Erosion of Sognefjord, Norway - Høgskulen i Sogn og Fjordane

ELSEVIER
Geomorphology, 9 ( 1994 ) 33-45
Erosion of Sognefjord, Norway
Atle Nesje a, Ian M. Whillans b
'~Department ~f Geography, UniversiO' of Bergen, Breiviken 2, N-5035 Bergen-Sandviken, Norwav
bByrd Polar Research Center, The Ohio State University, 1090 Carmaek Road, Columbus, OH 43210, USA
(Received April 29, 1993: accepted November 22, 1993)
Abstract
Sognefjord is formed by a combination of exploitation of rock structure and subaerial and subglacial processes. The fjord
system follows zones of rock-structural weakness. Above sea level the landforms are due mainly to subaerial processes, and
somewhat surprisingly not to glacial activity. Removal of erosive products and deepening of the fjord below sea level is due to
glacial erosion.
1. Introduction
Fjords, some of the most spectacular geomorphic
features on earth, are common along the coasts of Norway, Greenland, Alaska, British Columbia, Chile,
Antarctica, and New Zealand. The question of what are
the dominant processes responsible for fjord formation
is a classic problem in geomorphology. The debate has
revolved around the relative importance of glacial
activity, fractures associated with tectonism, and fluvial
downcutting.
One extreme view is held by Gregory 1913). Fjord
axes are observed to form a rectilinear pattern. This is
evidence that fjords developed along intersecting lines
of fracture. Noting this, Gregory argued that fjords were
largely of rock-structural origin and that fluvial and
glacial action were responsible for only slight modification of the landforms.
Another extreme view is due to Gjessing (1966).
The inner ends of fjords and fjord valleys commonly
end with steps or trough headwalls. Gjessing interpreted these as glacial rock steps, into which the action
of subglacial meltwater and subsequently subaerial
water have cut waterfall notches. In his view, the pres0169-555X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDIO I 6 9 - 5 5 5 X ( 9 3 ) E 0 0 2 2 - 5
ence of glaciers is necessary for carving out fjords.
The surest indication of glacial action is the overdeepening of fjord floors far below sea level and their
rock thresholds. This undoubtedly indicates that glaciers played a major role in fjord formation.
Somewhat of a puzzle is that the fjord walls along
Sognefjord are composed mainly of small V-shaped
valleys. These must have formed by fluvial-downslope
action and rock fall and rock-avalanche activity. Their
presence indicates that such fluvial action has been
dominant, despite the obvious linkage of fjords with
glaciers. However, the volume of debris in the valley
bottoms and on the fjord walls is grossly insufficient to
account for the amount of rock removed from these Vshaped valleys. Thus, the formation of the small Vshaped valleys must largely predate the last glaciation.
The presence of these small valleys leads to a difficult question. How could glaciers, on one hand, cut
deep fjords, and on the other, not destroy the gullies
and deeper V-shaped valleys formed during interstadials and interglacials? To our knowledge, we are the
first to emphasize the existence and significance of
these V-shaped valleys. The purpose of this contribution is to further the discussion on the formation of
34
A. Ne.sje, I.M. Whillans / Geomorphology 9 (1994) 3 3 4 5
fjords by proposing a model of fjord formation that is
consistent with all observations.
2. Physiography of Sognefjord
Sognefjord (Fig. 1) starts in the eastern part at Skjolden (210 km), becomes abruptly deeper westward to
reach a maximum depth at Vadheim of 1308 m below
present sea level (Fig. 2). Between 180 and 50 km it
is relatively flat-bottomed. As noted earlier by H. Holtedahl (1967), nearly all the tributary fjords "hang"
above the main fjord. The fjord bottom then rises to
form a high threshold at 30 km. The fjord walls are
farther apart at the threshold. Water depth at the threshold is 100 to 150 m.
The mountains along the sides of Sognefjord gradually decrease in elevation westward from about 2200
m in the Jotunheimen area to about 500 m in the coastal
~f~
~
region ( Fig. 2). The largest relief is 2850 m (at Bleia).
The average relief is about 2000 m. Between about 45
km and 170 km (Fig. 2) about 50% of the total relief
is below the threshold at the fjord mouth and 60%
below the present sea level.
There are three categories of bedrock in the Sognefjord region (Fig. 3). The inland, eastern part of
Sognefjord penetrates rocks of Caledonian age. In these
rocks the branches of the fjord system show obvious
connections with rock structure (see Fig. 6). Also,
some of the tributary fjords in the inner part are associated with a fold pattern (Aarseth, 1980a). |n the
middle and outer part of Sognefjord the bedrock
belongs to the west-Norwegian Precambrian gneiss
complex with E-W and NE-SW trending structures
(Sigmond et al., 1984), which also affect fjord orientation ( see Fig. 6). The western, Precambrian portion
of the drainage basin (west of Balestrand) is significantly narrower (about 30 km wide) than the eastern
SANDANE
ORDE
EIMEN
N
T
50km
F
Fig 1. Sognefjord drainage basin. The track, marked at 105 km intervals, follows the deepest portion of the fjord. Modern glaciers are drawn
with light stippling.
35
A. Ne,~je, I.M. Whillans I Geomorphology 9 (1994) 3 3 4 5
Jotunheimen
200ot
¢~x,'oo"
1600t
f
if
12001
ss S
s¢# S~'
800t
•
400-
Z
j
22
D
W
a
,<
~
--
<
w
~_
o
1
~_
<
-400-800-
k._~ord
-1200-
0
I
I
20
,
I
40
I
I
60
I
floor
~
0
I
~
10
,
I
120
I
I
140
,
I
160
I
~
180
I
I
200
I
~
I
220km
Fig. 2. Longitudinal profile of Sognefjord following the track shown in Fig. I. Summit levels and hanging tributary fjord bottoms are shown.
Caledonian portion (maximum width about 130 km)
(Fig. 1). In contrast, the fjord width increases, being 2
to 3 km in the eastern part, and 4 to 5 km in the western
portion. The coastal islands at Solund consist of Devonian sandstones and conglomerates.
Probably related to the width of the drainage basin
is the number of tributaries and distributaries. The inner
part has five branches (Fjaerlandsfjord, Sogndalsfjord,
Lustrafjord, ,~rdalsfjord, and Aurlandsfjord with
Naercyfjord; Fig. 1). There are few of these in outer
Sognefjord. The contrast is likely linked to structural
differences.
3. Geomorphic features
During the Mesozoic and early Tertiary, the landscape of Norway was eroded to a smooth erosion surface called the pal6ic (meaning " o l d " ) surface, close
to sea level (Reusch, 1901; Gjessing, 1967, 1978).
During the early Cenozoic, the crust was lifted asymmetrically (highest along the central mountain range
in Scandinavia) associated with the opening of the
Atlantic ocean. This old erosion surface is easily
mapped along Sognefjord as numerous summits and
plateaus (Fig. 4). These remnants form the basis for
reconstruction of the pal6ic surface in Fig. 5. This conservative reconstruction suggests a small-gradient valley 500 m above present sea level, with a wide eastern
basin and a narrow western part. The evidence does not
disallow gorges at the sites of the present fjords. There
are some mountains in the northeast. The shape of the
drainage basin is similar to the modern shape.
Nearly all the regolith has been removed from this
surface, perhaps by Pleistocene glaciers. However,
some of the pal6ic surface escaped glacial erosion
almost altogether. In these places, marked with stars in
Fig. 4, well-developed frost-patterned ground and
A. Nesje, LM. Whillans / Geomorphology 9 (1994) 33~5
36
Devonian
Caledonian
intrusive complexes
Lower Palaeozoic
metamorphic rocks
~
Valdres Nappe (Late
Precambrian - Mid. Ordovican)
~
Quartz - Sandstone Nappe (Late
I. -:.,":.:.: ] Precambrian - Mid. Ordovican)
~
Upper Jotun Nappe
Upper Bergsdalen Nappes
Lower
~
Anorthosite Complex (Bergen)
Dalsfjord Nappe (Sunnfjord)
Palaeozoic sediments
Precambrian basement
Fig. 3. Bedrock types around Sognefjord (adapted from Kolderup, 1960 and Sturt and Thon, 1978).
blockfields with remnant Tertiary soils and soil minerals are preserved (Roaldset et al., 1982; Nesje et al.,
1988).
At the western end of Sognefjord, an undulating rock
surface and rock ledges close to modem sea level
belong to the Norwegian strandflat. Its genesis is
largely unknown. It has been suggested that the strandflat is a composite feature, formed by the action of
waves, sub-aerial denudation, frost wedging in the surfzone, and glacial erosion (e.g. Larsen and Holtedahl,
1985).
The seaward entrance to Sognefjord is less than 200
m deep and only 3 km wide (Figs. 1 and 3). The
entrance is marked by the occurrence of rocky islets or
skerries. The threshold is basically bedrock and not a
moraine accumulation. It probably exists because of
decreased erosion at the fjord mouth where the ice
diverged and thinned and glacier velocity, or effective
basal ice pressure, became much reduced (Holtedahl,
1967).
Transverse elevation profiles of the fjord show a
break in slope at present sea level. An example is shown
in Fig. 7. Commonly, the portion above sea level has
medium steep slopes, as expected from subaerial
weathering and denudation. The portion below sea
level is steeper. We presume that this is because it has
always been partially supported by pressure from water
during interstadials and interglacials and by ice during
glacials. Significantly, despite the evident necessity of
glaciers to form the fjords, the landscape is importantly
controlled by sea level, a non-glacial influence.
The importance of subaerial processes is evident
A. Nesje, LM. Whillans / Geomorphology 9 (1994) 33~15
37
pe _
LEGEND
~b, Remnants of the
pal~ic surface
i p
"~, • ..)
^^ V-formed, fluvialgorges
r"
--~
J
/
.j./t"
0
50 km
)
(.j
Fig. 4. Remnants of the pal6ic surface (stippled) and V-shaped valleys and gorges (letters V) in the Sognefjord drainage basin. The pal6ic
surface is identified by correlating concordant uplands. The V-shaped valleys are identified from modern topographic maps. The stars identify
sites with Tertiary soil remnants (blockfields).
along the fjord walls. The walls are cut by numerous
small V-shaped valleys (Fig. 8). These indicate the
dominating importance of fluvial-downslope action.
At present, the most important means of downslope
movement are rockfall and rock and snow avalanches
and slope wash. These processes have formed avalanche and alluvial fans on the valley walls and bottoms. Minor tributary valleys and gullies are V-shaped,
as expected if due to such processes (Fig. 8). Some of
the gullies were invaded by glaciers after their formation, as shown by the presence of rounded forms on
interfluves (Rudberg, 1992, p. 239).
The inner ends of the fjords have steep headwalls.
Gjessing (1956) argues that the trough ends are of
Pleistocene glacial origin. The argument assumes that
a non-glacially formed valley would not have a steep
headwall. The headwall is taken to have developed
successively in glacial and interglacial periods by sub-
glacial stream erosion and interglacial avalanching.
However, it is difficult to understand why this process
would produce steep headwalls.
Rudberg (1992) argues on the basis of observations
from western Norway that the fjords and continuing
fjord valleys are the result mainly of glacial and fluvial
headward erosion.
It seems more logical to account for steep headwalls
as sites of structural weakness. Such sites are subject
to fast degradation under subaerial exposure as long as
the headwalls do not have a protective cover of regolith.
Today the most active downslope movement occurs at
valley heads and the largest fans have formed at the
base of headwalls. Such material is removed during
glacial cycles. Subaerial weathering and subsequent
removal during glaciations is an effective procedure,
so headwalls retreat relatively rapidly. The rock
beneath valley bottoms may be equally weak, but is
A. Nesje, I.M. Whillans / Geomorphology 9 (1994) 33-45
38
Nii!iiiiiii!iiiiiill
¸
........
iiiiiiiiiiiiii!iiiiiiiiiiiiii!iiii
iiii
!ii
iii~ii'iiiii i~i~ii~ii~i,iii,li!iiiiiiii!~i}ili~i~iliiiiii~!~i~:~,,, i:~iiii~i~i~ ~,
/
t
/
4 ~
J
16~0
~ .,\~\Q~--"f-~
...........~ ...........
IOOQ
11~
:~ii ii i: iii .....
i~iiiiiiiiiiiiiiiii~i
...... iiii
i
it
it
iI
1
J
,=
,
1400,
4
~J
i r
Fig, 5. Reconstruction of the pal6ic surface in the Sognefjord drainage basin. Elevation in meters above present sea level. The elevation contours
are constructed by manual interpolation of the remnants of the pal6ic surface shown in Fig. 4. The dashed line is the limit of the modern
Sognefjord drainage basin.
protected from subaerial weathering by fjord waters or
by colluvium or till left by retreating glaciers as well
as slides from valley walls. We think that these simple
processes can account for headwall retreat.
There is very little drift in the study area (Klakegg
et al., 1989). Moraines from the Little Ice Age and
Preboreal events are evident only at the valley heads
next to modern glaciers. Apart from some minor
Younger Dryas moraines near the mouth of the fjord,
only a few moraines associated with the last major
deglaciation have been recognized. Loose material is
generally limited to glaciofluvial sediments, marine
deposits, till, and avalanche fans. In the fjord bottom
the average thickness of postglacial sediments is about
200 m (Aarseth, 1980a, b, 1988). It seems that during
the maxima and retreat phases for the Weichselian and
Younger Dryas glaciations that the ice was rather
debris-free.
Glacially scoured and striated bedrock is common.
The oldest ice-flow directions reconstructed from striated bedrock indicate converging ice flow toward the
eastern part of the fjord, and diverging west- to northwest flow directions in the outer part (Klakegg et al.,
1989).
4. Known dates of events
Dor6 (1992) regards the age of the pal6ic surface to
be close to the Cretaceous/Tertiary boundary, and the
surface to have resulted from late Mesozoic levelling
and transgression. Warping of the pal6ic surface
occurred due to some regional stress during the Ceno-
A. Nesje, I.M. Whillans / Geomorphology 9 (1994) 3345
I
......
...................
i!!:XT!!i!
~.
i!::~,:,:. .
.
.
.
:.~
-,,
I~"
I
...............
39
x"
~
I
0
50 kin
Fig. 6. Bedrock fractures near Sognefjord (adapted from Gabrielsen and Ramberg, 1979 ).
800
N
f
S
600
400
200
0
-20C
"0
-40(]
-60C
-80C
-100
2 km
-121
-140
Sediments
Fig. 7. Transverse profile at 65 km, near Vadheim, where the fjord is deepest. Slopes above and below present sea level differ.
zoic and was accentuated by landmass erosion, rebound
to achieve isostasy, and flexural response to sediment
loading in the North Sea Basin mainly during the Quaternary glaciations.
The time period for glacial erosion is estimated from
the occurrence of ice rafted detritus (1RD) in marine
sediments on the mid-Norwegian continental shelf
(Jansen and Sjcholm, 1991 ). This work shows that the
first significant expansion of the Scandinavian ice sheet
occurred 2.57 million years ago. The period between
2.57 and i .2 million years ago produced a steady supply
of IRD, probably because of constant glacial activity
40
A. Nesje, LM. Whillans / Geomorphology 9 (1994) 33-45
Fig. 8. Lookingsouthwardacross Sognefjordat V-shapedvalleysand gorgesalong the northernslopeof Bleia. Photograph:CameraBergum.
and a small glacial to interglacial amplitude. During
the past 1.2 million years the amount of IRD increased,
indicating larger glacial activity in Scandinavia. The
last 0.6 million years were characterized by major fluctuations in the deposition of IRD, indicating large glacial cycles. During this time interval the amount of IRD
is larger. This is interpreted as evidence that ice sheets
frequently reached the edge of the continental shelf
(Jansen and Sjcholm, op. cit.). Thus, there has been
some 2.57 million years of glacial activity, culminating
with the last million years of large fluctuations.
The age and volume of large submarine fans off the
coast of Norway also indicate significant uplift and
glacial activity during the past 2.5 million years (Riis
et al., 1990). During this time there has been an areal
mean erosion of 600 to 1000 m from the coastal drainage basin (Riis, 1992). Thus, the fjords of Norway are
believed to have been eroded mainly during the past
2.5 million years (Andersen and Nesje, 1992), during
which the mean erosion rate was 2 to 3 mm/yr (Nesje
et al., 1992).
During the Weichselian and the Younger Dryas
event the fjord was completely inundated with glacial
ice (Andersen and Karlsen, 1986).
5. A model
Most authors agree that there has been a clear glacialerosive influence on the fjords (e.g. Helland, 1872,
1875, 1876, 1877, 1879; Ahlmann, 19 l 9; O. Holtedahl,
1929; Koechelin, 1947; Munday, 1948; Gjessing,
1956, 1966, 1987; R. Dahl, 1965; H. Holtedahl, 1967,
1975, 1988; Aarseth 1980a, b, 1988; Lind et al., 1981;
Klemsdal, 1982, 1984; Grosswald and Glazovskiy,
1984; Shoemaker, 1986; Syvitski, 1986; Domack,
1988; England, 1986, 1990; Syvitski et al., 1987;
Klemsdal and Sjulsen, 1988; Rudberg, 1988, 1992;
Porter, 1989; Eyles et al., 1990; Andersen and Nesje,
A. Nesje, I.M. Whillans / Geomorphology 9 (1994) 33-45
41
1
~
~ .~-
3
iiiiiiii~iiiiiiiii~ii
i~~iil~~~ ! i~,~ ~i,iii~i~i
Interglacial/interstadial
4
..... ........
!i!i!!i!!i~i ~ ~i~i~i! i!~?~~ ......
~
~
~
~i iii!~iiii!il
¸~!i~i!i
Fig. 9. Phases in proposed formation history of Sognefjord and a photograph of N~er0yfjord. Sea level is represented by the dashed line.
Photograph: Husmo-lbto.
42
A. Nesje, LM. Whillans / Geomorphology 9 (1994) 33~15
1992; Augustinus, 1992), but the importance of glacial
activity relative to other processes such as tectonism
(e.g. Kjerulf, 1863; Brogger, 1886; Gregory 1913;
Randall, 1961; Nicholson, 1963) and fluvial erosion
(e.g. Troll, 1957) has not been evaluated in detail.
We propose the following simple history to account
for the geomorphic features along the fjord (Fig. 9):
Phase 1. Erosion down to the pal~ic surface just
abol,e sea leL,el. During the Mesozoic and early Tertiary
the land surface of Norway was exposed to subaerial
weathering and erosion to form the surface shown in
the first panel of Fig. 9. As deduced by Roaldset et al.
(1982) and Nesje et al. (1988), a warm climate led to
extensive deep chemical weathering preserved today
as summit blockfields.
Phase 2. Uplift. During the Tertiary, the crust was
uplifted. The uplift was greatest in the west. It is probable, but not known for certain, that most of the regolith
was eroded at this time, and that fractured rock was
more deeply cut by fluvial and glacial processes to form
valleys (panel 2).
Phase 3. Interglacials and interstadials. Subaerial
processes produce most erosion from action on fractured rock above sea level, most especially at valley
heads. Valley bottoms partially fill with debris. There
is little erosion below sea level and little erosion of the
pal6ic surface.
Phase 4. Repeated Quaternary glaciations. The glaciations deepened and overdeepened the valleys (panel
4). In contrast, erosion of the pal6ic surface was nearly
uniform, as indicated by the survival of a recognizable
pal6ic surface. The survival of Tertiary soil remnants
indicates very small net erosion on this surface.
6. Discussion
Despite the visual drama of the Norwegian fjords,
their origin has been unclear. Prior studies have emphasized the role of rock structure or of glaciers and have
tended to emphasize one to the near exclusion of the
other. However, there is no question that both have
played crucial roles. Here, we add to that list downslope-mass wasting processes, because they have been
responsible for the dominant slope forms on the fjord
walls.
It has been difficult to understand the interplay of the
three principal processes (erosion along zones of weakness, glacial activity, and subaerial mass wasting). If
subaerial mass wasting is important, where are the colluvial products? If glacial activity is important, why are
major scoured surfaces so rare within the fjords, and
instead, the fjord walls so deeply cut with sharp Vshaped valleys? How can the fjords be so deep if glacial
activity were not the dominant process? How did the
steep headwalls to the fjord form? Why did remnants
of Tertiary soils survive on the Palric surface? Why is
there so little glacial drift in the fjords? In view of the
reputed former massive ice sheet over Norway, why
was glacial erosion not more spatially uniform? These
are questions that trouble scientists, and tourists with
geological training, and they led to the present contribution.
The importance of structure is highlighted by the
observation that today the largest accumulations of
coarse colluvium occur in fans at valley heads and
downstream deltas, below sites where fractured rock is
exposed. This indicates where erosion is occurring.
Other regolith is mainly restricted to raised marine
deposits and very limited glacial drift. Erosion is now
most effective on exposures of these fracture zones.
A major function of glaciers has been to remove
these fans at valley heads. With each glacial advance,
the protective fans are swept away and more of the
fractured bedrock is exposed to weathering during the
next subaerial phase. We think that this accounts for
nearly all erosion above sea level.
The deepening of the fjords is understood if one notes
that the debris picked up by the glacier becomes available as tools for glacial abrasion and plucking. Having
collected mainly at valley bottoms, the tools are carried
at the bed of the glacier and not at the sides. This, and
the weak rock in fjord bottoms, can explain why fjord
walls are so little affected by glacial erosion, yet the
fjords are deep.
Modem Norwegian glaciers are remarkable for the
small amount of debris that they carry. This cleanliness
is because there are no crumbling mountains towering
above the glaciers, and because the glaciers are now
too small to descend onto debris fans. Due to the inefficacy of the glaciers on valley walls, by default, the
dominant landforms are due to subaerial processes,
A. Ne.~je, LM. Whilhms / Geomorphology 9 (1994) 33-45
mainly downslope mass wasting. This mass wasting is
enhanced by removal of the debris by glacial activity.
Erosion of the uplands has been slow, by any means.
Slopes on the surface are evidently too gentle for gravity-driven mass wasting to be very effective. The regolith has been removed from most places (although
some survived), but so little bedrock has been removed
that the palric surface is still recognizable. Subaerial
erosive processes have dominated above sea level,
despite the recent widespread glaciation. It seems that
this last glaciation eroded very little of the t~jord walls
above sea level, except for removing loose material.
There are glacial striae on the interfluves and within
minor valleys, but not much rock was removed by the
most recent glaciation. The geomorphic changes that
may be expected with the next glaciation may be
inferred from the present-day distribution of detritus.
The accumulated debris at headwalls will be removed,
and the fjords so lengthened. Also, the fjord bottom
will be "freshened" by the flushing out of accumulated
debris and further erosion by debris passing in traction.
Elsewhere, geomorphic changes will be minor. The
upland surface now offers little debris for use as tools
by the next glaciation. In contrast to the headwalls, the
small volume of fans on the fjord sides points to the
very small amount of erosion during the current interglacial. The ice will flow across these valleys without
altering them greatly. The next glaciation will produce
a longer t]ord that is somewhat deeper.
The lower limit to fluvially-controlled mass wasting
seems to be at modern sea level. This is inconsistent
with the usual view about sea level. The cross section
in Fig. 7 indicates a close correspondence between the
slope break and modern sea level, and two of the three
valleys in Fig. 8 are graded to sea level. (The left-most
valley in Fig. 8 may have special structural control.)
The immediate conclusion is that past fluvial-downslope wasting has been dominant at times with the same
relative sea level as today. That is, fluvial activity dominates neither at higher stands, when the Antarctic ice
sheet may have been smaller, nor at the lower stands
of interstadials. The implication is that modern sea level
is the norm during the Quaternary. This finding conflicts with the standard interpretation of 6 ~80 values in
ocean foraminiferas in terms of global ice volume and
sea-level fluctuation. That interpretation suggests that
sea level has few consistent stands.
Discussion here has been limited to Sognefjord, but
43
the lessons may be widely applicable. Compared to
most sites of extreme '~glacial" scenery, the rock of
western Norway is very coherent. This is the reason
why glaciers today have very little supraglacial or
englacial debris. In other parts of the world glaciers
carry much more debris and so abrade the valley walls
more. Apart from this distinction, the genesis of other
glacial valleys may be similar to the model advanced
here.
In particular, we suspect that special flow patterns in
glaciers (e.g. Sugden and John, 1976, p. 168) are not
needed to account for cirques, paternoster lakes, and
troughs. We suggest that other workers experiment
with adaptations of the present model to account for
major 'glacial' landforms in terms of cyclic exposure
and removal of less competent rock.
Acknowledgements
The first draft of this paper was written in the Caf6
of the Hotel Mundal overlooking Fja~rlandsfjord. We
are grateful to the proprietors of the hotel for their
encouragement. S.E. White made many useful suggestions. This work was supported by a grant from the
Norwegian Research Council for Science and Humanities (NAVF) to O. Orheim. K.H. SjcstrOm prepared
the figures.
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