The Student's Elements of Geology 9

The Student's Elements of Geology 9

The very different levels at which the separated parts of the same strata are

found on the different sides of the fissure, in some faults, is truly

astonishing. One of the most celebrated in England is that called the "ninety-

fathom dike," in the coal-field of Newcastle. This name has been given to it,

because the same beds are ninety fathoms (540 feet) lower on the northern than

they are on the southern side. The fissure has been filled by a body of sand,

which is now in the state of sandstone, and is called the dike, which is

sometimes very narrow, but in other places more than twenty yards wide.

(Conybeare and Phillips Outlines, etc. page 376.) The walls of the fissure are

scored by grooves, such as would have been produced if the broken ends of the

rock had been rubbed along the plane of the fault. (Phillips Geology Lardner's

Cyclop. page 41.) In the Tynedale and Craven faults, in the north of England,

the vertical displacement is still greater, and the fracture has extended in a

horizontal direction for a distance of thirty miles or more.

 

GREAT FAULTS THE RESULT OF REPEATED MOVEMENTS.

 

It must not, however, be supposed that faults generally consist of single linear

rents; there are usually a number of faults springing off from the main one, and

sometimes a long strip of country seems broken up into fragments by sets of

parallel and connecting transverse faults. Oftentimes a great line of fault has

been repeated, or the movements have been continued through successive periods,

so that, newer deposits having covered the old line of displacement, the strata

both newer and older have given way along the old line of fracture. Some

geologists have considered it necessary to imagine that the upward or downward

movement in these cases was accomplished at a single stroke, and not by a series

of sudden but interrupted movements. They appear to have derived this idea from

a notion that the grooved walls have merely been rubbed in one direction, which

is far from being a constant phenomenon. Not only are some sets of striae not

parallel to others, but the clay and rubbish between the walls, when squeezed or

rubbed, have been streaked in different directions, the grooves which the harder

minerals have impressed on the softer being frequently curved and irregular.

 

(FIGURE 77. Faults and denuded coal-strata, Ashby de la Zouch. (Mammatt.))

 

The usual absence of protruding masses of rock forming precipices or ridges

along the lines of great faults has already been alluded to in explaining Figure

76, and the same remarkable fact is well exemplified in every coal-field which

has been extensively worked. It is in such districts that the former relation of

the beds which have been shifted is determinable with great accuracy. Thus in

the coal-field of Ashby de la Zouch, in Leicestershire (see Figure 77), a fault

occurs, on one side of which the coal-beds a, b, c, d must once have risen to

the height of 500 feet above the corresponding beds on the other side. But the

uplifted strata do not stand up 500 feet above the general surface; on the

contrary, the outline of the country, as expressed by the line z z, is uniformly

undulating, without any break, and the mass indicated by the dotted outline must

have been washed away. (See Mammatt's Geological Facts etc. page 90 and plate.)

 

The student may refer to Mr. Hull's measurement of faults, observed in the

Lancashire coal-field, where the vertical displacement has amounted to thousands

of feet, and yet where all the superficial inequalities which must have resulted

from such movements have been obliterated by subsequent denudation. In the same

memoir proofs are afforded of there having been two periods of vertical movement

in the same fault-- one, for example, before, and another after, the Triassic

epoch. (Hull Quarterly Geological Journal volume 24 page 318. 1868.)

 

The shifting of the beds by faults is often intimately connected with those same

foldings which constitute the anticlinal and synclinal axes before alluded to,

and there is no doubt that the subterranean causes of both forms of disturbance

are to a great extent the same. A fault in Virginia, believed to imply a

displacement of several thousand feet, has been traced for more than eighty

miles in the same direction as the foldings of the Appalachian chain. (H.D.

Rogers Geology of Pennsylvania page 897.) An hypothesis which attributes such a

change of position to a succession of movements, is far preferable to any theory

which assumes each fault to have been accomplished by a single upcast or

downthrow of several thousand feet. For we know that there are operations now in

progress, at great depths in the interior of the earth, by which both large and

small tracts of ground are made to rise above and sink below their former level,

some slowly and insensibly, others suddenly and by starts, a few feet or yards

at a time; whereas there are no grounds for believing that, during the last 3000

years at least, any regions have been either upheaved or depressed, at a single

stroke, to the amount of several hundred, much less several thousand feet.

 

It is certainly not easy to understand how in the subterranean regions one mass

of solid rock should have been folded up by a continued series of movements,

while another mass in contact, or only separated by a line of fissure, has

remained stationary or has perhaps subsided. But every volcano, by the

intermittent action of the steam, gases, and lava evolved during an eruption,

helps us to form some idea of the manner in which such operations take place.

For eruptions are repeated at uncertain intervals throughout the whole or a

large part of a geological period, some of the surrounding and contiguous

districts remaining quite undisturbed. And in most of the instances with which

we are best acquainted the emission of lava, scoria, and steam is accompanied by

the uplifting of the solid crust. Thus in Vesuvius, Etna, the Madeiras, the

Canary Islands, and the Azores there is evidence of marine deposits of recent

and tertiary date having been elevated to the height of a thousand feet, and

sometimes more, since the commencement of the volcanic explosions. There is,

moreover, a general tendency in contemporaneous volcanic vents to affect a

linear arrangement, extending in some instances, as in the Andes or the Indian

Archipelago, to distances equalling half the circumference of the globe. Where

volcanic heat, therefore, operates at such a depth as not to obtain vent at the

surface, in the form of an eruption, it may nevertheless be conceived to give

rise to upheavals, foldings, and faults in certain linear tracts. And marine

denudation, to be treated of in the next chapter, will help us to understand why

that which should be the protruding portion of the faulted rocks is missing at

the surface.

 

ARRANGEMENT AND DIRECTION OF PARALLEL FOLDS OF STRATA.

 

The possible causes of the folding of strata by lateral movements have been

considered in a former part of this chapter. No European chain of mountains

affords so remarkable an illustration of the persistency of such flexures for a

great distance as the Appalachians before alluded to, and none has been studied

and described by many good observers with more accuracy. The chain extends from

north to south, or rather N.N.E. to S.S.W., for nearly 1500 miles, with a

breadth of 50 miles, throughout which the Palaeozoic strata have been so bent as

to form a series of parallel anticlinal and synclinal ridges and troughs,

comprising usually three or four principal and many smaller plications, some of

them forming broad and gentle arches, others narrower and steeper ones, while

some, where the bending has been greatest, have the position of their beds

inverted, as before shown in Figure 73.

 

The strike of the parallel ridges, after continuing in a straight line for many

hundred miles, is then found to vary for a more limited distance as much as 30

degrees, the folds wheeling round together in the new direction and continuing

to be parallel, as if they had all obeyed the same movement. The date of the

movements by which the great flexures were brought about must, of course, be

subsequent to the formation of the uppermost part of the coal or the newest of

the bent rocks, but the disturbance must have ceased before the Triassic strata

were deposited on the denuded edges of the folded beds.

 

The manner in which the numerous parallel folds, all simultaneously formed,

assume a new direction common to the whole of them, and sometimes varying at an

angle of 30 degrees from the normal strike of the chain, shows what deviation

from an otherwise uniform strike of the beds may be experienced when the

geographical area through which they are traced is on so vast a scale.

 

The disturbances in the case here adverted to occurred between the Carboniferous

period and that of the Trias, and this interval is so vast that they may have

occupied a great lapse of time, during which their parallelism was always

preserved. But, as a rule, wherever after a long geological interval the

recurrence of lateral movements gives rise to a new set of folds, the strike of

these last is different. Thus, for example, Mr. Hull has pointed out that three

principal lines of disturbance, all later than the Carboniferous period, have

affected the stratified rocks of Lancashire. The first of these, having an

E.N.E. direction, took place at the close of the Carboniferous period. The next,

running north and south, at the close of the Permian, and the third, having a

N.N.W. direction, at the close of the Jurassic period. (Edward Hull Quarterly

Geological Journal volume 24 page 323.)

 

UNCONFORMABILITY OF STRATA.

 

(FIGURE 78. Unconformable junction of old red sandstone and Silurian schist at

the Siccar Point, near St. Abb's Head, Berwickshire.)

 

Strata are said to be unconformable when one series is so placed over another

that the planes of the superior repose on the edges of the inferior (see Figure

78.) In this case it is evident that a period had elapsed between the production

of the two sets of strata, and that, during this interval, the older series had

been tilted and disturbed. Afterwards the upper series was thrown down in

horizontal strata upon it. If these superior beds, d d Figure 78, are also

inclined, it is plain that the lower strata a a, have been twice displaced;

first, before the deposition of the newer beds, d d, and a second time when

these same strata were upraised out of the sea, and thrown slightly out of the

horizontal position.

 

(FIGURE 79. Junction of unconformable strata near Mons, in Belgium.)

 

It often happens that in the interval between the deposition of two sets of

unconformable strata, the inferior rock has not only been denuded, but drilled

by perforating shells. Thus, for example, at Autreppe and Gusigny, near Mons,

beds of an ancient (primary or palaeozoic) limestone, highly inclined, and often

bent, are covered with horizontal strata of greenish and whitish marls of the

Cretaceous formation. The lowest, and therefore the oldest, bed of the

horizontal series is usually the sand and conglomerate, a, in which are rounded

fragments of stone, from an inch to two feet in diameter. These fragments have

often adhering shells attached to them, and have been bored by perforating

mollusca. The solid surface of the inferior limestone has also been bored, so as

to exhibit cylindrical and pear-shaped cavities, as at c, the work of saxicavous

mollusca; and many rents, as at b, which descend several feet or yards into the

limestone, have been filled with sand and shells, similar to those in the

stratum a.

 

OVERLAPPING STRATA.

 

Strata are said to overlap when an upper bed extends beyond the limits of a

lower one. This may be produced in various ways; as, for example, when

alterations of physical geography cause the arms of a river or channels of

discharge to vary, so that sediment brought down is deposited over a wider area

than before, or when the sea-bottom has been raised up and again depressed

without disturbing the horizontal position of the strata. In this case the newer

strata may rest for the most part conformably on the older, but, extending

farther, pass over their edges. Every intermediate state between unconformable

and over-lapping beds may occur, because there may be every gradation between a

slight derangement of position, and a considerable disturbance and denudation of

the older formation before the newer beds come on.

 

 

CHAPTER VI.

 

DENUDATION.

 

Denudation defined.

Its Amount more than equal to the entire Mass of Stratified Deposits in the

Earth's Crust.

Subaerial Denudation.

Action of the Wind.

Action of Running Water.

Alluvium defined.

Different Ages of Alluvium.

Denuding Power of Rivers affected by Rise or Fall of Land.

Littoral Denudation.

Inland Sea-Cliffs.

Escarpments.

Submarine Denudation.

Dogger-bank.

Newfoundland Bank.

Denuding Power of the Ocean during Emergence of Land.

 

Denudation, which has been occasionally spoken of in the preceding chapters, is

the removal of solid matter by water in motion, whether of rivers or of the

waves and currents of the sea, and the consequent laying bare of some inferior

rock. This operation has exerted an influence on the structure of the earth's

crust as universal and important as sedimentary deposition itself; for

denudation is the necessary antecedent of the production of all new strata of

mechanical origin. The formation of every new deposit by the transport of

sediment and pebbles necessarily implies that there has been, somewhere else, a

grinding down of rock into rounded fragments, sand, or mud, equal in quantity to

the new strata. All deposition, therefore, except in the case of a shower of

volcanic ashes, and the outflow of lava, and the growth of certain organic

formations, is the sign of superficial waste going on contemporaneously, and to

an equal amount, elsewhere. The gain at one point is no more than sufficient to

balance the loss at some other. Here a lake has grown shallower, there a ravine

has been deepened. Here the depth of the sea has been augmented by the removal

of a sandbank during a storm, there its bottom has been raised and shallowed by

the accumulation in its bed of the same sand transported from the bank.

 

When we see a stone building, we know that somewhere, far or near, a quarry has

been opened. The courses of stone in the building may be compared to successive

strata, the quarry to a ravine or valley which has suffered denudation. As the

strata, like the courses of hewn stone, have been laid one upon another

gradually, so the excavation both of the valley and quarry have been gradual. To

pursue the comparison still farther, the superficial heaps of mud, sand, and

gravel, usually called alluvium, may be likened to the rubbish of a quarry which

has been rejected as useless by the workmen, or has fallen upon the road between

the quarry and the building, so as to lie scattered at random over the ground.

 

But we occasionally find in a conglomerate large rounded pebbles of an older

conglomerate, which had previously been derived from a variety of different

rocks. In such cases we are reminded that, the same materials having been used

over and over again, it is not enough to affirm that the entire mass of

stratified deposits in the earth's crust affords a monument and measure of the

denudation which has taken place, for in truth the quantity of matter now extant

in the form of stratified rock represents but a fraction of the material removed

by water and redeposited in past ages.

 

SUBAERIAL DENUDATION.

 

Denudation may be divided into subaerial, or the action of wind, rain, and

rivers; and submarine, or that effected by the waves of the sea, and its tides

and currents. With the operation of the first of these we are best acquainted,

and it may be well to give it our first attention.

 

ACTION OF THE WIND.

 

In desert regions where no rain falls, or where, as in parts of the Sahara, the

soil is so salt as to be without any covering of vegetation, clouds of dust and

sand attest the power of the wind to cause the shifting of the unconsolidated or

disintegrated rock.

 

In examining volcanic countries I have been much struck with the great

superficial changes brought about by this power in the course of centuries. The

highest peak of Madeira is about 6050 feet above the sea, and consists of the

skeleton of a volcanic cone now 250 feet high, the beds of which once dipped

from a centre in all directions at an angle of more than 30 degrees. The summit

is formed of a dike of basalt with much olivine, fifteen feet wide, apparently

the remains of a column of lava which once rose to the crater. Nearly all the

scoriae of the upper part of the cone have been swept away, those portions only

remaining which were hardened by the contact or proximity of the dike. While I

was myself on this peak on January 25, 1854, I saw the wind, though it was not

stormy weather, removing sand and dust derived from the decomposing scoriae.

There had been frost in the night, and some ice was still seen in the crevices

of the rock.

 

On the highest platform of the Grand Canary, at an elevation of 6000 feet, there

is a cylindrical column of hard lava, from which the softer matter has been

carried away; and other similar remnants of the dikes of cones of eruption

attest the denuding power of the wind at points where running water could never

have exerted any influence. The waste effected by wind aided by frost and snow,

may not be trifling, even in a single winter, and when multiplied by centuries

may become indefinitely great.

 

ACTION OF RUNNING WATER.

 

(FIGURE 80. Section through several eroded formations.

a. Older alluvium or drift.

b. Modern alluvium.)

 

There are different classes of phenomena which attest in a most striking manner

the vast spaces left vacant by the erosive power of water. I may allude, first,

to those valleys on both sides of which the same strata are seen following each

other in the same order, and having the same mineral composition and fossil

contents. We may observe, for example, several formations, as Nos. 1, 2, 3, 4,

in the diagram (Figure 80): No. 1, conglomerate, No. 2, clay, No. 3, grit, and

No. 4, limestone, each repeated in a series of hills separated by valleys

varying in depth. When we examine the subordinate parts of these four

formations, we find, in like manner, distinct beds in each, corresponding, on

the opposite sides of the valleys, both in composition and order of position. No

one can doubt that the strata were originally continuous, and that some cause

has swept away the portions which once connected the whole series. A torrent on

the side of a mountain produces similar interruptions; and when we make

artificial cuts in lowering roads, we expose, in like manner, corresponding beds

on either side. But in nature, these appearances occur in mountains several

thousand feet high, and separated by intervals of many miles or leagues in

extent.

 

In the "Memoirs of the Geological Survey of Great Britain" (volume 1), Professor

Ramsay has shown that the missing beds, removed from the summit of the Mendips,

must have been nearly a mile in thickness; and he has pointed out considerable

areas in South Wales and some of the adjacent counties of England, where a

series of primary (or palaeozoic) strata, no less than 11,000 feet in thickness,

have been stripped off. All these materials have of course been transported to

new regions, and have entered into the composition of more modern formations. On

the other hand, it is shown by observations in the same "Survey," that the

Palaeozoic strata are from 20,000 to 30,000 feet thick. It is clear that such

rocks, formed of mud and sand, now for the most part consolidated, are the

monuments of denuding operations, which took place on a grand scale at a very

remote period in the earth's history. For, whatever has been given to one area

must always have been borrowed from another; a truth which, obvious as it may

seem when thus stated, must be repeatedly impressed on the student's mind,

because in many geological speculations it is taken for granted that the

external crust of the earth has been always growing thicker in consequence of

the accumulation, period after period, of sedimentary matter, as if the new

strata were not always produced at the expense of pre-existing rocks, stratified

or unstratified. By duly reflecting on the fact that all deposits of mechanical

origin imply the transportation from some other region, whether contiguous or

remote, of an equal amount of solid matter, we perceive that the stony exterior

of the planet must always have grown thinner in one place, whenever, by

accessions of new strata, it was acquiring thickness in another.

 

It is well known that generally at the mouths of large rivers, deltas are

forming and the land is encroaching upon the sea; these deltas are monuments of

recent denudation and deposition; and it is obvious that if the mud, sand, and

gravel were taken from them and restored to the continents they would fill up a

large part of the gullies and valleys which are due to the excavating and

transporting power of torrents and rivers.

 

ALLUVIUM.

 

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 when it occurs in valleys the name of alluvium has been popularly

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 washed over low lands by a flood.

 

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-shore,

or the bed of a river. The drainage, moreover, may have been deranged again and

again by earthquakes, during which temporary lakes are caused by landslips, and

partial deluges occasioned by the bursting of the barriers of such lakes. 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 various. Besides, 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 and kept separate 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 volcanoes 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 constituted 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 fossil bones belonging to species

of land animals different from those which had previously flourished in the same

country and been buried in older gravels.

 

(FIGURE 81. Lavas of Auvergne resting on alluviums of different ages.)

 

Figure 81 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, although in some cases a narrow strip of solid lava shares

the bottom of the valley with the river.

 

The proportion of extinct species of quadrupeds is more numerous in the fossil

remains of the gravel No. 1 than in that indicated as No. 2; and in No. 3 they

agree more closely, sometimes entirely, with those of the existing fauna. The

usual absence or rarity of organic remains in beds of loose gravel and sand is

partly owing to the friction which originally ground down the rocks into small

fragments, and partly to the porous nature of alluvium, which allows the free

percolation through it of rain-water, and promotes the decomposition and removal

of fossil remains.

 

The loose transported matter on the surface of a large part of the land now

existing in the temperate and arctic regions of the northern hemisphere, must be

regarded as being in a somewhat exceptional state, in consequence of the

important part which ice has played in comparatively modern geological times.