on other rocks of a different kind. We should find in

the valleys the old moraines left as vast deposits of boulder and

shingle, which would be in time sawn through and sorted over by the

rivers. And if the sea-bottom outside were upheaved, and became dry

land, we should find on it the remains of the mud from under the

glacier, stuck full of stones and boulders iceberg-dropped. This mud

would be often very irregularly bedded; for it would have been

disturbed by the ploughing of the icebergs, and mixed here and there

with dirt which had fallen from them. Moreover, as the sea became

shallower and the mud-beds got awash one after the other, they would

be torn about, re-sifted, and re-shaped by currents and by tides, and

mixed with shore-sand ground out of shingle-beach, thus making

confusion worse confounded. A few shells, of an Arctic or northern

type, would be found in it here and there. Some would have lived

near those later beaches, some in deeper water in the ancient ooze,

wherever the iceberg had left it in peace long enough for sea-animals

to colonise and breed in it. But the general appearance of the dried

sea-bottom would be a dreary and lifeless waste of sands, gravels,

loose boulders, and boulder-bearing clays; and wherever a boss of

bare rock still stood up, it would be found ground down, and probably

polished and scored by the ponderous icebergs which had lumbered over

it in their passage out to sea.

 

In a word, it would look exactly as vast tracts of the English,

Scotch, and Irish lowlands must have looked before returning

vegetation coated their dreary sands and clays with a layer of brown

vegetable soil.

 

Thus, and I believe thus only, can we explain the facts connected

with these boulder pebbles. No agent known on earth can have stuck

them in the clay, save ice, which is known to do so still elsewhere.

 

No known agent can have scratched them as they are scratched, save

ice, which is known to do so still elsewhere.

 

No known agent--certainly not, in my opinion, the existing rivers--

can have accumulated the vast beds of boulders which lie along the

course of certain northern rivers; notably along the Dee about

Aboyne--save ice bearing them slowly down from the distant summits of

the Grampians.

 

No known agent, save ice, can have produced those rounded, and

polished, and scored, and fluted rochers moutonnes "sheep-backed

rocks"--so common in the Lake district; so common, too, in Snowdon,

especially between the two lakes of Llanberis; common in Kerry; to be

seen anywhere, as far as I have ascertained, around the Scotch

Highlands, where the turf is cleared away from an unweathered surface

of the rock, in the direction in which a glacier would have pressed

against it had one been there. Where these polishings and scorings

are found in narrow glens, it is, no doubt, an open question whether

some of them may not be the work of water. But nothing but the

action of ice can have produced what I have seen in land-locked and

quiet fords in Kerry--ice-flutings in polished rocks below high-water

mark, so large that I could lie down in one of them. Nothing but the

action of ice could produce what may be seen in any of our mountains-

-whole sheets of rock ground down into rounded flats, irrespective of

the lie of the beds, not in valleys, but on the brows and summits of

mountains, often ending abruptly at the edge of some sudden cliff,

where the true work of water, in the shape of rain and frost, is

actually destroying the previous work of ice, and fulfilling the rule

laid down (I think by Professor Geikie in his delightful book on

Scotch scenery as influenced by its geology), that ice planes down

into flats, while water saws out into crags and gullies; and that the

rain and frost are even now restoring Scotch scenery to something of

that ruggedness and picturesqueness which it must have lost when it

lay, like Greenland, under the indiscriminating grinding of a heavy

sheet of ice.

 

Lastly; no known agent, save ice, will explain those perched

boulders, composed of ancient hard rocks, which may be seen in so

many parts of these islands and of the Continent. No water power

could have lifted those stones, and tossed them up high and dry on

mountain ridges and promontories, upon rocks of a totally different

kind. Some of my readers surely recollect Wordsworth's noble lines

about these mysterious wanderers, of which he had seen many a one

about his native hills:

 

 

As a huge stone is sometimes seen to lie

Couched on the bald top of an eminence,

Wonder to all who do the same espy

By what means it could thither come, and whence;

So that it seems a thing endued with sense:

Like a sea-beast crawled forth, that on a shelf

Of rock or sand reposeth, there to sun itself.

 

 

Yes; but the next time you see such a stone, believe that the wonder

has been solved, and found to be, like most wonders in Nature, more

wonderful than we guessed it to be. It is not a sea-beast which has

crawled forth, but an ice-beast which has been left behind; lifted up

thither by the ice, as surely as the famous Pierre-a-bot, forty feet

in diameter, and hundreds of boulders more, almost as large as

cottages, have been carried by ice from the distant Alps right across

the lake of Neufchatel, and stranded on the slopes of the Jura, nine

hundred feet above the lake. {4}

 

Thus, I think, we have accounted for facts enough to make it probable

that Britain was once covered partly by an ice-sheet, as Greenland is

now, and partly, perhaps, by an icy sea. But, to make assurance more

sure, let us look for new facts, and try whether our ice-dream will

account for them also. Let us investigate our case as a good medical

man does, by "verifying his first induction."

 

He says: At the first glance, I can see symptoms a, b, c. It is

therefore probable that my patient has got complaint A. But if he

has he ought to have symptom d also. If I find that, my guess will

be yet more probable. He ought also to have symptom e, and so forth;

and as I find successively each of these symptoms which are proper to

A, my first guess will become more and more probable, till it reaches  practical certainty.