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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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breaking oif.

The Greenland ice sheet is a striking example. To the
best of our knowledge, almost the entire island is covered



WORK OF AVALANCHES AND GLACIERS 157

with ice and snow that have been accumulatiufi: during:
long periods of time. So far as known the only rock
that reaches above the surface of the ice is found near
the coast, where the ice-covering is thinnest.

Along the southern coast much of the ice and snow dis-
appears by melting. Farther north, hoAvever, the ice
reaches the coast — sometimes descending into the fjords,
sometimes presenting an unbroken wall from five to fifty
miles in extent. In places the fiow of the ice is compara-




lilKlH OF THE ICEBERG
The buoyant force of the water is shearing the fragments, and the latter float away.



tively rapid — as much as forty or fift}' feet a day. The frag-
ments broken from the ice front are icebergs. Sometimes
tliey tumble from the top; in other instances the edge of
tlie shet.'t is pushed out so far that the buoyant force of the
water breaks a fragment from the sheet, and it floats off.

Hund)oldt (llacier, on the west coast of Greenland, is a
strikiug example of the ice-slieet. For a distance of about
sixty miles, its ragged front, broken here and there by
rock-cliffs, forms a sea-wall in ])laces several hundred feet



158 PHYSICAL GEOGRAPHY

high. By far the most stupendous examples, however,
are those of antarctic regions. Apparently the ice-sheet
is continental in size and, judging from the thickness of
the icebergs, it is probably several miles thick.

Occurrence of Glaciers. — In general, glaciers begin
above the line of perpetual snow and extend usually a
short distance below it. In high latitudes, where the
weather is cold, they occur at no great altitude above sea-
level, but the nearer they are to tropical regions, the
higher the altitude. In low latitudes they rarely occur
below the altitude of fifteen -thousand feet, while in polar
regions they usually flow into<the sea. '

The largest stream or ravine glaciers known are in the
Himalaya Mountains; the best known are those of the
Alps. Along the northern coast of Norway there are fine
examples ; in the Patagonian Andes, and along the
Alaskan coast, almost every arm of the sea contains one or
more of them. Study the character of these coasts on a
good map. In the Rocky Mountains there are numerous
glaciers, but none of them is of great size. Several of the
glaciers of Mounts Shasta and Tacoma (Rainier) rival the
Alpine ice-streams in extent. Muir Glacier, Alaska, has
a frontage of two miles on the sea.

Most of the rivers flowing from the high slopes of
mountains that reach above the snow-line have their sources
in glaciers. Find examples in the Alps.

Physiographic Effects of Glaciers. — The results of
glacial action are readily observed in the glaciers of the
present time and, indeed, they are so full of character that
they are a most excellent key Avhereby the stupendous
efl"ects of the glaciers of prior geological times have been
studied.

The chief efi'ects of glacial action are erosion and trans-
portation. Ice alone is so soft that it has little or no



WORK OF AVALANCHES AND GLACIERS 159

wearing effect on hard rock, but if a moving mass of ice
drags or pushes fragments of rock along at the sides and
bottom it becomes a cutting tool of great power. It
planes, gouges, or scratches, according to the character of
the rock over which it moves.

All through the northern United States and Canada,
nearly to the Eocky Mountains, the surface has been
scoured by glacial ice, and many thousand lake basins




REGION OF GLACIATION IN THK UNITED STATES

rhe heavy line shows the limit of terminal moraines : erratic bowlders occur in occasional
localities a little farther south of the line.

have been made or shaped. In the exposed rock of New
England and New York, the grooved and rounded surfaces
are one of the most marked features, and every Avhere the
erosion is so charact(iristic as to reveal its origin. The
northern Appalachian Mountains were worn and broken,
and the wide gap between the Adirondack and Catskill
ranges — both groups being parts of the A])palachiaii folds
— was probably made at this time. What lias been tho



160



PHYSICAL GEOGRAPHY



effect of this gap on the cominercial development of New
York? That the siirfuco of the ice-sheet did not reach
quite to the top of the highest peaks of the Adirondack
and White Mountains is inferred from the fact that certain
alpine species are still found at their summits that do not
occur at a lower level.

The same markings are equally plain throughout north-
ern Europe, and the coasts of Norway and the British
Isles probably received their present frayed and ragged
appearance at the same time that so much of North
Amercia was covered with glacial ice.





'oiisfetfeS



A DRUMLIN
III many instances the surface is covered with fertile soil.

The transportation of material is a still more noticeable
effect of glaciation, and the rock waste that has been
removed, is commonly known as drift. Glacial drift is
unsorted material, and in size the pieces vary from grains
of sand to bowlders weighing several thousand tons. In
character, the gravel of drift differs materially from stream
gravel ; for while the latter is composed of uniformly
rounded pieces the fragments of the former are rough and
angular, with one or more faces planed smooth.

Glacial rock waste or detritus has been deposited in
various forms. Much of it has been spread over the sur-



WORK OF AVALANCHES AND GLACIERS 161



face as an imperfectly mixed mass of clay, sand, and gi'avel.
These deposits are the well-known till plains of northern
Europe and the United States. Not infrequently the
material occurs in rounded hillocks or drumlins, or per-
haps in loug
ridge -shaped
bars, called
esJcers. The
former are very
common in the
New England
Plateau, the
northern lake
region, and
also in Eng-
land and Scot-
land. Several
of the islands
in Boston har-
bor are drum-
lins.

Near the
southern limit
of the glacial
ice - sheet the
drift occasion-
ally takes the
form of long
ridges — perhaps many miles in extent, and one hundred
feet or more in height. In nearly every instance these
heaps are moraines. A part of Long Island is probably
a terminal moraine, and several of the ridges that cross
New Jersey are of similar origin. Many of the low ridges
extending into the valleys of Colorado are moraines.




SPLIT ROCK : AN KRRATIC BOWLDER

Tlic bittlcnntl-tn'c, growing from the cliff, is forty yiars old.



1G2 PHYSICAL GEOGRAPHY

A remarkable form of drift is found in the rounded
blocks of stone strewn over the surface of the New England
and Middle Atlantic States and a few other localities.
These are commonly known as erratic bowlders. With re-
spect to mineral character the bowlders are of many kinds;
those of the northeastern United States are mainly of
granite. The most interesting feature about them is
the fact that they are unhke the rock in the locality
where they are found ; in some instances they certainly
have been brought from a long distance. Some of them
are of enormous size ; one, Split Eock,"^ near Mount Ver-
non, New York, weighs not far from five hundred tons.

Icebergs. — The formation of icebergs along the sea-
front of glaciers becomes an important factor in several
ways. The icebergs from the west coast of Greenland float
southward during late spring, and during May and June
cross the routes of trans-atlantic steamships, thus becom-
ing a menace to navigation. Sometimes several hundred
of them are drifting about in the vicinity of the New-
foundland Banks, and remain there until they melt or are
broken up by storms. The huge blocks broken from the
Antarctic ice-sheet drift about over a very large area, some-
times being found as far north as latitude 40° S. In the
North Pacific Ocean the icebergs are small and are rarely
found beyond the partly enclosed waters of the Alaskan
coast and Bering Sea.

QUESTIONS AND EXERCISES. -Describe any effects you have
noticed with relation to snowslides on the roofs of buildings or steep
slopes.

A mass of snow weighing ten thousand tons moves with a velocity
of twenty-five feet per second ; what is its momentum in foot-
pounds ? Would this force be sufficient to break off or uproot large
trees ?

In a previous paragraph it is stated that the water issuing from the
end of a glacier is muddy ; account for the presence of the mud.



WORK OF AVALANCHES AND GLACIERS 163

Explain the way in which rock fragments may get to the bottom of a
glacier. Why are the scratches made by these fragments parallel ?

Why are there no glaciers in the Appalachian Mountains ?

The map on p. 157 shows the terminal moraine of the great ice-sheet ;
describe its course and location. Name two large lakes situated in the
basin of former Lake Agassiz.

Describe any evidence of glaciation in the neighborhood in which you
live, noting drumlins, eskers, moraines, markings and scratches, erratic
bowlders, or drift. If possible delineate them on a map.

COLLATERAL READING AND REFERENCE.

Tyndall. — For'uis of Water.

Tyndall. — Hours of Exercise in the Alps.

Le Coxte. — Eleiuents of Geology, pp. 569-583.

NOTES

' It is rare that snow accumulates to a depth of more than ten
or twelve feet on a level area. On mountain slopes the snow is
not evenly distributed, most of it finally lodging in ravines and
places not exposed to the sweep of the wind. In laying the foun-
dations for the observatory at the summit of Mont Blanc, the
snow and ice were so deep that no rock bottom could be found at
a depth of sixty feet. On the western slope of the Sierra Nevada
Mountains the fall of snow sometimes reaches twenty feet on
the level, while the drift may be several times as great.

■' Wet clothing hung out to dry in very cold weather first
freezes and then gradually dries. An inspection of Table III.,
Appendix, shows that at a temperature of —40° F. a small amount
of moisture may still exist in the atmosphere.

' The power of wind in drifting loose .soil has already been
noted. But snow is less than one-quarter as heavy as soil of
average material ; hence the work of wind is far more effective.

* In certain parts of the Rocky, Cascade, and Sierra Nevada
Mountains avalanches are of frequent occurrence, but they are
by no means so common as in the Alps. In the latter ranges the
slopes are steeper and the snowfall is considerably greater. It is
not improbal)le that such snow-slides are just as common .tnd
quite as destructive in the Caucasus and the Himalaya Moun-
tains as they are in the Alps.



164 PHYSICAL GEOGRAPHY

'■ These are the poiulreuses (powdery snow), and they are the
most dreaded of all snowslides. Damp snow does not shear and
move readily ; it is the gatliering of light, dry snow, little by
little, until finally the whole mass is in motion, that is the dis-
tinctive feature of this form of avalanche.

" This form is known as the avalanche de fond. It is rarely
destructive.

' The bands are alternate layers of ice and dirty snow. The ice
is formed of snow that has been subjected to great pressure. Be-
cause of the pressure all the air has been squeezed out, and for
this reason the ice is tolerably clear and blue. The bands of snow
contain air and are therefore whitish and opaque.

" The 'ice hunnnocks are conical in shape and, if present, are
found almost always at the lower end. Not infrequently one of
these hummocks is surmounted by a bowlder of several tons
weight. The bowlder protects its support from the heat of the
sun, while the latter melts the ice around the lower end of the
coluuni. Sooner or later the ice column breaks and the bowlder
falls to a lower level, where the same process is again repeated.

° This peculiar feature at one time gave rise to the opinion that
there might be an up-stream motion to a glacier. The reason for
their direction, however, is evident ; the crack or break is neces-
sarily at right angles to the direction of the strain. Now the
movement of the ice is twofold — down stream and away from the
bank. Therefore when the ice breaks the crack points diagonally
up the stream.

'" Many years since this bowlder broke into two parts along a
cleavage plane. A butternut-tree sprang up in the cleft and in
time its trunk has wedged the two fragments apart in the form
of a V-shaped opening. In the northern part of Westchester
County a large erratic block has been deposited on the top of
three sinaller stones, the latter forming a very firm tripod. In a
number of instances one bowlder has been deposited on the top
of a boss of rock in such a position that the equilibrium, while
more or less unstable, cannot be readily overthrown. Examples
are found throughout the New England States, and they are pop-
ularly known as rocking stones. There is a fine example in
Bronx Park, New York City. Rocking stones are also common
in the glaciated regions of northern Europe.



CHAPTEE X

THE WASTING OF THE LAND : THE RESULTS OF
IMPERFECT AND OBSTRUCTED DRAINAGE. LAKES

AND MARSHES

In floAving from higher to lower levels along lines of
least resistance, the water may find its passage temporarily
obstructed, or perhaps wholly blocked by obstacles.
Sometimes a ridge of land prevents its progress ; in other
cases a landslide or, perhaps, a stream of lava athwart
the channel prevents its progress. The water therefore
spreads out, forming a hike^ pond, or marsh. In places
where the flow is obstructed, one of two things must occur
— either the water will collect until its surface is high
enough to flow over the lowest part of the rim, or else it
will spread over the surface until the amount that evapo-
rates just equals that which flows in. The area whose
waters flow into the lake constitutes its basin. A large
basin usually has several rivers and many small streams
that are its tributaries or feeders.

Marsh Lakes. — In a region of considerable rain-fall, if
the general slo[)e be very decided, perhaps there may be no
lakes and ponds, for the reason that the water flows oft',
meeting no obstructions which cause it to collect in basins.^
On the contrary, if the surface be flat, the water, finding
no definite channels, spreads over the surface and forms a
multitude of small ponds. In Florida and along the Gulf
Coast there are excellent examples, and they are commonly
called marsh lakes.'^

165



166



PHYSICAL GEOGRAPHY



A marsh lake of large size or considerable depth could
not form in perfectly flat lands, for the reason that, after
reaching a certain height, the water would flow ofi^ as fast
as it was supplied. For a similar reason, such lakes could
not be very numerous on a surface that had a consider-
able slope.^ But while many — perhaps most— of the
lakes have been formed by the surplus of raiufall over
drainage, there are many thousand lakes that are the
result of factors with which rain-fall has no direct con-




MARSH LAKES, FLORIDA.



nection. The most important are those whose basins have
been shaped largely by the action of moving streams of
ice — that is, by glaciers.

Glacial Lakes. — A glance at a good map of the north-
ern part of North America shows that the lakes of this
region are its most remarkable surface feature. As a rule
they are long and narrow, and if a group of them be con-
sidered, it is at once apparent that their axes, or lines of
greatest length, are nearly parallel. Careful investigations



IMPERFECT AND OBSTRUCTED DRAINAGE 167

Lave shown that not only are such hikes comparatively
much deeper than the marsh lakes previously described,
but that also, in most instances, their basins have been
wrought in the hardest rocks. In many instances, too,
their rims are walls of bowlders that could scarcely have
been more regular, had the courses of rock been laid by
human hands.^

Very frequentl}^ such lakes occur in chains, a river fol-
lowing the course of each chain ; indeed, these lakes are




GLACIAL LAKLS
A group III the Adiioiiitack Mountains, New York.

merely incidents in tlie histcny of the river. In a few
instances a cluster of such lakes apparently radiates from
a central point, as is seen in the " finger " lakes of New
York. Lakes of this type are closely associated with the
great accumulation of glacial ice " that formerly covered a
large part of the northern hemisphere. The lakes them-
selves are found in glaciated regions only — never else-
where. They are therefore called glacial lakes or, in the
British Isles, tarns.



168



PHYSICAL GEOGRAPHY



Accidental Lakes. — There are other lakes whose
origin is the result of accident ; such as the destruction
of a river loop, the damming of a stream,^ the formation
of a bar across an estuary or cove, or the sinking of an
area of land. Very man}- lakes have come suddenly into
existence through one or another of the causes named.

In the illustrations pp. 108 and 110 there is shown a
type of lake that is common along tlie bottom lands of the
Mississippi and other rivers that flow through level plains.
The origin of such lakes is very apparent. The lakes
themselves are manifestly the abandoned loops of rivers,
and they are formed when the river straightens its channel.
The moat thus formed remains filled with Avater. Perhaps
a bayou or small stream may be left as a feeder, but more
likely the moat becomes a stagnant pool, sooner or later
to disappear — possibly overgrown by vegetation, possibly
buried under the sediment brought down by floods.

Another type of accidental lake occurs along low, flat
coasts. These are the lagoons of the sea-shore or the lake-

shore. The
south coast of
Marthas Vine-
yard furnishes
an excellent il-
lustration of la-
goons of this
type. In times
past, this shore
was a succes-
sion of coves
and small bays.
But the water
on this side of the island is so shallow that the waves,
dragging heavily on the bottom, have pushed enough sand



w Chop

f Chop




LAGOONS, MARTHAS VINEYARD



IMPERFECT AND OBSTRUCTED DRAINAGE 169

before them to throw barriers across the coves, and shut
them off from the oceau.^

Any good map of the United States or of Europe will
show a multitude of wave-formed lagoons of this character.
Those near the shore often have more the nature of sounds
than of lagoons.^ But as the coast, little by little, extends
seaward, many of them now near the shore Avill ulti-
mately be at a considerable distance inland.

Salt Lakes. — Salt lakes have no outlets, and for that
reason they are salt.'*' Nearly all soil contains more or
less mineral salts that are soluble in water. Even the
hardest granites and igneous rocks contain a minute pro-
portion of soluble matter. So when the water flows to
the basin, it carries with it any soluble matter with which
it comes in contact. If the lake or pond has an outlet,
both the water and the salt flow off together." If there
be no outlet, however, the water is removed by evapora-
tion, while the mineral salts, Avliich cannot evaporate,
remain in the basin. In time, the water becomes decid-
edly salt, and finally, a brine that Avill dissolve nothing
more. After this, unless there is an inflow of fresh water,
the salt sinks to the bottom, and forms also a wide mar-
gin of crusted salt along the shore.

Temperature and atmospheric moisture are also factors
in the origin of salt lakes. High temperature and dry-
ness of the atmosphere both promote evaporation, and
doubtless there are regions, whose lakes are now fresh,
that would become regions of salt lakes were the tem-
perature and dryness to increase materially.

Although salt lakes have no outlets, it is not necessarily
true that lakes without outlets are salt. As a matter of
fact, there are many such lakes whose waters are almost
as sweet and pure as when they fell from the clouds. Of
this apparent contradiction there are two explanations.



170 PHYSICAL GEOGRAPHY

In the first place the lakes may be young. In this case,
time only is required to change the fresh lake to one of
brine, and the time will be long or short, according as
the soil through which the feeders flow contains little or
nnicli soluble matter. In the Great Basin, west of the
Rocky Mountains, there are several young lakes, whose
waters are comparatively fresh,^^ situated almost alongside
lakes of briny saltness. In the second place, all the
soluble matter may have been leached from the soil at
some prior time when the lake overflowed its basin.
There are many such lakes in Canada and the United
States. They are not salt, and unless their conditions of
existence are changed they will not become salt.

There are certain lakes, mainly in arid regions, that are
periodic in character. During the rainy season they may
be of considerable size ; they have no great depth, how-
ever, and in the dry season their waters evaporate, leaving
in each basin a thick crust of salt. There are numerous
small lakes of this character in the western part of the
United States; some of those in southern Russia are of
considerable area. Lakes of this kind are commonly called
playa lakes. Commercially some of them are important
on account of the enormous amount of salt they yield.

Physiographic Aspect of Lakes.— Lakes are the most
transitory features of the earth's surface. Rivers and the
various relief features of the earth are seldom entirely
obliterated ; but as time is reckoned a lake is the creation
of a very brief period. Its life is almost ephemeral, and
various forces are constantly at work to destroy it. Physi-
ographic agents that have no effect on other features of
the earth are often fatal to the existence of lakes.

Among the various agents, glaciers are, perhaps, the
chief. Glaciers have been energetic factors in making
lakes, it is true ; they have also been quite as effective in



IMPERFECT AXD OBSTRUCTED DRAINAGE 171

causing their destruction. The glacier blocks the channel
of a river with ice or with gravel, and in a short time a lake
is formed. Later it forces a passage through the obstruc-
tions made, and in a little while the lake has disappeared.'"
A few old shore marks and, perhaps, a delta or tAvo are all
that remain to tell the storv.



A BURIED LAKE BASIN
The basin has been filled with sediments brought into it by the river.

A change in the level of the lake-bed by elevation or by
depression always produces great changes in the lake. Such
a change may throw up a ridge so as to form a basin for a
new lake, but it may also lower the land at the foot of the
lake and destroy the l)asin of an old one. Long before the
existence of the lakes whose remnants are now found in
the Great Basin, a vast body of water covered much of



172 PHYSICAL GEOGRAPHY

this region. But a change in the level of the basin
occnired, and this, together with probable changes in
climate, caused the great internal seas gradually to dis-
appear.'^

Rapidly growing vegetation is also a potent factor in
the destruction of lakes. Vegetation has but little effect
on deep lakes, but in the case of marsh lakes it has a great
deal. The process is verj^ simple : the roots, stalks, and
leaves of the dead j^lants fill the basin until there is no
more room for the lodgement of water. Usually the plant
begins its growth at the edge of the lake and spreads
toAvard the centre, gj-adually filling the basin, until a deep
hole is all that remains. The struggle of the lake may be
a long one, but in the end the vegetation conquers. Buried
and partly obliterated lakes of this character are common
in all coast plains and level lands. One near Goshen,
New York, covering an area of about sixty square miles,
has disappeared within recent times and most of its former
bed is now cultivated land — the famous "onion fields" of
the State.

Winds are sometimes very effective in the destruction
of lakes, especially the lagoons along the seashore. The
manner in which they operate is very simple ; they merely
carry enough fine rock waste into the basin to fill it.'^
The rock waste is piled upon the windward shore, and
the latter advances, little by little, until finally it meets
the opposite shore. The lagoon is filled, and at the
same time an estuary becomes a part of the coast plain.
Such instances are common on coasts that are swept by
constant winds.

The foregoing are the most apparent agencies that con-
tribute to the destruction of lakes ; and although in many
instances they operate continuously and systematically,


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 11 of 25)