SURFACE-RELIEF AND THE PROCESS OF EROSION
It is not necessary here to consider the various formal definitions of
geography which have been proposed in the last few years. As is only
natural with a developing subject, much discussion has taken place as to
the exact limits of its field of action, and many definitions have been
proposed with the object of setting forth these limits as clearly as
possible. But it is sufficient for our purpose to note that geography
deals with the surface-relief of the earth, and with the influence which
that relief exercises upon the distribution of other phenomena, and
especially upon the life of man. Before we proceed to study detailed
problems, then, it is obviously necessary to look at some general points
connected with the relief of the earth's surface and its causes.
In the words of the physical geographer, the earth's surface consists
of the solid crust, or lithosphere, of the mass of water forming the
seas and oceans and constituting the hydrosphere, and of that envelope
of gas which we call the atmosphere. Considered separately, each of
these is the concern of special sciences, and not of the geographer
proper. His business it is to take the facts furnished by the
meteorologists, the physicists, the geologists, and so forth, and with
these facts in hand to proceed to consider the effect of the interaction
of earth and water and air in a way which the separate sciences cannot
do. We must further note that it is the interactions of these three
which make the earth a possible home for life as we know it, and it is
these interactions therefore which influence the distribution of life on
the surface of the globe.
There may have been a period when the crust of the earth was clothed in
a uniform sheet of water, just as the globe is now enveloped in a
complete covering of air, but at present, as through the long ages of
geological time, the lithosphere consists of elevations and hollows, and
it is in the hollows that the water accumulates, so that we can
distinguish between the dry land and the ocean beds. Both chemically
and physically the fluid hydrosphere differs markedly from the solid
lithosphere, and it is, above all, the physical differences which are of
supreme importance to the geographer. Because of them sea and land
respond differently to the stream of solar energy which pours down upon
our globe, and it is this different response which is the predominating
factor in the production of the different climates, which again
determine in its main outlines the distribution of living organisms.
This being so, it is clear that it is of great importance to the
geographer to know exactly the distribution of land and water over the
surface of the earth. As the North Polar regions are still inadequately
known, and the South Polar regions hardly known at all, we cannot as yet
determine exactly this distribution, but any globe will show that land
and ocean are very unequally distributed. The great land masses cluster
round the North Pole, while the southern hemisphere consists largely of
water. We thus have a land hemisphere and a water one. According to
recent calculations the oceans occupy some 72 per cent. of the entire
surface of the globe, leaving only 28 per cent. of land. But while in
the northern hemisphere there is about one and a half times more water
than land, in the southern there is about six times more water, both
figures being liable to error, as indicated above, owing to our
uncertainty as to the land and water of the Polar zones.
This distribution is of great importance in connection with certain
theories as to the actual plan of the earth, but this is a difficult
subject which need not concern us here. It is discussed in Prof. J. W.
Gregory's volume on _The Making of the Earth_. More interesting is the
effect which the arrangement of land and water has had upon that part of
the life of the earth which was evolved in late geological time. Though
the geographer for convenience' sake recognizes three separate
continents in the Old World--Europe, Asia and Africa--yet these form
practically one land mass, which in its turn approaches America very
nearly at Bering Straits, and, less nearly, in the North Atlantic
through the intervention of the British Isles, the Faeroes, Iceland,
Greenland, etc. The centre of this land mass lies in Europe, a point not
without its importance.
In this great land mass of the northern hemisphere life has reached its
highest degree of development, both as regards animal form and as
regards human societies. It was in the northern hemisphere that the
highest mammals, the placentals, arose. There are many remarkable
resemblances between the faunas of Europe, of Asia and of Africa, and a
similar, if less marked, resemblance between those of North America on
the one hand and of Europe and temperate Asia on the other. On the other
hand, the two great land masses which occur in the southern hemisphere,
South America and Australia, show very marked differences in their
fauna, both from each other and from the northern land masses, and in
both cases the fauna has a primitive aspect, which is best marked in
Australia.
When we come to consider man, somewhat similar conditions present
themselves. The great civilisations developed in the land mass of the
Old World, though the waterless desert of the Sahara cut off much of
Africa from participation in them. America developed a relatively high
civilisation of its own, but as the icefields and ice-pack of the north
formed a greater barrier to the migrations of man than to those of the
northern animals, this American civilisation was for long cut off from
that of the Old World, and when free communication became possible, it
went down before that of the eastern world.
We must connect these facts directly with the peculiar distribution of
land and water in the northern hemisphere, which made free intercourse
possible, alike for the land animals and for man. The importance of this
intercourse may be suggested in a few words. When a group of organisms
is limited, from whatever cause, to a particular zone of the earth's
surface, the members of the group tend to acquire characters fitting
them for this restricted area. But if the area is open, constantly or
periodically, to incursions of organisms from adjacent areas, then, with
the widening of the environment, and the greater intensity of the
struggle for existence, evolution is quickened and new characters
appear. The men of the Eurasian continent learnt, on the fierce
battle-grounds of that continent, lessons which enabled them to conquer
without difficulty the more isolated human groups of the southern
hemisphere. The fact that they took south with them the mammals of the
north, who also have thriven at the expense of the native forms, shows
that the hold of the southern animals upon their habitat was no less
precarious than that of man himself.
One other point is worth notice in connection with the distribution of
land and water over the surface of the globe. We have seen that the
northern hemisphere is the region where organic evolution has been most
marked. It is, as it were, a great biological laboratory. On the other
hand, in the southern hemisphere, which has fewer land masses to
interfere with the circulation of the atmosphere, many physical
phenomena occur in a more marked and orderly fashion than to the north.
The westerly winds of the south blow with a force and a constancy which
makes it impossible to compare them with the more variable westerlies of
the north. Even the ocean currents of the south seem to show more
constancy than those of the north. If the northern hemisphere is a great
biological laboratory, the southern may be described as a physical one,
and one of the great interests of the further exploration of the
Antarctic is that it will probably cast light upon some important
meteorological problems. (See Dr. W. S. Bruce's volume on _Polar
Exploration_.)
The distribution of land and water, with all its effects on climate and
on the distribution of life, is, as we have seen, caused by the main
features of the relief of the earth, by the existence of vast
depressions in which the water accumulates, and of relative elevations
from which it flows. But the minor details of relief, hill and valley,
ocean depth and continental shelf, are also important, and exercise a
very marked effect upon distribution. They therefore demand in their
turn some consideration.
Taking first the prime distinction between land surface and ocean floor,
we note that the two differ from one another markedly, alike in their
characteristics and in the conditions to which they are exposed. The
land is subjected to constantly varying conditions: to the alternation
of day and night, and to the changes of the seasons, with corresponding
variations in temperature; to the fluctuations of the weather; to
running water, and so forth. In the great ocean depths at least, on the
other hand, the conditions are remarkably uniform. Neither diurnal nor
seasonal changes have here any effect; the temperature seems to
fluctuate but little; the water is almost still. This uniformity of
physical conditions is reflected in the uniformity of the surface over
wide areas. While the land surface shows marked irregularities, the
ocean floor has a monotonous character, with more gentle outlines.
In its most general form the characters of the sea bottom may be briefly
stated. Round the great land masses there is an area of relatively
shallow water, which is sometimes only a few miles wide, and at other
times extends outwards for hundreds of miles. This region is the
_Continental Shelf_, and its seaward boundary for convenience' sake is
taken at a depth of 100 fathoms, or 600 feet. Within this zone the
influence of the land is still felt, and some of the characters of land
surfaces appear. Thus we sometimes find that river valleys are prolonged
outwards over the Continental Shelf, giving a markedly irregular
appearance to the ocean floor. The British Islands lie upon a
Continental Shelf of this kind, and this is one of our reasons for
knowing that they are really only a part of the continent of Europe,
separated from it by a slight depression.
The Continental Shelf slopes away from the land gently, and is widest
where it fringes low continents, and narrowest where mountains approach
the coast. Over it is spread the waste of the land, the coarser lying
near the shore-line, the finer extending outwards to the steep seaward
slope. This rapid slope leads down to the more or less uniform ocean
plateau, whose surface is broken by the great ocean abysses, the
greatest of which has a depth of about six miles. Relative but not
absolute uniformity thus characterises all that part of the ocean floor
which lies below about 100 fathoms.
Again, though the ocean floor is doubtless being slowly raised by the
deposition upon it of the oceanic oozes, yet it is also true that as
compared with the land surface it displays great constancy. While the
land surface is constantly changing owing to the varying forces which
act upon it, the floor of the ocean can vary but little from age to age,
unless it is acted upon by the internal forces of the earth.
Turn now to the land. We note at once the two characters of marked
irregularity of surface, and of changeableness. The changeableness is
due to the forces of erosion which act upon the surface, and of these
forces the most important to the geographer is running water. It is
running water, aided by other agents, which carves the land into hill
and valley, which produces gorge and lake, only ultimately to fill up
the lake and plane away the gorge. It is running water which spreads out
on the lower ground the waste of the higher, and thus prepares the way
for the operations of man.
The result of the long-continued action of the varied forces of erosion
must necessarily be to reduce the surface to an almost level condition.
The denuding agents first produce irregularities and then finally remove
these, until the whole surface is once again almost level. The whole
globe would thus be reduced to the condition of a plain were it not for
the intervention of the internal forces which raise up the surface anew
into folds, or which produce volcanoes and outbursts of molten rock.
This constantly repeated series of changes may be said to be chiefly the
concern of the geologist, especially as it is a series which has
repeated itself in all time. But it is to be noted that at various parts
of the surface of the globe at the present time every stage in the
process occurs, and everywhere the question whether a particular land
area has been exposed for a relatively long or for a relatively short
period to the forces of erosion, has a profound influence upon life. It
is therefore important for the geographer to be able to recognise the
different stages. This he cannot hope to do without some detailed
knowledge of the effects of erosion.
Theoretically every land surface elevated above sea-level should pass
through what has been called a cycle of erosion. There should be a
period when the active forces are working upon a surface as yet but
little modified; this is the period called by analogy youth. At a later
stage the drainage has been well established, and the rivers run in
broad valleys, from which lakes and waterfalls have largely disappeared.
To this condition the term mature has been applied. At a still later
stage the land surface has been so worn by the eroding forces that the
whole process of erosion is slackened, and an uplift must occur before
the erosive forces regain their lost strength. This is the so-called
"cycle of normal erosion," but it is constantly liable to variations due
to local crust movements, to changes in climate, and to local
conditions, though at the same time the distinction of the various
stages has value for the geographer because of their varying effects
upon human life. It is necessary for us, therefore, to consider how the
different stages may be recognized, and how the forces of erosion act.
Let us begin our study of erosion by a general survey of the striking
features of the earth's surface at the present day. We know that at
various parts of the surface there rise lofty mountain chains, whose
summits are often permanently snow-clad, and which, from the sharpness
of their forms and from the masses of rock rubbish which are
accumulating round them, have obviously only been exposed for a
geologically short period to the action of the atmosphere and of running
water. When examined such mountain chains are all found to have the same
peculiarities of internal form, the rocks composing them being
elaborately folded and fractured. Careful investigation has convinced
geologists that all the existing great chains owe their origin to a
series of earth movements which occurred in the period called Tertiary,
that is, in the third of the great geological periods, the one
immediately preceding that in which we live.
These lofty mountain chains of Tertiary origin are most familiar in the
great series of folds which appear at the surface to form the Pyrenees,
the Alps, the Caucasus and the Himalayas, but the Atlas Mountains belong
to the same series, as does also that great mountain chain which, under
various names, runs down the western coast of the American continent.
[Illustration: Fig. 1.--The main points in regard to the structure of
Europe. The shaded areas (1) are regions of ancient rocks, much folded
and crumpled, which once formed mountain regions but are now mostly worn
down to uplands. The lines (2) show the regions affected by Tertiary
folding, largely occupied by mountain chains. The unshaded areas are
mostly plains and basins, only slightly affected by folding, and made up
of rocks which are often almost horizontal.]
As already indicated, these areas are recognised not only by the fact
that there appear at the surface a great number of peaks forming a
mountain chain, but also by the internal structure, the
characteristically complex folding of the rocks. Now outside of these
recently elevated areas in, for example, the continent of Europe, we
find two conditions. On the one hand, there are regions of upland type
but with rounded and smoothed forms, which are sometimes almost reduced
to the condition of a plain. Such regions occur in Ireland, in the west
of Great Britain generally, in Brittany, in the central plateau of
France, in the Ardennes, in Bohemia, in the central plateau of Spain, in
Scandinavia, and so forth. Between these relatively elevated areas we
have plains and low-lying river basins, such as the London basin, the
Paris basin, and so on. When the rocks are examined in both cases it is
found that in the basins and plains the rocks, as a general rule, are
only slightly inclined, while in the uplands and plateaux there are
obvious remnants of folding, and the rocks are of ancient types, not
relatively modern like those of the Alps, Himalayas, etc. (see fig. 1).
If, then, the existing mountain chains show complex folding in their
constituent rocks, and though geologically but of yesterday have been
already deeply affected by the denuding agents, must we not suppose that
the folded and contorted uplands of Europe and elsewhere are the last
remnants of very ancient mountain chains? It is they which form the
framework of the continents, and by their wear and tear the low grounds
have been formed, owing to the filling in of the great gulfs which ran
between the old mountain chains.
We may elaborate a little further this very interesting subject. Let us
first note that the geologists group the rocks composing the earth's
crust into three great divisions. We have, first, the Primary rocks,
which are the oldest, and include as their most generally interesting
member the Carboniferous rocks, with their coal-bearing beds, so
important in the modern industrial world. Second, we have the Secondary
beds, the most interesting members of which is the Chalk, so
well-developed in parts of England and France. Finally, the Tertiary
series includes the rocks of the period immediately preceding that in
which the first undoubted remains of man occur.
Each of these periods was of enormous length, and the labours of
successive generations of geologists have brought to light, at least in
broad outline, the general appearance of the globe in so far as affected
by the distribution of land and water, and the main earth movements, in
each separate period. Thus we know that during that long period of time
which is included in the Primary epoch, very extensive earth movements,
resulting in extensive folding and mountain formation, took place. The
geologists distinguish no less than three separate periods of folding in
Primary times. It is not necessary for us to consider these in detail;
their total result was to produce the mountain regions whose worn-down
stumps now form those uplands which we have described in Europe. But
they do not occur in Europe alone. That vast and relatively infertile
area in Eastern Canada which geologists call the Canadian Shield is a
region of very old rocks, once folded into a mountain region, but long
since worn down to an upland. In the eastern United States that long,
but interrupted, range of hills, which, under various names, runs from
the mouth of the St. Lawrence to Alabama and Georgia, and partially
shuts the seaboard off from the prairies and plains beyond, is a region
where the folding is still well marked, in spite of long denudation.
The Secondary period seems to have been one in which comparatively
little folding took place, while, as already indicated, the Tertiary was
one in which there was enormous folding in almost all parts of the
globe, the result being the appearance at the surface of the great
mountain chains of the present day. The structure of these chains makes
them relatively unstable, and the forces of erosion are now acting upon
them with extraordinary activity, beginning that process of wearing down
which has reduced their prototypes of the Primary period to mere
remnants of their former greatness.
Extensive as the Tertiary folding was, however, it left great areas
unaffected, or but slightly affected, and such areas form plains or
basins, where the rocks are but slightly tilted, or show a very simple
form of folding. In Europe such slightly modified rocks occur, _e. g._
in the Paris basin, and in the fertile plains of south-eastern England.
In the United States beds of a similar character occur right over the
great plains, filling what seems once to have been a great gulf between
the old highlands to the east and the towering modern mountain chains of
the west.
It must be realised that this is only a very summary and partial account
of a difficult and complicated problem; but from the standpoint of pure
geography it seems desirable to distinguish between those remnants of
ancient mountains which form the backbone of the continents, the
recently elevated mountain chains where enormously rapid erosion is
taking place, and the largely unmodified rocks which often form fertile
plains.
Let us next proceed to consider how the eroding agents act upon the
surface of the land as soon as it is exposed. We may begin with the
effect of running water upon a recently exposed surface, _e. g._ upon
land slowly emerging above sea-level, or even with the effect of heavy
rain upon sloping ground unprotected by a covering of vegetation. Alike
in the one case and in the other the first effect is the formation of a
number of shallow rills, which at first run parallel to one another.
Sooner or later, however, these parallel channels tend to converge, and
a torrent is formed such as may be seen in any mountain region.
[Illustration: Fig. 2.--An ideal profile of a mature river (AC), showing
the increase in the slope towards the source. The dotted line BC shows
an earlier stage, when there are smooth reaches and rapid reaches with
waterfalls, etc. Note that progressive erosion causes the source to
retreat (_i. e._ from B to A).]
[Illustration: Fig. 3.--An actual profile of the Loire. It will be noted
that the Loire is a mature river, its profile nearly coinciding with the
"ideal" condition. (After de Martonne.)]
Such a torrent consists of three often well-defined parts. First we have
the numerous tiny rills which collect together to form what the French
physiographers call a receiving basin (_bassin de réception_); then
there is the stream proper forming a canal which drains the basin, while
below, where the torrent debouches on the low ground, we find that it
spreads out fanwise and throws down its load of débris to form a cone
(_cône de déjection_). The torrent therefore already imitates a
full-grown river, with its threefold division into mountain track,
valley track, and plain track. It further illustrates the twofold work
of the river, that of erosion and deposition.
Observation on an unprotected surface after a heavy rainfall will
illustrate another point which is of much interest in connection with
the work of rivers. This is that the water has most excavating power,
not, as might be supposed, in the collecting basin, but in the valley
region, where the slope is still great, where the volume of the water is
at its maximum, and where it has acquired a load of débris by means of
which it carves out its bed. The excavation of the bed therefore
proceeds from below upwards towards the collecting basin. The result is
that the slope of the valley floor diminishes as we pass from the upper
region to the lower, owing to the levelling effect of erosion. The
process of levelling down cannot be carried beyond a certain point, the
so-called base level of erosion, which in a lateral stream is
determined by the point of junction with the main stream, and in a main
stream by the point which marks sea or lake level, for obviously no
point in the river valley can be worn down much below its mouth.
When the work of a river is completed, the line which marks the profile
of its bed should have a gentle and continuous slope downwards to base
level. The existence of irregularities, of breaks in the smoothness of
the slope, means that the work of excavation has not proceeded far, that
the river is young. But it is not necessary to proceed to the laborious
drawing of a profile in order to determine the extent to which the
process of excavation has been carried. The existence of rapids, of
waterfalls, the alternation of swift and slow-flowing reaches are all
proofs that it has not been carried far. In short, if a river is
navigable, the navigable reach at least is mature; if it is capable of
furnishing power, that region at least is youthful. If, as sometimes
happens, the middle course is navigable and slow-flowing, and the lower
course broken by rapids and falls, then the probability is that earth
movements have occurred, so that the two regions are of different age.
This is a condition which occurs relatively often in the case of large
rivers.
One other point is worth notice, because it illustrates another way in
which the analogy of youth and maturity holds good. The youthful river,
with its interrupted slope, its lakes and falls, does not permit the
water to flow off with the same regularity as the mature river with its
smoothed outlines. The mature river is thus a more perfect instrument of
drainage.
It is not necessary for our purpose to consider in detail the
characteristic forms of river erosion. It may be sufficient to notice
that rapids and waterfalls are due to the varying hardness of the rocks
forming the bed of the river, and that the normal course of events is
the transition from waterfall to rapid, and from rapid to stream flowing
quietly at the bottom of a rocky gorge. Long gorges or canyons tend to
occur in regions where river erosion is not greatly assisted by the
other eroding agents. As a general rule, as the river cuts its way down,
the other agents cut back the walls so exposed, so that a wide valley is
formed.
But a river does not only eat out its bed in its valley track. A
necessary consequence of this erosion is that it is also able to eat
back the slope on which it is rising, as a result of the smoothing out
of the curves of its bed, so that its source retreats further and
further into the mountain. In regions of abundant rainfall every slope
is abundantly supplied with streams, and therefore those streams which
cut back their region of origin most rapidly will necessarily encroach
upon their neighbours' territory. They therefore tend to tap some of the
tributaries of the other streams, a phenomenon which has sometimes
considerable human importance, and has been extensively studied of late
years under the name of river-capture.
Some examples may serve to make the phenomenon clear. Every one who has
travelled up the Rhone valley in Switzerland has noted the enormous
number of lateral streams, of all sizes, which tumble down the mountain
sides into the Rhone. These streams on, _e. g._, the south side, are,
roughly speaking, parallel to each other, and to a large extent enter
the main stream independently. That is, for the most part they are very
youthful streams. In some cases, however, _e. g._ in the case of the
Dranse and the Visp, the drainage is of a more advanced character, and
we find a large stream with tributaries of considerable size as
distinct from mere torrents. A glance at any great river system on the
map, _e. g._ the Mississippi, the Amazon, etc., will show that the
condition of a great stream with many tributaries is normal in a
district where the drainage is of the developed type. How are the two
conditions, that of numerous parallel mountain torrents and that of a
great river system, related to one another? There is no doubt that
capture, the encroachment of one stream upon the territory of another,
has played an important part in the process.
[Illustration: Fig. 4.--Sketch-map to illustrate river-capture.]
A very simple example of this widespread phenomenon may be taken in
illustration. The accompanying sketch-map, drawn by Mr. Lionel Hinxman,
shows part of the course of the River Feshie, one of the tributaries of
the Spey, and part of the Geldie Burn, one of the tributaries of the
Aberdeenshire Dee. It will be noted that the Feshie shows a very curious
bend, or elbow. Mr. Hinxman points out that this curious condition can
be explained on the supposition that the River Eidart, shown on the map
to the north of the bend, once formed the headwaters of the Feshie,
which cut its valley back until it captured the headwaters of the
Geldie, and thus brought water which formerly flowed into the Dee into
the Spey valley. The boundary between the two counties of Aberdeen and
Inverness is shown on the map by a dotted line, and it is seen that the
march follows the watershed, which between the present Geldie and the
bend on the Feshie is very low. Formerly, however, this watershed lay
much further to the west, and its shifting is due to the capture.
A careful study of large scale maps will show many examples of similar
river-capture, some old and some recent. A sharp bend, the so-called
elbow of capture, on a river in close proximity to another stream
affords in itself a certain presumption that capture has taken place,
though this presumption can only be verified by study on the spot.
It may be noted that before the capture is finally accomplished there
may be an intermediate stage when the water has the choice of two
channels, both of which may be utilised in a time of flood. A very
curious case is that of the Casiquiare, a river in South America which
connects together the two systems of the Amazon and the Orinoco, while
another is the connection recently discovered by Captain Lenfant, a
French explorer, between the systems of the Shari and the Niger in
Africa. Such conditions are obviously unstable, for one stream must
sooner or later predominate over the other, and deprive it even of flood
water.
Another example may help to explain the evolution of a complex river
system with many tributaries. A glance at the map of England (see
diagram) shows that while the rivers of Northumberland and Durham flow
independently into the sea, those of Yorkshire are united into a
characteristic bunch, and all reach the ocean by means of the Humber.
This estuary breaks through the high ground formed by the Wolds of
Yorkshire and Lincolnshire, which consists of hard rock. At one time it
is probable that the rivers of Yorkshire entered the sea separately,
while the other great factor of the Humber, the Trent, mingled its
waters with the present Witham. At this time the weathering of the land
surface had not reached its present stage so the land would lie higher.
In what is now the vale of York the rocks are much softer than where the
Wolds now stand, and the present Ouse, which was at first a longitudinal
tributary of a transverse stream, eating its way back through these soft
rocks, tapped successively the streams flowing eastwards from the
Pennines, and with the help of the abundant water so obtained was
enabled to cut out the wide estuary of the Humber.
[Illustration: Fig. 5.--Sketch-map of northern England, to show the
position of the Tyne and Aire Gaps, and the peculiar character of the
rivers of Yorkshire. The black areas are heights above 600 feet.]
One other important point in connection with river-capture has been
already suggested in the account given of the Feshie. In the little
sketch-map we see clearly the shift of the watershed to the east. The
ultimate cause of this shift is doubtless the fact that in Great
Britain the rainfall diminishes to the east, so that, generally
speaking, the westerly streams have more erosive power than the
easterly. But the special interest of the case is simply that it may
serve to suggest a fact not at first sight obvious, which is that
water-partings are excessively unstable features. One set of streams is
continually striving to encroach upon the others, and by capturing their
headwaters to reduce their erosive power. A very striking example of
capture on the large scale is seen in southern Patagonia, where the
water-parting does not lie near the summit of the chain of the Andes, as
might be expected, but considerably to the east, the western streams (or
glaciers) having captured all the headwaters of the eastern streams,
which lie in a region of much lower rainfall.
The net result is that running water not only scours valleys in the
sides of mountain chains, but also, sooner or later, wears away the
crest itself, and with the assistance of the other agents of denudation
tends to reduce the mountains to plains--or at least "peneplains." The
deduction is, of course, old enough, but the recent emphasis placed upon
river-capture helps us to realise it, showing us the actual "shift of
the divide," or, in other words, the wearing down of the summit levels.
This is a theoretical matter, but there is another point which has
practical significance. Referring once again to the sketch-map on p. 43,
we note that just at the sharp bend in the Feshie, that is, at the elbow
of capture, there is a narrow region, crossed by the boundary line,
which was once traversed by the headwaters of the Geldie, but is now a
dry valley. Such "gaps," as they are called, are present where recent
capture has occurred, and where they occur in hilly country they
sometimes form useful passes, permitting the construction of an easy
road across the hills. A good example is the Aire Gap (see fig. 5) in
the Pennine range of Great Britain, apparently connected with the fact
that the Ribble has captured the headwaters of the Aire. Another
interesting example is the so-called Tyne Gap, that breach in the
Pennines which occurs near the present head of the South Tyne; it was
traversed by the Roman wall, and is now crossed by the road and the
railway from Newcastle to Carlisle.
As we shall see, ice appears to have this power of cutting passes
through mountain chains to a much greater extent than running water; but
here, as in many other respects, there does not appear to be a sharp
breach between the action of the two.