TRANSPORTATION OF SOLID MATTER BY ICE.
Carrying power of river-ice--Rocks annually conveyed into the St.
Lawrence by its tributaries--Ground-ice; its origin and transporting
power--Glaciers--Theory of their downward movement--Smoothed and
grooved rocks--The moraine unstratified--Icebergs covered with mud
and stones--Limits of glaciers and icebergs--Their effects on the
bottom when they run aground--Packing of coast-ice--Boulders drifted
by ice on coast of Labrador--Blocks moved by ice in the Baltic.
The power of running water to carry sand, gravel, and fragments of rock
to considerable distances is greatly augmented in those regions where,
during some part of the year, the frost is of sufficient intensity to
convert the water, either at the surface or bottom of rivers, into ice.
This subject may be considered under three different heads:--first, the
effect of surface-ice and ground-ice in enabling streams to remove
gravel and stones to a distance; secondly, the action of glaciers in the
transport of boulders, and in the polishing and scratching of rocks;
thirdly, the floating off of glaciers charged with solid matter into the
sea, and the drifting of icebergs and coast-ice.
_River-ice._--Pebbles and small pieces of rock may be seen entangled in
ice, and floating annually down the Tay in Scotland, as far as the mouth
of that river. Similar observations might doubtless be made respecting
almost all the larger rivers of England and Scotland; but there seems
reason to suspect that the principal transfer from place to place of
pebbles and stones adhering to ice goes on unseen by us under water. For
although the specific gravity of the compound mass may cause it to sink,
it may still be very buoyant, and easily borne along by a feeble
current. The ice, moreover, melts very slowly at the bottom of running
streams in winter, as the water there is often nearly at the freezing
point, as will be seen from what will be said in the sequel of
ground-ice.
As we traverse Europe in the latitudes of Great Britain, we find the
winters more severe, and the rivers more regularly frozen over. M.
Lariviere relates that, being at Memel on the Baltic in 1821, when the
ice of the river Niemen broke up, he saw a mass of ice thirty feet long
which had descended the stream, and had been thrown ashore. In the
middle of it was a triangular piece of granite, about a yard in
diameter, resembling in composition the red granite of Finland.[283]
When rivers in the northern hemisphere flow from south to north, the ice
first breaks up in the higher part of their course, and the flooded
waters, bearing along large icy fragments, often arrive at parts of the
stream which are still firmly frozen over. Great inundations are thus
frequently occasioned by the obstructions thrown in the way of the
descending waters, as in the case of the Mackenzie in North America, and
the Irtish, Obi, Yenesei, Lena, and other rivers of Siberia. (See map,
fig. 1, p. 79.) A partial stoppage of this kind lately occurred (Jan.
31, 1840) in the Vistula, about a mile and a half above the city of
Dantzic, where the river, choked up by packed ice, was made to take a
new course over its right bank, so that it hollowed out in a few days a
deep and broad channel, many leagues in length, through a tract of
sand-hills which were from 40 to 60 feet high.
In Canada, where the winter's cold is intense, in a latitude
corresponding to that of central France, several tributaries of the St.
Lawrence begin to thaw in their upper course, while they remain frozen
over lower down, and thus large slabs of ice are set free and thrown
upon the unbroken sheet of ice below. Then begins what is called the
packing of the drifted fragments; that is to say, one slab is made to
slide over another, until a vast pile is built up, and the whole being
frozen together, is urged onwards by the force of the dammed up waters
and drift-ice. Thus propelled, it not only forces along boulders, but
breaks off from cliffs, which border the rivers, huge pieces of
projecting rock. By this means several buttresses of solid masonry,
which, up to the year 1836, supported a wooden bridge on the St.
Maurice, which falls into the St. Lawrence, near the town of Trois
Rivieres, lat. 46 degrees 20 minutes, were thrown down, and conveyed by
the ice into the main river; and instances have occurred at Montreal of
wharfs and stone-buildings, from 30 to 50 feet square, having been
removed in a similar manner. We learn from Captain Bayfield that anchors
laid down within high-water mark, to secure vessels hauled on shore for
the winter, must be cut out of the ice on the approach of spring, or
they would be carried away. In 1834, the Gulnare's bower-anchor,
weighing half a ton, was transported some yards by the ice, and so
firmly was it fixed, that the force of the moving ice broke a
chain-cable suited for a 10-gun brig, and which had rode the Gulnare
during the heaviest gales in the gulf. Had not this anchor been cut out
of the ice, it would have been earned into deep water and lost.[284]
[Illustration: PLATE II.
BOULDERS DRIFTED BY ICE ON SHORES OF THE ST. LAWRENCE
View taken by Lieut. Bowen, from the N. E., in the Spring of 1835, at
Richelieu Rapid, lat. 46 degrees N.]
The scene represented in the annexed plate (pl. 2), from a drawing by
Lieutenant Bowen, R. N., will enable the reader to comprehend the
incessant changes which the transport of boulders produces annually on
the low islands, shores, and bed of the St. Lawrence above Quebec. The
fundamental rocks at Richelieu Rapid, situated in lat. 46 degrees N., are
limestone and slate, which are seen at low-water to be covered with
boulders of granite. These boulders owe their spheroidal form chiefly to
weathering, or action of frost, which causes the surface to exfoliate
in concentric plates, so that all the more prominent angles are removed.
At the point _a_ is a cavity in the mud or sand of the beach, now filled
with water, which was occupied during the preceding winter (1835) by the
huge erratic _b_, a mass of granite, 70 tons' weight, found in the
spring following (1836) at a distance of several feet from its former
position. Many small islands are seen on the river, such as _c d_, which
afford still more striking proofs of the carrying and propelling power
of ice. These islets are never under water, yet every winter ice is
thrown upon them in such abundance, that it _packs_ to the height of 20,
and even 30 feet, bringing with it a continual supply of large stones or
boulders, and carrying away others; the greatest number being deposited,
according to Lieutenant Bowen, on the edge of deep water. On the island
_d_, on the left of the accompanying view, a lighthouse is represented,
consisting of a square wooden building, which having no other foundation
than the boulders, requires to be taken down every winter, and rebuilt
on the reopening of the river.
These effects of frost, which are so striking on the St. Lawrence above
Quebec, are by no means displayed on a smaller scale below that city,
where the gulf rises and falls with the tide. On the contrary; it is in
the estuary, between the latitudes 47 degrees and 49 degrees, that the
greatest quantity of gravel and boulders of large dimensions are carried
down annually towards the sea. Here the frost is so intense, that a
dense sheet of ice is formed at low water, which, on the rise of the
tide, is lifted up, broken, and thrown in heaps on the extensive shoals
which border the estuary. When the tide recedes, this packed ice is
exposed to a temperature sometimes 30 degrees below zero, which freezes
together all the loose pieces of ice, as well as the granitic and other
boulders. The whole of these are often swept away by a high tide, or
when the river is swollen by the melting of the snow in Spring. One huge
block of granite, 15 feet long by 10 feet both in width and height, and
estimated to contain 1500 cubic feet, was conveyed in this manner to
some distance in the year 1837, its previous position being well known,
as up to that time it had been used by Captain Bayfield as a mark for
the surveying station.
_Ground-ice._--When a current of cold air passes over the surface of a
lake or stream it abstracts from it a quantity of heat, and the specific
gravity of the water being thereby increased, the cooled portion sinks.
This circulation may continue until the whole body of fluid has been
cooled down to the temperature of 40 degrees F., after which, if the cold
increase, the vertical movement ceases, the water which is uppermost
expands and floats over the heavier fluid below, and when it has
attained a temperature of 32 degrees Fahr. it sets into a sheet of ice. It
should seem therefore impossible, according to this law of congelation,
that ice should ever form at the bottom of a river; and yet such is the
fact, and many speculations have been hazarded to account for so
singular a phenomenon. M. Arago is of opinion that the mechanical action
of a running stream produces a circulation by which the entire body of
water is mixed up together, and cooled alike, and the whole being thus
reduced to the freezing point, ice begins to form at the bottom for two
reasons, first, because there is less motion there, and secondly,
because the water is in contact with solid rock or pebbles which have a
cold surface.[285] Whatever explanation we adopt, there is no doubt of
the fact, that in countries where the intensity and duration of the cold
is great, rivers and torrents acquire an increase of carrying power by
the formation of what is called ground-ice. Even in the Thames we learn
from Dr. Plott that pieces of this kind of ice, having gravel frozen on
to their under side, rise up from the bottom in winter, and float on the
surface. In the Siberian rivers, Weitz describes large stones as having
been brought up from the river's bed in the same manner, and made to
float.[286]
_Glaciers._--In the temperate zone, the snow lies for months in winter
on the summit of every high mountain, while in the arctic regions, a
long summer's day of half a year's duration is insufficient to melt the
snow, even on land just raised above the level of the sea. It is
therefore not surprising, since the atmosphere becomes colder in
proportion as we ascend in it, that there should be heights, even in
tropical countries, where the snow never melts. The lowest limit to
which the perpetual snow extends downwards, from the tops of mountains
at the equator, is an elevation of not less than 16,000 feet above the
sea; while in the Swiss Alps, in lat. 46 degrees N. it reaches as low as
8,500 feet above the same level, the loftier peaks of the Alpine chain
being from 12,000 to 15,000 feet high. The frozen mass augmenting from
year to year would add indefinitely to the altitude of alpine summits,
were it not relieved by its descent through the larger and deeper
valleys to regions far below the general snow-line. To these it slowly
finds its way in the form of rivers of ice, called glaciers, the
consolidation of which is produced by pressure, and by the congelation
of water infiltered into the porous mass, which is always undergoing
partial liquefaction, and receiving in summer occasional showers of rain
on its surface. In a day of hot sunshine, or mild rain, innumerable
rills of pure and sparkling water run in icy channels along the surface
of the glaciers, which in the night shrink, and come to nothing. They
are often precipitated in bold cascades into deep fissures in the ice,
and contribute together with springs to form torrents, which flow in
tunnels at the bottom of the glaciers for many a league, and at length
issue at their extremities, from beneath beautiful caverns or arches.
The waters of these streams are always densely charged with the finest
mud, produced by the grinding of rock and sand under the weight of the
moving mass. (See fig. 18.)
[Illustration: Fig 18.
Glacier with medial and lateral moraines and with terminal cave.]
The length of the Swiss glaciers is sometimes twenty miles, their width
in the middle portion, where they are broadest, occasionally two or
three miles; their depth or thickness sometimes more than 600 feet. When
they descend steep slopes, and precipices, or are forced through narrow
gorges, the ice is broken up, and assumes the most fantastic and
picturesque forms, with lofty peaks and pinnacles, projecting above the
general level. These snow-white masses are often relieved by a dark
background of pines, as in the valley of Chamouni; and are not only
surrounded with abundance of the wild rhododendron in full flower, but
encroach still lower into the region of cultivation, and trespass on
fields where the tobacco-plant is flourishing by the side of the
peasant's hut.
The cause of glacier motion has of late been a subject of careful
investigation and much keen controversy. Although a question of physics,
rather than of geology, it is too interesting to allow me to pass it by
without some brief mention. De Saussure, whose travels in the Alps are
full of original observations, as well as sound and comprehensive
general views, conceived that the weight of the ice might be sufficient
to urge it down the slope of the valley, if the sliding motion were
aided by the water flowing at the bottom. For this "gravitation theory"
Charpentier, followed by Agassiz, substituted the hypothesis of
dilatation. The most solid ice is always permeable to water, and
penetrated by innumerable fissures and capillary tubes, often extremely
minute. These tubes imbibe the aqueous fluid during the day, which
freezes, it is said, in the cold of the night, and expands while in the
act of congelation. The distension of the whole mass exerts an immense
force, tending to propel the glacier in the direction of least
resistance--"in other words, down the valley." This theory was opposed
by Mr. Hopkins on mathematical and mechanical grounds, in several able
papers. Among other objections, he pointed out that the friction of so
enormous a body as a glacier on its bed is so great, that the vertical
direction would always be that of least resistance, and if a
considerable distension of the mass should take place, by the action of
freezing, it would tend to increase its thickness, rather than
accelerate its downward progress. He also contended (and his arguments
were illustrated by many ingenious experiments), that a glacier can move
along an extremely slight slope, solely by the influence of gravitation,
owing to the constant dissolution of ice in contact with the rocky
bottom, and the number of separate fragments into which the glacier is
divided by fissures, so that freedom of motion is imparted to its
several parts somewhat resembling that of an imperfect fluid. To this
view Professor James Forbes objected, that gravitation would not supply
an adequate cause for the sliding of solid ice down slopes having an
inclination of no more than four or five degrees, still less would it
explain how the glacier advances where the channel expands and
contracts. The Mer de Glace in Chamouni, for example, after being 2000
yards wide, passes through a strait only 900 yards in width. Such a
gorge, it is contended, would be choked up by the advance of any solid
mass, even if it be broken up into numerous fragments. The same acute
observer remarked, that water in the fissures and pores of glaciers
cannot, and does not part with its latent heat, so as to freeze every
night to a great depth, or far in the interior of the mass. Had the
dilatation theory been true, the chief motion of the glacier would have
occurred about sunset, when the freezing of the water must be greatest,
and it had, in fact, been at first assumed by those who favored that
hypothesis, that the mass moved faster at the sides, where the melting
of ice was promoted by the sun's heat, reflected from boundary
precipices.
Agassiz appears to have been the first to commence, in 1841, aided by a
skilful engineer, M. Escher de la Linth, a series of exact measurements
to ascertain the laws of glacier motion, and he soon discovered,
contrary to his preconceived notions, that the stream of ice moved more
slowly at the sides than at the centre, and faster in the middle region
of the glacier than at its extremity.[287] Professor James Forbes, who
had joined Mr. Agassiz during his earlier investigations in the Alps,
undertook himself an independent series of experiments, which he
followed up with great perseverance, to determine the laws of glacier
motion. These he found to agree very closely with the laws governing the
course of rivers, their progress being greater in the centre than at the
sides, and more rapid at the surface than at the bottom. This fact was
verified by carefully fixing a great number of marks in the ice,
arranged in a straight line, which gradually assumed a beautiful curve,
the middle part pointing down the glacier, and showing a velocity there,
double or treble that of the lateral parts.[288] He ascertained that the
rate of advance by night was nearly the same as by day, and that even
the hourly march of the icy stream could be detected, although the
progress might not amount to more than six or seven inches in twelve
hours. By the incessant though invisible advance of the marks placed on
the ice, "time," says Mr. Forbes, "was marked out as by a shadow on a
dial, and the unequivocal evidence which I obtained, that even while
walking on a glacier we are, day by day, and hour by hour, imperceptibly
carried on by the resistless flow of the icy stream, filled me with
admiration." (Travels in the Alps, p. 133.) In order to explain this
remarkable regularity of motion, and its obedience to laws so strictly
analogous to those of fluids, the same writer proposed the theory that
the ice, instead of being solid and compact, is a viscous or plastic
body, capable of yielding to great pressure, and the more so in
proportion as its temperature is higher, and as it approaches more
nearly to the melting point. He endeavors to show that this hypothesis
will account for many complicated phenomena, especially for a ribboned
or veined structure which is everywhere observable in the ice, and might
be produced by lines of discontinuity, arising from the different rates
at which the various portions of the semi-rigid glacier advance and pass
each other. Many examples are adduced to prove that a glacier can model
itself to the form of the ground over which it is forced, exactly as
would happen if it possessed a certain ductility, and this power of
yielding under intense pressure, is shown not to be irreconcilable with
the idea of the ice being sufficiently compact to break into fragments,
when the strain upon its parts is excessive; as where the glacier turns
a sharp angle, or descends upon a rapid or convex slope. The increased
velocity in summer is attributed partly to the greater plasticity of the
ice, when not exposed to intense cold, and partly to the hydrostatic
pressure of the water in the capillary tubes, which imbibe more of this
liquid in the hot season.
On the assumption of the ice being a rigid mass, Mr. Hopkins attributed
the more rapid motions in the centre to the unequal rate at which the
broad stripes of ice, intervening between longitudinal fissures,
advance; but besides that there are parts of the glacier where no such
fissures exist, such a mode of progression, says Mr. Forbes, would cause
the borders of large transverse rents or "crevasses," to be jagged like
a saw, instead of being perfectly even and straight-edged.[289] An
experiment recently made by Mr. Christie, secretary to the Royal
Society, appears to demonstrate that ice, under great pressure,
possesses a sufficient degree of moulding and self-adapting power to
allow it to be acted upon, as if it were a pasty substance. A hollow
shell of iron an inch and a half thick, the interior being ten inches in
diameter, was filled with water, in the course of a severe winter, and
exposed to the frost, with the fuze-hole uppermost. A portion of the
water expanded in freezing, so as to protrude a cylinder of ice from the
fuze-hole; and this cylinder continued to grow inch by inch in
proportion as the central nucleus of water froze. As we cannot doubt
that an outer shell of ice is first formed, and then another within, the
continued rise of the column through the fuze-hole must proceed from the
squeezing of successive shells of ice concentrically formed, through the
narrow orifice; and yet the protruded cylinder consisted of entire, and
not fragmentary ice.[290]
The agency of glaciers in producing permanent geological changes
consists partly in their power of transporting gravel, sand, and huge
stones to great distances, and partly in the smoothing, polishing, and
scoring of their rocky channels, and the boundary walls of the valleys
through which they pass. At the foot of every steep cliff or precipice
in high Alpine regions, a talus is seen of rocky fragments detached by
the alternate action of frost and thaw. If these loose masses, instead
of accumulating on a stationary base, happen to fall upon a glacier,
they will move along with it, and, in place of a single heap, they will
form in the course of years a long stream of blocks. If a glacier be 20
miles long, and its annual progression about 500 feet, it will require
about two centuries for a block thus lodged upon its surface to travel
down from the higher to the lower regions, or to the extremity of the
icy mass. This terminal point remains usually unchanged from year to
year, although every part of the ice is in motion, because the
liquefaction by heat is just sufficient to balance the onward movement
of the glacier, which may be compared to an endless file of soldiers,
pouring into a breach, and shot down as fast as they advance.
The stones carried along on the ice are called in Switzerland the
"moraines" of the glacier. There is always one line of blocks on each
side or edge of the icy stream, and often several in the middle, where
they are arranged in long ridges or mounds, often several yards high.
(See fig. 18, p. 223.) The cause of these "medial moraines" was first
explained by Agassiz, who referred them to the confluence of tributary
glaciers.[291] Upon the union of two streams of ice, the right lateral
moraine of one of the streams comes in contact with the left lateral
moraine of the other, and they afterwards move on together, in the
centre, if the confluent glaciers are equal in size, or nearer to one
side if unequal.
All sand and fragments of soft stone which fall through fissures and
reach the bottom of the glaciers, or which are interposed between the
glacier and the steep sides of the valley, are pushed along, and ground
down into mud, while the larger and harder fragments have their angles
worn off. At the same time the fundamental and boundary rocks are
smoothed and polished, and often scored with parallel furrows, or with
lines and scratches produced by hard minerals, such as crystals of
quartz, which act like the diamond upon glass.[292] This effect is
perfectly different from that caused by the action of water, or a muddy
torrent forcing along heavy fragments; for when stones are fixed firmly
in the ice, and pushed along by it under great pressure, in straight
lines, they scoop out long rectilinear furrows or grooves parallel to
each other.[293] The discovery of such markings at various heights far
above the surface of the existing glaciers and for miles beyond their
present terminations, affords geological evidence of the former
extension of the ice beyond its present limits in Switzerland and other
countries.
The moraine of the glacier, observes Charpentier, is entirely devoid of
stratification, for there has been no sorting of the materials, as in
the case of sand, mud, and pebbles, when deposited by running water. The
ice transports indifferently, and to the same spots, the heaviest blocks
and the finest particles, mingling all together, and leaving them in one
confused and promiscuous heap wherever it melts.[294]
_Icebergs._--In countries situated in high northern latitudes, like
Spitzbergen, between 70 degrees and 80 degrees N., glaciers, loaded with
mud and rock, descend to the sea, and there huge fragments of them float
off and become icebergs. Scoresby counted 500 of these bergs drifting
along in latitudes 69 degrees and 70 degrees N., which rose above the
surface from the height of 100 to 200 feet, and measured from a few
yards to a mile in circumference.[295] Many of them were loaded with
beds of earth and rock of such thickness, that the weight was
conjectured to be from 50,000 to 100,000 tons. Specimens of the rocks
were obtained, and among them were granite, gneiss, mica-schist,
clay-slate, granular felspar, and greenstone. Such bergs must be of
great magnitude; because the mass of ice below the level of the water is
about eight times greater than that above. Wherever they are dissolved,
it is evident that the "moraine" will fall to the bottom of the sea. In
this manner may submarine valleys, mountains, and platforms become
strewed over with gravel, sand, mud, and scattered blocks of foreign
rock, of a nature perfectly dissimilar from all in the vicinity, and
which may have been transported across unfathomable abysses. If the
bergs happen to melt in still water, so that the earthy and stony
materials may fall tranquilly to the bottom, the deposit will probably
be unstratified, like the terminal moraine of a glacier; but whenever
the materials are under the influence of a current of water as they
fall, they will be sorted and arranged according to their relative
weight and size, and therefore more or less perfectly stratified.
In a former chapter it was stated that some ice islands have been known
to drift from Baffin's Bay to the Azores, and from the South Pole to the
immediate neighborhood of the Cape of Good Hope, so that the area over
which the effects of moving ice may be experienced, comprehends a large
portion of the globe.
We learn from Von Buch that the most southern point on the continent of
Europe at which a glacier comes down to the sea is in Norway, in lat.
67 degrees N.[296] But Mr. Darwin has shown, that they extend to the
sea, in South America, in latitudes more than 20 degrees nearer the
equator than in Europe; as, for example, in Chili, where, in the Gulf of
Penas, lat. 46 degrees 40 minutes S., or the latitude of central France;
and in Sir George Eyre's Sound, in the latitude of Paris, they give
origin to icebergs, which were seen in 1834 carrying angular pieces of
granite, and stranding them in fiords, where the shores were composed of
clay-slate.[297] A large proportion, however, of the ice-islands seen
floating both in the northern and southern hemispheres, are probably not
generated by glaciers, but rather by the accumulation of coast ice. When
the sea freezes at the base of a lofty precipice, the sheet of ice is
prevented from adhering to the land by the rise and fall of the tide.
Nevertheless, it often continues on the shore at the foot of the cliff,
and receives accessions of drift snow blown from the land. Under the
weight of this snow the ice sinks slowly if the water be deep, and the
snow is gradually converted into ice by partial liquefaction and
re-congelation. In this manner, islands of ice of great thickness and
many leagues in length, originate, and are eventually blown out to sea
by off-shore winds. In their interior are inclosed many fragments of
stone which had fallen upon them from overhanging cliffs during their
formation. Such floating icebergs are commonly flat-topped, but their
lower portions are liable to melt in latitudes where the ocean at a
moderate depth is usually warmer than the surface water and the air.
Hence their centre of gravity changes continually, and they turn over
and assume very irregular shapes.
In a voyage of discovery made in the antarctic regions in 1839, a
dark-colored angular mass of rock was seen imbedded in an iceberg,
drifting along in mid-ocean in lat. 61 degrees S. That part of the rock
which was visible was about 12 feet in height, and from 5 to 6 in width,
but the dark color of the surrounding ice indicated that much more of
the stone was concealed. A sketch made by Mr. Macnab, when the vessel
was within a quarter of a mile of it, is now published.[298] This
iceberg, one of many observed at sea on the same day, was between 250
and 300 feet high, and was no less than 1400 miles from any certainly
known land. It is exceedingly improbable, says Mr. Darwin, in his notice
of this phenomenon, that any land will hereafter be discovered within
100 miles of the spot, and it must be remembered that the erratic was
still firmly fixed in the ice, and may have sailed for many a league
farther before it dropped to the bottom.[299]
Captain Sir James Ross, in his antarctic voyage in 1841, 42, and 43, saw
multitudes of icebergs transporting stones and rocks of various sizes,
with frozen mud, in high southern latitudes. His companion, Dr. J.
Hooker, informs me that he came to the conclusion that most of the
southern icebergs have stones in them, although they are usually
concealed from view by the quantity of snow which falls upon them.
In the account given by Messrs. Dease and Simpson, of their recent
arctic discoveries, we learn that in lat. 71 degrees N., long. 156
degrees W., they found "a long low spit, named Point Barrow, composed of
gravel and coarse sand, in some parts more than a quarter of a mile
broad, which the pressure of the ice had forced up into numerous mounds,
that, viewed from a distance, assumed the appearance of huge boulder
rocks."[300]
This fact is important, as showing how masses of drift ice, when
stranding on submarine banks, may exert a lateral pressure capable of
bending and dislocating any yielding strata of gravel, sand, or mud. The
banks on which icebergs occasionally run aground between Baffin's Bay
and Newfoundland, are many hundred feet under water, and the force with
which they are struck will depend not so much on the velocity as the
momentum of the floating ice-islands. The same berg is often carried
away by a change of wind, and then driven back again upon the same bank,
or it is made to rise and fall by the waves of the ocean, so that it may
alternately strike the bottom with its whole weight, and then be lifted
up again until it has deranged the superficial beds over a wide area. In
this manner the geologist may account, perhaps, for the circumstance
that in Scandinavia, Scotland, and other countries where erratics are
met with, the beds of sand, loam, and gravel are often vertical, bent,
and contorted into the most complicated folds, while the underlying
strata, although composed of equally pliant materials, are horizontal.
But some of these curvatures of loose strata may also have been due to
repeated alternations of layers of gravel and sand, ice and snow, the
melting of the latter having caused the intercalated beds of
indestructible matter to assume their present anomalous position.
There can be little doubt that icebergs must often break off the peaks
and projecting points of submarine mountains, and must grate upon and
polish their surface, furrowing or scratching them in precisely the same
way as we have seen that glaciers act on the solid rocks over which they
are propelled.[301]
To conclude: it appears that large stones, mud, and gravel are carried
down by the ice of rivers, estuaries, and glaciers, into the sea, where
the tides and currents of the ocean, aided by the wind, cause them to
drift for hundreds of miles from the place of their origin. Although it
will belong more properly to the seventh and eighth chapters to treat of
the transportation of solid matter by the movements of the ocean, I
shall add here what I have farther to say on this subject in connection
with ice.
The saline matter which sea-water holds in solution, prevents its
congelation, except where the most intense cold prevails. But the
drifting of the snow from the land often renders the surface-water
brackish near the coast, so that a sheet of ice is readily formed there,
and by this means a large quantity of gravel is frequently conveyed from
place to place, and heavy boulders also, when the coast-ice is packed
into dense masses. Both the large and small stones thus conveyed usually
travel in one direction like shingle-beaches, and this was observed to
take place on the coast of Labrador and Gulf of St. Lawrence, between
the latitudes 50 degrees and 60 degrees N., by Capt. Bayfield, during
his late survey. The line of coast alluded to is strewed over for a
distance of 700 miles with ice-borne boulders, often 6 feet in diameter,
which are for the most part on their way from north to south, or in the
direction of the prevailing current. Some points on this coast have been
observed to be occasionally deserted, and then again at another season
thickly bestrewed with erratics.
[Illustration: Fig. 19.
Boulders, chiefly of granite, stranded by ice on the coast of Labrador,
between lat. 50 degrees and 60 degrees N. (Lieut. Bowen, R. N.)]
The accompanying drawing (fig. 19), for which I am indebted to Lieut.
Bowen, R. N., represents the ordinary appearance of the Labrador coast,
between the latitudes of 50 degrees and 60 degrees N. Countless blocks,
chiefly granitic, and of various sizes, are seen lying between high and
low-water mark. Capt. Bayfield saw similar masses carried by ice through
the Straits of Belle Isle, between Newfoundland and the American
continent, which he conceives may have travelled in the course of years
from Baffin's Bay, a distance which may be compared in our hemisphere to
the drifting of erratics from Lapland and Iceland as far south as
Germany, France, and England.
It may be asked in what manner have these blocks been originally
detached? We may answer that some have fallen from precipitous cliffs,
others have been lifted up from the bottom of the sea, adhering by their
tops to the ice, while others have been brought down by rivers and
glaciers.
The erratics of North America are sometimes angular, but most of them
have been rounded either by friction or decomposition. The granite of
Canada, as before remarked (p. 221 ), has a tendency to concentric
exfoliation, and scales off in spheroidal coats when exposed to the
spray of the sea during severe frosts. The range of the thermometer in
that country usually exceeds, in the course of the year, 100 degrees, and
sometimes 120 degrees F.; and, to prevent the granite used in the
buildings of Quebec from peeling off in winter, it is necessary to oil
and paint the squared stones.
In parts of the Baltic, such as the Gulf of Bothnia, where the quantity
of salt in the water amounts in general to one fourth only of that in
the ocean, the entire surface freezes over in winter to the depth of 5
or 6 feet. Stones are thus frozen in, and afterwards lifted up about 3
feet perpendicularly on the melting of the snow in summer, and then
carried by floating ice-islands to great distances. Professor Von Baer
states, in a communication on this subject to the Academy of St.
Petersburg, that a block of granite, weighing a million of pounds, was
carried by ice during the winter of 1837-8 from Finland to the island of
Hockland, and two other huge blocks were transported about the years
1806 and 1814 by packed ice on the south coast of Finland, according to
the testimony of the pilots and inhabitants, one block having travelled
about a quarter of a mile, and lying about 18 feet above the level of
the sea.[302]
More recently Dr. Forchhammer has shown that in the Sound, the Great
Belt, and other places near the entrance of the Baltic, ground-ice forms
plentifully at the bottom and then rises to the surface, charged with
sand and gravel, stones and sea-weed. Sheets of ice, also, with included
boulders, are driven up on the coast during storms, and "packed" to a
height of 50 feet. To the motion of such masses, but still more to that
of the ground-ice, the Danish professor attributes the striation of
rocky surfaces, forming the shores and bed of the sea, and he relates a
striking fact to prove that large quantities of rocky fragments are
annually carried by ice out of the Baltic. "In the year 1807," he says,
"at the time of the bombardment of the Danish fleet, an English
sloop-of-war, riding at anchor in the roads at Copenhagen, blew up. In
1844, or thirty-seven years afterwards, one of our divers, known to be
a trustworthy man, went down to save whatever might yet remain in the
shipwrecked vessel. He found the space between decks entire, but covered
with blocks from 6 to 8 cubic feet in size, and some of them heaped one
upon the other. He also affirmed, that all the sunk ships which he had
visited in the Sound, were in like manner strewed over with blocks."
Dr. Forchhammer also informs us, that during an intense frost in
February, 1844, the Sound was suddenly frozen over, and sheets of ice,
driven by a storm, were heaped up at the bottom of the Bay of Taarbeijk,
threatening to destroy a fishing-village on the shore. The whole was
soon frozen together into one mass, and forced up on the beach, forming
a mound more than 16 feet high, which threw down the walls of several
buildings. "When I visited the spot next day, I saw ridges of ice, sand,
and pebbles, not only on the shore, but extending far out into the
bottom of the sea, showing how greatly its bed had been changed, and how
easily, where it is composed of rock, it may be furrowed and streaked by
stones firmly fixed in the moving ice."[303]