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THE monuments of the
glaciation of the regions about Bering Sea and the northern shores of
Siberia and Alaska are in general much broken and obscured on account of the
intensity of the action of the agents of destruction in these low, moist
regions, together with the perishable character of the rocks of which most
of the monuments consist. Lofty headlands, once covered with clear glacial
inscriptions, have been undermined and cast down in loose, draggled taluses,
while others, in a dim, ruinous condition, with most of their surface
records effaced, are rapidly giving way to the weather. The moraines, also,
and the grooved, scratched, and polished surfaces are much blurred and
wasted, while glaciated areas of great extent are not open to observation at
all, being covered by the shallow waters of Bering Sea and the Arctic Ocean,
and buried beneath sediments and coarse detritus which has been weathered
from the higher grounds, or deposited by the ice itself when it was being
melted and withdrawn towards the close of the main glacial period. But amid
this general waste and obscurity a few legible fragments, favorably situated
here and there, have escaped destruction - patches of polished and striated
surfaces in a fair state of preservation, with moraines of local glaciers
that have not been exposed to the heavier forms of water or avalanche
action. And had these fading vestiges perished altogether, yet would not the
observer be left without a sure guide, for there are other monuments of ice
action in all glaciated regions that are almost indestructible, enduring for
tens of thousands of years after those simpler traces that we have been
considering have vanished. These are the material of moraines, though
scattered, washed, crumbled, and re-formed over and over again; and the
sculpture and configuration of the landscape in general, cafions, valleys,
mountains, ridges, roches moutonnées with forms and correlations
specifically glacial. These, also, it is true, suffer incessant waste, being
constantly written upon by other agents; yet, because the glacial characters
are formed on so colossal a scale of magnitude, they continue to stand out
free and clear through every after inscription whether of the torrent, the
avalanche, or universal eroding atmosphere; opening grand and comprehensive
views of the vanished ice, and the geographical and topographical changes
effected by its action in the form of local and distinct glaciers.
River-like, they flowed from the mountains to the sea, and, as a broad,
undulating mantle, crawled over all the landscape through unnumbered
centuries; crushed and ground and spread soil-beds; fashioned the features
of mountain and plain; extended the domain of the sea; separated continents;
dotted new coasts with islands, fringed them with deep inreaching fords, and
impressed their peculiar style of sculpture on all the regions over which
they passed.
A general exploration of the
mountain ranges of the Pacific Coast shows that there are about sixty- five
small residual glaciers on the Sierra Nevada of California, between latitude
36° 30' and 39°, distributed singly or in small groups on the north sides of
the highest peaks at an elevation of about eleven to twelve thousand feet
above the level of the sea, representatives of the grand glaciers that once
covered all the range. More than two thirds of these lie between latitude
37° and 38º, and form the highest sources of the San Joaquin, Tuolumne,
Merced, and Owens Rivers.
Mount Shasta, near the
northern boundary of California, has a few shrinking glacier remnants, the
largest about three miles in length. We find that, to the north of
California, groups of active glaciers still exist on all the highest
mountains - Mounts Jefferson, Adams, Saint Helens, Hood, Rainier, Baker, and
others. Of these Mount Rainier is the highest and iciest. Its summit is
fairly capped with ice, and eight glaciers, from seven to fifteen miles
long, radiate from it as a center and form the sources of the principal
streams. The lowest descends to about thirty-five hundred feet above sea
level, pouring a stream opaque with glacial mud into the head of Puget
Sound.
On through British Columbia
and southeastern Alaska the broad sustained mountain chain extending along
the coast is generally glacier-bearing. The upper branches of nearly every
one of the main canons are occupied by glaciers, which gradually increase in
size and descend lower until the lofty region between Mount Fairweather and
Mount St. Elias is reached, where a considerable number discharge into the
waters of the ocean.
This is the region of
greatest glacial abundance on the continent. To the northward from here the
glaciers gradually diminish in size and depth and melt at higher levels
until the latitude of about 62° is reached, beyond which few, if any,
glaciers remain in existence, the ground being comparatively low and the
annual snowfall light.
Between latitude 56° and 60º
there are probably more than five thousand glaciers, great and small,
hundreds of the largest size, descending through the forests nearly to the
level of the sea, though, as far as I know after a pretty thorough
exploration of the region, not more than twenty-five discharge into the sea.
All the long, high-walled
fords into which these great glaciers of the first class flow are of course
crowded with icebergs of every conceivable form, which are detached at
intervals of a few minutes. But these are small as compared with those of
Greenland, and only a few escape from the intricate labyrinth of channels,
with which this portion of the coast is fringed, into the open sea. Nearly
all of them are washed and drifted back and forth in the fords by wind and
tide until finally melted by sunshine and the copious warm rains of summer.
The southmost of the glaciers
that reach the sea occupies a narrow fiord about twenty miles to the
northwest of the mouth of the Stikine River, in latitude 56° 50'. It is
called "Hutli" [Now known as Le Conte Glacier; also the Bay into which it
discharges. Both were so named in 1887 by Lieutenant-Commander Charles M.
Thomas, U.S.N., presumably in honor of Joseph Le Conte, the well- known
California geologist. "Huth" is the Tlingit Indian name for the mythical
bird which produces thunder by the flapping of its wings. The word,
therefore, means "The Thunderer."] by the natives, from the noise made by
the icebergs in rising and falling from the inflowing glacier. About one
degree farther north there are four of these complete glaciers at the heads
of branches of Holkham Bay, at the head of Taku Inlet one, and at the head
and around the sides of a bay trending in a general northerly direction from
Cross Sound, first explored by myself in 1879, there are no less than five
of these complete glaciers reaching tide-water, the largest of which, the
Muir, is of colossal size, having upwards of two hundred tributaries and a
width of trunk below the confluence of the main tributaries of three to
twenty-five miles. Between the west side of this icy bay and the ocean all
the ground, high and low, with the exception of the summits of the mountain
peaks, is covered by a mantle of ice from one to three thousand feet thick,
which discharges to the eastward and westward through many distinct mouths.
This ice-sheet, together with
the multitude of distinct glaciers that load the lofty mountains of the
coast, evidently once formed part of one grand, continuous ice-sheet that
flowed over all the region hereabouts, extending southward as far as the
Straits of Juan de Fuca, for all the islands of the Alexander Archipelago,
great and small, as well as the headlands and promontories of the mainland,
are seen to have forms of greatest strength with reference to the action of
a vast press of oversweeping ice, and their surfaces have a smooth, rounded,
over-rubbed appearance, generally free from angles. The canals, channels,
straits, passages, sounds, etc., between the islands —a marvelous labyrinth
- manifest in their forms and trends and general characteristics the same
subordination to the grinding action of a continuous ice-sheet, and they
differ from the islands, as to their origin, only in being portions of the
general pre-glacial margin of the continent, more deeply eroded, and,
therefore, covered with the ocean waters which flowed into them as the ice
was melted out of them.
That the dominion of the sea
is being extended over the land by the wearing away of its shores is well
known. But in these northern regions the coast rocks have been so short a
time exposed to wave- action that they are but little wasted as yet, the
extension of the sea affected by its own action in post-glacial time in this
region being probably less than the millionth part of that affected by
glacial action during the last glacial period.
Traces of the ancient
glaciers made during the period of greater extension abound on the
California Sierra a.s far south as latitude 360. Even the most evanescent of
them, the polished surfaces, are still found, in a marvelously perfect state
of preservation, on the upper half of the middle portion of the range. They
occur in irregular patches, some of which are several acres in extent, and,
though they have been subjected to the weather with all its storms for
thousands of years, their mechanical excellence is such that they reflect
the sunbeams like glass, and attract the attention of every observer.
The most perfect of these
shining pavements lie at an elevation of about seven to eight thousand feet
above the level of the sea, where the rock is close-grained, siliceous
granite. Small fading patches may be found at from three to five thousand
feet elevation on the driest and most enduring portions of vertical walls,
where there is protection from the drip and friction of water; also, on
compact swelling bosses partially protected by a covering of boulders.
On the north half of the
Sierra the striated and polished surfaces are rarely found, not only because
this portion of the chain is lower, but on account of the surface rocks
being chiefly porous lavas subject to rapid waste. The moraines, also,
though well preserved on the south half of the range, seem to be nearly
wanting over a considerable portion of the north half, but the material of
which they were composed is found in abundance, scattered and disintegrated,
until its glacial origin is not obvious to the unskilled observer.
A similar blurred condition
of the superficial records obtains throughout most of Oregon, Washington,
British Columbia, and Alaska, due in great part to the action of excessive
moisture. Even in southeastern Alaska, where the most extensive glaciers
still exist, the more evanescent of the traces of their former greater
extension, though comparatively recent, are more obscure than those of the
ancient glaciers of California, where the climate is drier and the rocks
more resisting. We are prepared, therefore, to find the finer lines of the
glacial record dim or obliterated altogether in the Arctic regions, where
the ground is mostly low and the action of frost and moisture specially
destructive.
The Aleutian chain of islands
sweeps westward in a regular curve, about a thousand miles long, from the
Alaska Peninsula toward Kamchatka, nearly uniting the American and Asiatic
continents. A very short geological time ago, just before the coming on of
the glacial winter, the union of the two continents was probably complete.
The entire chain appears to be simply a degraded portion of the North
Pacific pre-glacial coast mountains, with its foothills and lowest portions
of the connecting ridges between the peaks a few feet under water, the
submerged ridges forming the passes between the islands as they exist
to-day, while the broad plain to the north of the chain is now covered by
the shallow waters of Bering Sea.
Now the evidence seems
everywhere complete that this segregating degradation has been effected
almost wholly by glacial action. Yet, strange to say, it is held by most
observers who have made brief visits to different portions of the chain that
each island is a distinct volcanic upheaval, but little changed since the
period of emergence from the sea, an impression made no doubt by the
volcanic character of most of the rocks, ancient and recent, of which they
are composed, and by the many extinct, or feebly active volcanoes occurring
here and there along the summits of the highest masses. But, on the
contrary, all the evidence we have seen goes to show that the amount of
glacial denudation these rocks have undergone is very great, so great that,
with the exception of the recent craters, almost every existing feature is
distinctly glacial. The comparatively featureless pre-glacial rocks have
been heavily sculptured and fashioned into the endless Variety they now
present of peak and ridge, valley and fiord and clustering islets,
harmoniously correlated in accordance with glacial law.
On Mount Makushin, [Muir
probably adopted current estimates of the altitude of this volcano.
Gannett's Altitudes in Alaska (1900) gives the elevation as 5474 feet; and
the United States Coast and Geodetic Survey Map, No. 8860 (1916) as 5691
feet.] whose summit reaches an elevation of about nine thousand feet above
the sea, several small glaciers still exist, while others yet smaller may be
hidden in the basins of other mountains not yet explored. The summit of
Makushin, at the time my observations were made, was capped with heavy
clouds, and from beneath these the glaciers were seen descending imposingly
into the open sunshine to within a thousand or fifteen hundred feet of the
sea level, the largest perhaps about six miles in length. After the clouds
cleared away the summit was seen to be heavily capped with ice, leaving only
the crumbling edges of the dividing ridges and subordinate peaks free. The
lower slopes of the mountain and the wide valleys proceeding from the
glaciers present testimony of every kind to show that these glaciers now
lingering on the summit once flowed directly into the sea. The adjacent
mountains, though now mostly free from ice, are covered with glacial
markings, extending over all the low grounds about their bases and the
shores of the fords, and over many of the rocks now under water. But besides
this evidence of recent local glacial abundance, we find traces of far
grander glacial conditions on the heavily abraded rocks along the shores of
the passes separating the islands, and also in the low wide valleys
extending in a direction parallel with the passes across the islands,
indicating the movement of a vast ice-sheet from the north over the ground
now covered by Bering Sea.
The amount of degradation
this island region has undergone is only partially manifested by the
crumbling, sharpened condition of the ridges and peaks, the abraded surfaces
that have been overswept, and by the extent of the valleys and fords, and
the gaps between the mountains and islands.
That these valleys, fords,
forges, and gaps, great and small, like those of the Sierra, are not a
result of local subsidences and upheavals, but of the removal of the
material that once filled them, is shown by the broken condition and the
similarity of the physical structure and composition of their contiguous
sides, just as the correspondence between the tiers of masonry on either
side of a broken gap in a wall shows that the missing blocks required to
fill it up have been removed.
The chief agents of erosion
and transportation are water and ice, each being regarded as the more
influential by different observers, though the phenomena to which they give
rise are widely different. All geologists recognize the fact that glaciers
wear away the rocks over which they move, but great vagueness prevails as to
the size of the fragments of erosion, and the way they are detached and
removed; and if possible still greater vagueness prevails as to the forms
and characteristics in general of the mountains, hills, rocks, valleys,
etc., resulting from this erosion.
Towards the end of summer,
when the snow is melted from the lower portions of the glaciers, particles
of dust and sand may be seen scattered over their surfaces, together with
angular masses of rocks, derived from the shattered storm-beaten cliffs
above their fountains. The separation of these masses, which vary greatly in
size, is due only in part to the action of the glacier, though they are all
transported on its surface like floating drift on a river, and deposited
together in moraines. The winds supply a portion of the sand and dust, some
of the larger fragments are set free by the action of frost, rains, and
general weathering agents, considerable quantities are swept down in
avalanches of snow where the inclination of the slopes is favorable to their
action, and shaken down by earthquake shocks, while the glacier itself plays
an important part in the production of these superficial effects by
undermining the cliffs from whence the fragments fall.
But in all moraines boulders
and small dust particles may be recognized that have not been thus derived
from the weathered cliffs and dividing ridges projecting above the glaciers,
but from the rocks past which and over which the glaciers flow. The streams
which drain glaciers are always turbid with finely ground mud particles worn
off the bed-rocks by a sliding motion, accompanied by great pressure, giving
rise to polished surfaces, and keeping up a waste that never for a moment
ceases while the glacier exists; and besides these small particles boulders
are found that may be traced to their origin in the bottoms or sides of the
channels. Accordingly, an abrupt transition is discovered from the polished
and plain portions of the channels to the more or less angular and fractured
portions, showing that glaciers degrade the rocks over which they pass in at
least two different ways, by grinding them into mud, and by crushing,
breaking, and splitting them into a coarse detritus of chips and boulders,
the forms and sizes of which are in great part determined by the divisional
planes the rocks possess, and the intensity and direction of application of
the force brought to bear on them. The quantity of this coarser material
remaining in the channels along the lines of dispersal, and the probable
rate of movement of the glaciers that quarried and transported it, form data
from which some approximation to the rate of this method of degradation may
be reached.
The amount of influence
exerted on the Aleutian region by running water in its various forms, and by
the winds, avalanches, and the atmosphere in degrading and fashioning the
surface subsequent to the melting of the ice, is as yet scarcely more
appreciable than it is in the upper middle portion of the Sierra; for,
besides being much feebler in their action, the time during which the region
has been exposed to their influence is comparatively short.
On the other hand, the
quantity of material quarried and carried away by the force of the ice, in
process of bringing the region into its present condition, can hardly be
overestimated; for, with the exception of the recent volcanic cones, almost
every noticeable feature, great and small, has evidently been ground down
into the form of greatest strength in relation to the stress of oversweeping
floods of ice. And that these present features are not the pre-glacial
features merely smoothed and polished and otherwise superficially altered,
but an entirely new set sculptured from a surface comparatively featureless,
is manifested by the relationship existing between the spaces that separate
them and the glacier fountains. The greater the valley or hollow of any
sort, the greater the snow-collecting basin above it whence flowed the ice
that created it, not a fiord or valley being found here or on any portion of
the Pacific Coast that does not conduct to fountains of vanished or residual
glaciers corresponding with it in size and position as cause and effect.
And, furthermore, that the
courses of the present valleys were not determined by the streams of water
now occupying them, nor by pre-glacial streams, but by the glaciers of the
last or of some former glacial period, is shown by the fact that the
directions of the trends of all these valleys, however variable, are
resultants of the forces of the main trunk glaciers that filled them and
their inflowing tributary glaciers, the wriggling fortuitous trends of
valleys formed by the action of water being essentially different from those
formed by ice; and therefore not liable to be confounded with them. Neither
can we suppose pre-existing fissures or local subsidences to have exercised
any primary determining influence, there being no conceivable coincidence
between the trends of fissures and subsidences and the specific trends of
ice-created valleys and basins in general, nor between the position and
direction of extension of these hypothetical fissures, subsidences, and
foldings and the positions of ice- fountains.
The Pribilof Islands, St.
Paul, St. George, Walrus, and Otter, appear in general views from the sea as
mere storm-beaten remnants of a once continuous land, wasted into bluffs
around their shores by the action of the waves, all their upper surfaces
being planed down by a heavy oversweeping ice-sheet, slightly roughened here
and there with low ridges and hillocks that alternate with shallow valleys.
None of their features, as far as I could discover without opportunity for
close observation, showed any trace of local glaciation or of volcanic
action subsequent to the period of universal glaciation.
St. Lawrence Island, the
largest in Bering Sea, is situated at a distance of about one hundred and
twenty miles off the mouths of the Yukon, and forty miles from the nearest
point on the coast of Siberia. It is about a hundred miles long from east to
west, fifteen miles in average width, and is chiefly composed of various
kinds of granite, slate, and lava.
The highest portion along the
middle is diversified with groups of volcanic cones, some of which are of
considerable size and clearly post-glacial in age, presenting well-defined
craters and regular slopes down to the base, though I saw no evidence of
their having poured forth extensive streams of molten lava over the adjacent
rocks since the close of the glacial period; for, with the exception of the
ground occupied by the cones, all the surface is marked with glacial
inscriptions of the most telling kind - moraines, erratic boulders, roches
nwutonnées, in great abundance and variety as to size, and alternating
ridges and valleys with wide U-shaped cross- sections, and with nearly
parallel trends across the island in a general north to south direction,
some of them extending from shore to shore, and all showing subordination to
the grinding, furrowing action of a broad over-sweeping ice-sheet.
Some of the widest gap-like
valleys have been eroded nearly to the level of the sea, indicating that if
the ice action had gone on much longer the present single island would have
been eroded into a group of small ones; or the entire mass of the island
would have been degraded beneath the sea level, obliterating it from the
landscape to be in part restored perhaps by the antagonistic elevating
volcanic action. The action of local glaciers has been comparatively light
hereabouts, not enough greatly to obscure or interrupt the overmastering
effects of the ice-sheet, though they have given marked character to the
sculpture of some of the higher portions of the island.
The two Diomede Islands and
Fairway Rock are mostly residual masses of granite brought into relief and
separated from one another and from the general mass of the continent, by
the action of ice in removing the missing material, while the islands remain
because of superior resistance offered to the universal degrading force.
That they are remnants of a once continuous land now separated by Bering
Strait is indicated by the relative condition of the sides of the islands
and of the contiguous shoulders of the continents, East Cape and Cape Prince
of Wales, while the general configuration of the islands shows that they
have been subjected to a glaciation of the most comprehensive kind, leaving
them as roches moutonnées on a grand scale.
I discovered traces of local
glaciation on the largest of the three, but the effects produced by this
cause are comparatively slight, while the action of excessive moisture in
the form of almost constant fogs and rains throughout the summer months,
combined with frost and thaw, has effected a considerable amount of
denudation, manifested by groups of crumbling pinnacles occurring here and
there on the summits.
Sledge, King, and Herald
Islands are evidently of similar origin, displaying the same glacial traces,
and varying chiefly in the amount of post-glacial waste they have suffered,
and in the consequent degree of clearness of the testimony they present.
During our visit to Herald Island an exceptionally favorable opportunity
offered as to the time of year, state of the weather, etc., for observation.
Kellett, who first discovered
this island and landed on it under adverse circumstances, describes it as an
inaccessible rock. The sides are indeed precipitous in the main, but
mountaineers would find many slopes and gullies by which the summit could be
easily attained. We landed on the southwest side, opposite the mouth of a
small valley, the bed of a vanished glacier. A short gully which conducts
from the water's edge to the mouth of the valley proper is very steep, and
at the time of our visit was blocked with compacted snow, in which steps had
to be cut, but beyond this no difficulty was encountered, the ice having
graded a fine broad way to the summit. Thence following the highest ground
nearly to the northwestern extremity, we obtained views of most of the
surface. The highest point is about twelve hundred feet above the sea, about
a mile and a half from the northwest end of the island, and four and a half
miles from the southeast. This makes the island about six miles long, the
average width being about two miles.
Near the middle of the island
there is a low gap, where the width is only about half a mile, and the
height of the summit of this portion of the water-shed between the two sides
is only about two hundred and fifty feet. The entire island as far as seen
is a mass of granite, with the exception of a patch of metamorphic slates
near the middle, which no doubt owes its existence, with so considerable a
height, to the superior resistance it offered to the degrading action of
ice, traces of which are presented in the general moutonnée form of the
island, and in the smooth parallel ridges and valleys trending north and
south. These evidently have not been determined as to size, form, position,
or the direction of their trends by subsidences, upheavals, foldings, or any
structural peculiarity of the rocks in which they have been eroded, but
simply by the mechanical force of an oversweeping ice-sheet.
The effects of local glaciers
are seen in short valleys of considerable depth as compared with the area
from which their fountain snows were derived. We noticed four of these
valleys that had been occupied by residual glaciers; and on the hardest and
most enduring of the upswelling rock bosses several patches of the ancient
scored and polished surface were discovered, still in a good state of
preservation. That these local glaciers have but recently vanished is
indicated by the raw appearance of the surface of their beds, while one
small glacier remnant occupying a sheltered hollow and possessing a well-
characterized terminal moraine seems to be still feebly active in the last
stage of decadence. This small granite island, standing solitary in the
Polar Ocean, we regard as one of the most interesting and significant of the
monuments of geographical change effected by general glaciation.
Our stay on Wrangell Land was
too short to admit of more than a hasty examination of a few square miles of
surface near the eastern extremity. The rock here is a close-grained clay
slate, cleaving freely into thin flakes, with occasional compact metamorphic
masses rising above the general surface or forming cliffs along the shore.
The soil about the banks of a river of considerable size, that enters the
ocean here, has evidently been derived in the main from the underlying
slates, indicating a rapid weathering of the surface. A few small deposits
of moraine material were discovered containing traveled boulders of quartz
and granite, no doubt from the mountains in which the river takes its rise,
while the valley now occupied by the river manifests its glacial origin in
its form and trends, the small portion in the middle eroded by the river
itself being clearly distinguished by its abrupt angular sides, which
contrast sharply with the glacial outlines.
In general views obtained in
sailing along its southern coast the phenomena presented seemed essentially
the same as have been described elsewhere - hills, valleys, and sculptured
peaks, testifying in all their main trends and contours to the action of
ice. A range of mountains of moderate height extends from one extremity of
the island to the other, a distance of about sixty-five miles, the highest
point as measured by Lieutenant Berry being twenty-five hundred feet above
the sea.
All the coast region of
Siberia that came under our observation, from the Gulf of Anadir to North
Cape, presents traces in great abundance and variety of universal as well as
local glaciation. Between Plover and St. Lawrence Bays, where the mountains
attain their greatest elevation and where local glaciation has been
heaviest, the coast is lacerated with deep fords, on the lofty granite walls
of which the glacial records are in many places well preserved, and offer
evidence that could hardly be overlooked by the most careless observer.
Our first general views of
this region were obtained on June 7, when it was yet winter, and the
landscape was covered with snow down to the water's edge. After several days
of storm the clouds lifted, exposing the heavily abraded fronts of
outstanding cliffs; then the smooth overswept ridges and slopes at the base
of the mountains came in sight, and one angular peak after another, until a
continuous range forty to fifty miles long could be seen from one
standpoint. Many of the peaks are fluted with the narrow channels of
avalanches, and hollowed with névé amphitheaters of great beauty of form,
while long withdrawing fords and valleys may be traced back into the
recesses of the highest groups, once the beds of glaciers that flowed in
imposing ranks to the sea.
Plover Bay, which I examined
in detail, may be taken as a good representative of the fords of this
portion of the coast. The walls rise to an average height of about two
thousand feet, and present a severely desolate and bedraggled appearance,
owing to the crumbling condition of the rocks, which in most places are
being rapidly disintegrated, loading the slopes with loose, shifting
detritus whenever the angle is low enough to allow it to come to rest. When
examined closely, however, this loose material is found to be of no great
depth. The solid rock comes to the surface in many places, and on the most
enduring portions rounded glaciated surfaces are still found grooved,
scratched, and polished in small patches from the sea level up to a height
of a thousand feet or more.
Large taluses with their
bases under the water occur on both sides of the fiord in front of the side
canons that partially separate the main mountain masses that form the walls.
These taluses are composed in great part of moraine material, brought down
by avalanches of snow from the terminal moraines of small vanished glaciers
that lay at a height of from one to five thousand feet, in recesses where
the snow accumulated from the surrounding slopes, and where sheltered from
the direct action of the sun the glaciers lingered longest. These recent
moraines are formed of several concentric masses shoved together, showing
that the glaciers to which they belonged melted and receded gradually with
slight fluctuations of level and rate of decadence, in accordance with
conditions of snowfall, temperature, etc., like those of lower latitudes.
When the main central glacier
that filled this fiord was in its prime as a distinct glacier it measured
about thirty miles in length and from five to six miles in width, and was
from two to three thousand feet in depth. It then had at least five main
tributaries, which, as the trunk melted, became independent glaciers; and,
again, as the trunks of these main tributaries melted, their smaller
tributaries, numbering about seventy-five, and from less than a mile to
several miles in length, became separate glaciers and lingered probably for
centuries in the high, cool fountains. These also, as far as we have seen,
have vanished, though possibly some wasting remnant may still exist in the
highest and best- protected recesses about the head of the fiord.
Along the coast, a distance
of fifteen or twenty miles to the eastward and southward of the mouth of
Metchigme Bay, interesting deposits occur of roughly stratified glacial
detritus in the form of sand, gravel, and boulders. They rise from the shore
in raw, wave-washed bluffs about forty feet high and extend to the base of
the mountains as a gently inclined plain, with a width in some places of two
or three miles. Similar morainal deposits were also observed on the American
coast at Golofnin Bay, Kotzebue Sound, Cape Prince of Wales, and elsewhere.
At Cape Prince of Wales the formation rises in successive well-defined
terraces.
The peninsula, the extremity
of which forms East Cape, trends nearly in an easterly direction from the
mainland, and consequently occupies a telling position with reference to ice
moving from the northward. I was therefore eager to examine it to see what
testimony it might have to offer. We landed during favorable weather on the
south side at a small Eskimo village built on a rough moraine, and pushed on
direct to the summit of the watershed, from which good general views of
nearly all the surface of the peninsula were obtained.
The dividing ridge along the
high eastern portion is traversed by a telling series of parallel grooves
and small valleys trending north and south approximately, the curves on the
north commencing nearly at the water's edge, while the south side is more or
less precipitous. The culminating point of the elevated eastern portion of
the peninsula is about twenty-five hundred feet high, and has been cut from
the mainland and added as another island to the Diomede group, the wide gap
of low ground connecting it with the adjacent mountainous portion of the
mainland being only a few feet above tide-water. Out in the midst of this
low, flat region smooth upswelling roches moutonnées were discovered here
and there like groups of small islands, with trends and contours
emphatically glacial, all telling the action of a universal abrading
ice-sheet moving southward.
Hence along the coast to Cape
North, which is the limit of our observations in this direction, the same
class of ice phenomena was discovered - moraine material, washed and
re-formed, moutonnée masses of the harder rocks standing like islands in the
low, mossy tundra, and traveled boulders and pebbles lying stranded on the
summits of rocky headlands.
These enduring monuments are
particularly abundant and significant in the neighborhood of Cape Wankarem,
where the granite is more compact and resisting than is commonly found in
the Arctic regions we have visited, and consequently has longer retained the
more evanescent of the glacial markings. Cape Wankarem is a narrow,
flat-topped, residual mass of this enduring granite, on the summit of which
two patches of the original polished surface were discovered that still
retains the fine stria and many erratic boulders of slate, quartz, and
various kinds of lava, which, from the configuration and geographical
position of the cape with reference to the surrounding region, could not
have been brought to their present resting-places by any local glacier.
Cape Serdzekamen is another
of these residual island masses, brought into relief by general glacial
denudation, manifesting its origin in every feature, and corroborating the
testimony given at Cape Wankarem and elsewhere in the most emphatic manner.
All the sections of the
tundra seen either on the Siberian or Alaskan coast lead towards the
conclusion that the ground is glacial, re-formed under the action of running
water derived in broad, shallow currents from the melting, receding edges of
the ice- sheet, and also in some measure from ice left on the high lands
after the main ice-sheet had been withdrawn; for these low, flat deposits
differ in no particular of form or composition that we have been able to
detect from those still in process of formation in front of the large
receding glaciers of southeastern Alaska. On many of the so-called "mud-
flats" extending from the snouts of glaciers that have receded a few miles
from the shore, mosses and lichens and other kinds of tundra vegetation are
being gradually acquired, and when thus clothed these patches of tundra are
not to be distinguished from the extensive deposits about the shores of the
Arctic regions.
The phenomena observed on the
American coast from St. Michael to Point Barrow differ in no essential
particular from those which have been described on the opposite shores of
Siberia. Moraines more or less wasted, and re-formations of moraine
material, smooth overswept ridges with glacial trends and the corresponding
valleys, roches moutonnées, and the fountain amphitheaters of local glaciers
were observed almost everywhere on the mountainous portions of the coast,
though in general more deeply weathered, owing mainly to the occurrence of
less resisting rocks, limestones, sandstones, porous lavas, etc.
A number of
well-characterized moraines so situated with reference to topographical
conditions as to have escaped destructive washing were noticed near Cape
Lisburne, and moraine deposits of great extent at Kotzebue Sound and
Golofnin Bay, of which many fine sections were exposed. At the latter
locality, judging from the comparatively fresh appearance of the rock
surfaces and deposits around the head of the bay, and the height and extent
of the ice-fountains, the glacier that discharged here was probably the last
to vanish from the American shore of Bering Sea.
As to the thickness attained
by the ice-sheet over the regions that we have been examining during the
period of greatest glacial development, we have seen that it passed heavily
over the islands of Bering Sea and the adjacent mountains on either side,
especially at East Cape and Cape Prince of Wales, at a height of twenty-five
hundred feet or more above the bottom of Bering Sea and Strait, the average
depth of water here being about a hundred and fifty feet. And though the
lowest portion of the land beneath the ice may have been degraded to a
considerable depth subsequent to the time when these highest portions were
left bare, on the other hand the level of the ice must have been
considerably higher than the summits over which it passed, inasmuch as they
give evidence of having been heavily abraded. It appears, therefore, that
the thickness of the general northern ice-sheet throughout a considerable
portion of its history was not less than twenty-five hundred feet, and
probably more, over the northern portion of the region now covered by Bering
Sea and part of the Arctic Ocean.
In view of this colossal
ice-flood grinding on throughout the hundreds of thousands of years of the
glacial period, the excavation of the shallow basins of Bering Sea and
Strait and the Arctic Ocean must be taken as only a small part of the
erosion effected; for so shallow are these waters, were the tallest sequoias
planted on the bottom where soundings have been made, their tops would rise
in most places a hundred feet or more above the surface. The Plover Bay
glacier, as we have shown, eroded the granite in the formation of its
channel to a depth of not less than two thousand feet, and the amount of
erosion effected by the ice-sheet was probably much greater.
It appears, therefore, in
summing up the results of our observations along the North Pacific and
Arctic coasts: -
(1) That the southernmost
glacier lies on the Sierra near latitude 36°; the northernmost, with perhaps
a few exceptions, near 62°.
(2) That the region of
greatest glaciation lies between 56° and 61°, where the mountains are
highest and the snowfall greatest.
(3) That an ice-sheet flowed
from the Arctic regions, from beyond the end of the continent, pursuing a
general southerly direction, and discharged into the Pacific Ocean south of
the Aleutian Islands.
(4) That of this continuous
ice-sheet, extending from the Arctic Ocean beyond the northern extremity of
the continent, the glaciers, great and small, now existing are the remnants.
(5) That the basins of Bering
Sea and Strait and of the adjacent portion of the Arctic Ocean are simply
those portions of the bed of the ice- sheet which were eroded to a moderate
depth beneath the level of the sea, and over which the ocean waters were
gradually extended as the ice-sheet was withdrawn, thus separating the
continents of Asia and America, at the close of the glacial period.
We are now better prepared to
read the changes that have taken place on the Sierra, and fortunately, as we
have already seen, nowhere is the glacial record clearer. |
