DTD 5
ARTICLE IN PRESS
Geomorphology xx (2005) xxx – xxx
www.elsevier.com/locate/geomorph
Re-exposed basement landforms in the Disko region, West
Greenland — disregarded data for estimation of glacial
erosion and uplift modelling
Johan M. Bonow*,1
Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden
Received 26 March 2004; received in revised form 12 May 2005; accepted 17 May 2005
Abstract
Classifications of large-scale landscapes in Greenland have traditionally been based on type and intensity of glacial erosion,
with the general idea that present landforms are mainly the result of erosion from ice sheets and glaciers. However, on southern
Disko and in areas offshore in Disko Bugt, a basement surface has preserved remnants of weathered gneiss and pre-Paleocene
landforms, recently exhumed from Paleocene basalt. Isolated hills and lineaments have been mapped in a digital terrain model
and aerial photographs. Offshore have hills been mapped from seismic lines. The medium size bedrock forms on southern
Disko as tors, clefts and roche moutonées have been studied in the field. Remnant saprolites were inventoried, sampled and
analysed according to grain size and clay mineralogy. The basement surface retains saprolites up to 8 m thick in close relation to
the cover rocks. The landforms in the basement rocks belong essentially to an etched surface only slightly remodelled by glacial
erosion and, below the highest coastline, also by wave action. The outline of hills is governed by two lineament directions,
ENE–WSW representing the schistocity of the gneiss and NW–SE fracture zones. These structures are thus interpreted to have
been exploited by the deep weathering while the frequent N–S lineaments have not and thus might be younger. Main ice-flow
has been from the NE and has resulted in plucking of SW facing lee sides, however the resulting bedrock forms are mainly
controlled by structures and orientation of joints. The identification of re-exposed sub-Paleocene etch forms on Disko and the
hills of similar size offshore, forming a hilly relief, have implications for identification of a hilly relief south of Disko Bugt, its
relation to younger planation surfaces as well as for conclusions of uplift events.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Etch surface; Glacial erosion; Kaolin; Landforms; Weathering; West Greenland
1. Introduction
* Present address: Geological Survey of Denmark and Greenland
(GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
Tel.: +45 38142251; fax: +45 38142050.
E-mail addresses: [email protected], [email protected].
1
Tel.: +46 8 164864; fax: +46 8 164818.
Basement landforms in Greenland have generally
been considered as glacially formed and thus classified as glacially scoured, with characteristics as nu-
0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2005.05.006
GEOMOR-01742; No of Pages 22
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
merous lake basins, roche moutonées and striated
surfaces (Sugden, 1972, 1974; Gordon, 1981; Funder,
1989; Glasser and Warren, 1990). In former glaciated
areas in Canada, Scandinavia and Scotland, the
long-term non-glacial landform development has
by a few researchers been regarded as important
Greenland
Kuk
Ka
7800000
Nuussuaq
Ice covered areas
Ku
7700000
Disko
41
Disko Bugt
42
20 km
Onshore
Depth (m)
0 200 400 600 800
Borehole
Hills offshore,
<25, 25-50, >50 m
Eocene and
younger sediments
Paleogene basalt
Cretaceous-Paleocene
sediments
Precambrian basement
5
4
46
3
45
N
W
E
S
600000
1
700000
Fig. 1. Regional onshore and offshore geology map together with bathymetry and hills offshore along selected seismic Brandal numbered lines.
Ka: Kangâmiut, Ku: Kuugannguaq. Grid in UTM, zone 22n.
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
for the present shape of bedrock forms and landforms
have been associated with processes active in the
pre-Quaternary (Chalmers, 1898; Lidmar-Bergström,
1982, 1989, 1995; Hall and Sugden, 1987; Hall
and Mellor, 1988; Rudberg, 1988; Hall, 1991; Bouchard and Jolicoeur, 2000; Hall and Bishop, 2002;
Olvmo and Johansson, 2002; Hall, 2003; Hall and
Auton, 2003).
Sub-aerial pre-Quaternary deep weathering has
been encountered at several sites in Greenland (e.g.,
Escher and Watt, 1976; Pulvertaft, 1979). These
saprolites are of various ages in different regions.
Basement rocks and saprolites in situ on southern
Disko, West Greenland, have recently been re-exposed by erosion of its cover of Late Paleocene basalt.
The exposed basement therefore offers a unique opportunity to study the development of landforms associated with both deep weathering (etching) and
glacial reshaping.
The aim of this study is to analyse landforms in
basement rocks in relation to saprolite remnants
and cover rocks in order to identify and describe
primary etch forms and their characteristics. The
influence on landform development of bedrock
structures such as schistosity, joints and fractures
is examined and these data are subsequently analysed in a landscape context, where the large landforms are in focus. Finally, the identified etch
forms are compared with forms reshaped by glacial
processes in order to evaluate the effect of glacial
erosion.
3
2. Geological context
2.1. Basement and cover rocks
This study was performed within a limited area on
southern Disko between Fortunebay (west) and
Qeqertarsuaq (east) (Figs 1, 2). Here, basement consisting mainly of highly metamorphic Proterozoic
gneiss (Pedersen et al., 2000), is exhumed from
Palaeogene basalts. The study area is part of an 18
km wide, north to south trending basement ridge,
which runs below the basalt from northern Disko
and southwards below Disko Bugt (bay). The gneiss
ridge is believed to have existed as a paleohigh predating the basalt flows in the Paleocene (Chalmers et
al., 1999). The gneiss is exposed in windows and
occurs up to 700 m a.s.l. in central Disko (Pedersen
et al., 2001), but it outcrops also at the bottom of the
Disko Bugt (Fig. 1). Here it is exposed on a number of
skerries and small islands. East of the ridge in Disko
Bugt, 1 to 2 km of Cretaceous sediments rest directly
on the basement, but west of the ridge it is instead
basalt that rests directly on the basement (Chalmers et
al., 1999). Most of Disko is covered by flatlying
Lower/Upper Palaeocene basalt (61–59 Ma) of the
Maligât formation (Fig. 1; GGU, 1971; Clark and
Pedersen, 1976; Henriksen et al., 2000; Pedersen et
al., 2000), but for the east and southeast coasts, which
are built up of Cretaceous to Danian terrestrial sediments (Clark and Pedersen, 1976; Fig. 1). The basalt
is mainly of sub-aqueous type in the Qeqertarsuaq
Projected gneiss profile line
Qeqertarsuaq
Fortunebay
Basement
Basalt
400
300
200
100
0
Disko Bugt
1 km
(m)
Fig. 2. The study area with general geology on southern Disko. A part of the basement ridge is exposed up to 300 m. Rectangles mark location
for Figs. 8 and 9. See rectangle in Fig. 1 for location.
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
area, while it is of sub-aerial type at Fortunebay (Clark
and Pedersen, 1976; Pedersen et al., 2000).
The relief in the Precambrian basement of the
study area is characterised by an undulating terrain
from sea level up to 320 m a.s.l. (Fig. 2). This terrain
type is also reflected by several small islands, up to 50
m high, formed in the basement along the coast. The
basement disappears beneath the basalt both to the
east and to the west (Fig. 3), and the basalt is locally
present also at sea level. The basalt forms vertical
cliffs above the exposed basement and extends as a
slightly undulating plateau further inland at 600 to
700 m a.s.l. (Fig. 3).
2.2. Saprolites
The nature of the basement at the contact to the
cover rocks has been documented in offshore and
onshore wells as well as from exposures in West
Greenland (Fig. 1). In the offshore Kangâmiut well
(overview map in Fig. 1), approximately 350 km
south of Disko, basement was struck at about 3800
m depth (Bate, 1997). An approximately 150 m thick
section on top of the basement contained kaolin and
was interpreted as heavily altered (deeply weathered)
basement. In a borehole in the Kuugannguaq valley,
central Disko (Fig. 1), Lower Cretaceous sediments
were found overlaying gneiss basement, which was
struck at 161 m a.s.l. (Chalmers et al., 1999). The
section on top of basement was heavily altered
(Gregers Dam, pers. comm. 2003). Pulvertaft
(1979) reported a 35 m thick sequence of deeply
kaolinised basement below Cretaceous strata at
Kûk, northern Nuussuaq (Fig. 1). Within the study
area, weathered basement with probable kaolinisation beneath Paleocene basalt has been reported
from Fortunebay (e.g., Møller-Nielsen, 1985).
2.3. Faults and fractures
The major fault systems in the region are dominated by three directions, namely N–S, WNW–ESE and
NW–SE (Chalmers et al., 1999) and are primarily
thought to be a consequence of activation of the
Labrador rift system and the Ungava fault zone initiated in the Early Cretaceous (Chalmers and Pulvertaft,
2001). Detailed mapping of fracture systems in the
study area has been made by K-E. Klint (in Japsen et
al., 2002), who recognised four major systems, viz. E–
W, NE–SW, NW–SE and N–S. The latter two were
regarded as regional systems, younger than the basalt
and connected to the large regional N–S fault system
(cf. Chalmers et al., 1999).
2.4. Glacial history
During Quaternary glacial periods, Disko was
probably completely covered by ice (Weidick, 1976;
Funder, 1989). Outlet glaciers draining wide areas of
the Greenland ice sheet flowed through the Disko
Bugt towards the southwest, creating large offshore
troughs (Funder and Hansen, 1996; Long and Roberts,
2003; Fig. 1). Today local ice caps cover central
Disko. After the last deglaciation, ~10,300 years
Fig. 3. 3D view over the study area from the SSE. Numerous hills exist in the gneiss. The high cliffs of basalt end abruptly in a plateau, which
continue inland (cf. Fig. 6). Dashed line shows the interpreted border between gneiss and basalt.
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ago, the relative sea level has lowered about 120 m in
the southern Disko area (Funder, 1989; Long and
Roberts, 2003).
3. Methods
Contour maps were created from a digital terrain
model obtained from the Geological Survey of Denmark and Greenland (GEUS). The model has a
resolution of 8 by 8 m in areas below ~300 m
a.s.l.. Other areas are covered by a 250 by 250 m
grid resolution square grid, constructed by National
Survey and Cadastre (Kort og Matrikelstyrelsen),
Copenhagen (Ekholm, 1996; Bamber et al., 2001),
which was resampled to a smoothed 8 m grid. The
gneiss is exposed from sea level to slightly above
300 m a.s.l. and hence mainly covered by the detailed dataset.
Hills were mapped in the gneiss area from aerial
photographs, at the scale 1 : 40,000, but for the Qeqertarsuaq peninsula aerial photographs at the scale
1 : 8000 were available (Fig. 4). The outline of hills
was defined by coherent rock masses, which were
delimited by clear break-of-slope. The hills were plotted together with contours and geology. The height
was measured from the hill foot to the highest point.
Detailed interpretation of the offshore geology, especially the outline of basement rocks in selected seismic profiles was provided by J.A. Chalmers (pers.
comm. 2003). The height of individual hills in base-
5
ment areas offshore in southern Disko Bugt, were
mapped and their height measured from eight single-channel analogue, unmigrated, seismic profiles
(GEUS archive, 1972, unpublished data). The breakof-slope in the sea-bed topography was used to identify the foot of the hill and from that point the height
was measured to the highest reflector of that hill. The
height was measured with an accuracy of 0.01s twoway travel time, equal to ~7.5 m. The hills were
classified in 0.01s classes. The exact position of the
vessel carrying the equipment for seismic line sampling, and its speed in-between control points is however not well known. Therefore was it not possible to
map the precise length of hills from this dataset (J.A.
Chalmers, pers. comm. 2003).
Lineaments in the gneiss area were mapped from
the aerial photographs and plotted together with contours and hills (Fig. 4). Furthermore, a field inventory
was carried out in the area between Qeqertarsuaq and
Fortunebay and the whole area was documented by
photographs. Clefts were inventoried and investigated
in the gneiss areas, especially along the coast and the
detailed forms were described.
A field inventory of saprolites was carried out in
the area between Qeqertarsuaq and Fortunebay. The
inventory was focused on areas where the gneiss/
basalt contact was indicated on the geological map
(Pedersen et al., 2000) and along streams deeply
incised in the basalt, where the basalt/gneiss contact
might be exposed, as the contact usually is covered by
talus. Encountered saprolites were sampled and the
100 m
Fig. 4. Stereogram covering a small part of the Qeqertarsuaq peninsula, demonstrating the appearance of lineaments and hills in aerial
photographs. See Fig. 9 for location. Detail of aerial photographs 091 and 093; route no. J2/229; 1958. nKort and Matrikelstyrelsen, Denmark.
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
?
?
?
Fortunebay
7684000
Qeqertarsuaq
N
Land
Sea
W
Disko Bugt
Interpreted gneiss border
Gneiss hill
Contour 25 m
392000
E
S
398000
1 km
Fig. 5. Distribution of hills in the study area.
weathering forms within the gneiss were studied in
detail. To be able to compare the Disko saprolites with
saprolites elsewhere and to use them for discussion on
the origin of the topography, they were analysed for
grain size and clay mineralogy.
The saprolites were studied by X-ray diffraction
analysis (XRD) with two slightly different methods
at two laboratories. The clay fraction of six samples
were analysed according to a method described in
Lidmar-Bergström et al. (1997). One sample was
analysed according to the method described by
Ernstsen (1996). Both methods resulted in semiquantative estimates of clay minerals.
To evaluate the glacial erosional effect on the
basement rock, two reference surfaces were used for
qualitative estimation: the sub-basalt surface and the
glacial surface. Ice flow directions, indicated by striae
and chattermarks, were also measured.
4. Results
4.1. Gneiss hills and lineations
Eighty individual hills formed in basement were
identified (Fig. 5). A few of them are covered by a
Basalt
Basement
Fig. 6. Re-exposed gneiss hills in the Tini area beneath high cliffs of basalt. The cliffs are mainly covered with talus. Some of the gneiss hills are
buried by deposits. The dashed lines indicate the limit of observed gneiss and the interpreted gneiss/basalt contact respectively.
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7
25
20
Hills onshore
Frequency
Hills offshore
15
10
5
0
5
7
22
10 15 20 25 30
52
37 40 45 50
60
67
75
70
82
80
90
100
height (m)
Fig. 7. Height and frequency of hills onshore southern Disko and offshore in the southern Disko Bugt (Fig .1).
post-glacial cover including talus, till and marine
sediments (Fig. 6). The hills have a diameter of 50
to 250 m, and range from 5 to 100 m in height above
the hill foot with an average of 30 m. Most hills are
less than 45 m high (Fig. 7).
In the Fortunebay area lineaments in three directions dominate; N–S (most), NW–SE and ENE–
WSW (least) (Fig. 8a, b). The latter direction follows
approximately the schistocity strike in the gneiss.
For lineaments limiting hills, the ENE–WSW direction was dominant, followed by NW–SE and N–S
(Fig. 8c).
In the Qeqertarsuaq area two lineament directions
dominate; N–S (most) and NW–SE, and a third direc-
a)
?
?
7685500
?
Fortunebay
b)
c) Limiting lineaments
Total Lineaments
N
W
0
5
10
20
20
30
E
0
S
2
4
6
8
Contours 100 m
n=266
Gneiss hill
390000
391000
Lineament
Gneiss
Quaternary
500 m
n=58
Basalt
Fig. 8. Lineaments and hills in the Fortunebay area mapped from aerial photographs at the scale 1 : 40,000 (a). Total lineaments (b) and limiting
lineaments (c) are shown in rose diagrams.
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are located in areas between large glacial troughs and
channels (Fig. 1). Some channels are overdeepened
and have water depths exceeding 800 m, and thus
reach down to 600 m below the surface of the gneiss
ridge.
tion, generally around ENE–WSW can be noted (Fig.
9a, b). The direction of lineaments limiting the hills is
not as dominant as in Fortunebay. Only the NW–SE
direction dominates and almost all other directions are
represented (Fig. 9c). Minor elongated valleys follow
along lineaments (Figs 3, 5).
4.3. Clefts
4.2. Offshore landforms
Clefts were only found in areas below the highest
post-glacial marine limit and relatively far from the
basalt outcrops (Fig. 10). The clefts follow vertical
joints and are therefore more or less straight and
have steep walls. Horizontal joints divide the walls
A total of 123 hills were identified along seismic
lines, crossing the submerged gneiss area in southern
Disko bay (Fig. 1). Their height range from 7 to 82 m
with an average of 36 m (Fig. 7). All identified hills
7684000
a)
Qeqertarsuaq
Fig. 4
b) Total lineaments c) Limiting lineaments
Gneiss hill
Lineament
Quaternary
N
W
E
0
5
15 25 35
0
2
4
6
8
S
Gneiss
Contours 100 m
Basalt
500 m
n=593
n=54
399500
Fig. 9. a) Lineaments and hills in the Qeqertarsuaq area mapped from aerial photographs at the scale 1 : 8,000 (a). Total lineaments (b) and
limiting lineaments (c) are shown in rose diagrams.
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9
2
1
4
5
6 7a,b
,b
3
8
7685000
Fortunebay
N
W
E
Saprolite
S
Basalt
Gneiss
Sea
Qeqertarsuaq
Saprolite, sampled
Contour 20m
1
Sample no. (table 3)
Clefts
Straie/Chattermarks
394000
1 km
Fig. 15
Fig. 10. Saprolite sampling sites and observed occurrences of saprolites, clefts, striae and chattermarks. All saprolite sites are close to the basalt/
gneiss contact. The direction for clefts is shown by the orientation of the line. Location of striae and interpretation of ice flow direction is
marked by arrows.
x
x
3
6
x
x
1
8
x
5
3
x
x
x
x
x
x
x
x
2
1
x
4
7
Fig. 11. Sketch of forms
2) Stapled boulders with
in profile. 5) Weathered
grains on the cleft wall
x
x
x
x
documented in clefts formed in basement rock on southern Disko. 1) Weathered horizontal and vertical joints.
weathered rounded edges. 3) Cleft floor, with only some unconsolidated material. 4) Weathered horizontal joint
joint transformed to P-form (cf. no 4). 6) Stoss-side – lee-side topography, occasionally with striae. 7) Quartz
polished by wave action. 8) Hand-sized hollows with beach sand.
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
into sheet slabs, with more closely spaced joints
towards the upper part of the cleft (Fig. 11). Weathering along the joint system has caused the formation
of tor-like pillars where boulders with rounded edges
are placed on top of each other. Boulders with
rounded edges are common at the floor of the
a)
S
cleft, often together with some unconsolidated
sandy to gravelly material.
4.3.1. Case study of two cleft areas
In the Fortunebay area several clefts were identified (Fig. 10). They are about 100 m from the base-
b)
~2m
c)
d)
Fig. 12. a) Overview of clefts in Fortunebay area (Fig. 10, cleft at sample site 1). Photograph is towards the south. b) Detail of forms in one of
the cleft walls in Fortunebay. Both vertical and horizontal joints are weathered and have rounded edges. Large boulders have fallen from the
wall. Signs of glacial activity could not be identified here. c) Cleft at Qeqertarsuaq peninsula. The boulders have weathered forms and the cleft
wall has a rough surface (white arrows). d) Cleft at Qeqertarsuaq peninsula. Tor-like pillars and large boulders occur frequently. High up in the
wall a P-form has exploited one of the pre-weathered horizontal joints (white arrows). Note person for scale.
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11
Table 1
Clefts and detailed forms on southern Disko
Location
Depth/Width/Length (m)
Strike
Forms in lower part
Forms in upper part
Forms summit surface
Qeqertarsuaq
Qeqertarsuaq
Qeqertarsuaq
Qeqertarsuaq
Qeqertarsuaq
Qeqertarsuaq*
Fortunebay
Fortunebay**
10/1–2/30
3/2/10
1/0.5/15
4/1–2/35
15/3/200
5/5?/35
12/1–2/30
10/5/100
N318E
N318E
N148W
N478E
N328E
N
N288E
N578W
Dmw, Ps, Bre, Cwj,
Dmw, Ps
Dmw
Dmw
Dmw, Ps, Bre, Tlp
Dmw
Tlp, Bre, Wp
Bse
Pf?
–
–
Pf,
Bre
–
–
Bse
Rm
–
–
Rm
Rm
Rm
–
–
Bse: Boulders with sharp edges, Bre: Boulders with rounded edges, Cwj: Convex-edged sheet slabs with weathered joints, Dmw:
differential mineral weathering, Pf: P-forms, Ps: Polished surface, Rm Roche moutonée, Tlp: Tor like pillars, Wp: Weathering pits.
*One sided cleft, ** Maybe young cleft.
ment/basalt contact, and about 200 m from the basalt
cliffs. The clefts are five to ten metres deep, a few
metres wide and run mainly along NNW–SSE lineaments for 30 to 50 m (Fig. 12a). The cleft walls have
rounded, weathered forms and both vertical and
horizontal joints are extensively weathered (Table
1). The vertical joints divide the cleft wall into
separate blocks, which gives the wall the impression
of pillars of corestones on top of each other (Fig.
12b). Some unconsolidated material is present in
joints or along the cleft floor. Small amounts of
sorted fine sand were encountered in hand-sized
hollows in the cleft wall. The top surface may
have been glacially plucked, but no striae or chattermarks could be identified.
On the peninsula south of Qeqertarsuaq several
clefts were encountered within a small area (Fig. 10).
All of them lie more than 2 km from the present
gneiss/basalt contact. The clefts are five to ten metres
deep, a few metres wide and run mainly along NE–
SW lineaments for 10 to 50 m, although one is about
200 m long. The vertical and horizontal joints are
mainly free from weathered material and their spacing along the cleft wall resembles pillars with corestones piled on top of each other. Boulders with
rounded corners and small weathered forms are
spread along the cleft floor. They consist of the
same bedrock as the cleft wall and seem to have
been transported only a short distance (Fig. 12c).
The top surface above the cleft has roche moutonées
with plucked lee-sides. Due to post-glacial surface
weathering few striae are observed. The post-glacial
weathering is estimated to one to three cm, indicated
by weathering-resistant quartzite veins, which still
have the glacially polished surface intact. In the
higher part of the clefts, concave, smooth and gently
Table 2
Sampling sites, saprolite types and result of semi-quantative estimates of clay minerals (b0.002 mm) from XRD, southern Disko (Fig. 10)
1**
2*
3
4
5*
6*
7a*
7b*
8*
Location/Position
Saprolite type
Sm/Ve
Kao
Ill
Gib
%ff
%cf
Geusno
Fortunebay/Weathering front
Fortunebay/Weathering front
North of Tini/Slided, gneiss/basalt
Tuuapassuit valley/Landslided?
Tuuapassuit valley/Weathering front?
Akuarut/Weathering front
South of Lyngmarksfjeld/Exposed valley side
South of Lyngmarksfjeld/Exposed valley side
Lyngmark water well/Weathering front
Not measured
Sandy gravelly
Not measured
Not measured
Gravelly/sandy
Sandy/gravelly
Sandy
Sandy
Sandy
++++
++++w
–
–
+++
++
+++
+++
++++w
+
+
–
–
++
++
++
+
(+)
(+)
0
–
–
0
+
0
0
0
0
0
–
11
–
–
11
8
20
13
16
–
1
–
–
1
1
2
1
2
4791-29
4791-44
4791-51
4791-49
4791-50
4791-47
4791-45
4791-46
4791-48
–
–
+
0
(+)
+
(+)
Sm/Ve=Smectite/vermiculite, Kao=Kaolinite, Ill=Illite, Gib=Gibsite, %ff= % fine fraction, %cf= %clay fraction, 0= non, (+)= very small
amounts, ++++=dominant, w smectite+ vermiculite as indicated by non-swelling by treatment with glycerol. *Analysis by Siv Olsson, Lund.
**Clay mineral analysis by Vibeke Ernstsen, GEUS, Copenhagen.
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winding so-called P-forms have formed (Fig. 12d),
preferably along horizontal joins. The cleft walls are
often affected by differential weathering, which has
caused a rough wall surface. Occasionally, in the
lowest part of the cleft some reddish quartz grains
are polished and mainly unaffected by this weathering (Table 1).
4.4. Saprolites and basement landforms
Saprolites were encountered at twelve sites between Qeqertarsuaq and Fortunebay (Fig. 10), and
are distributed all over the study area. Samples were
collected at nine different locations, and seven samples were selected for detailed clay mineral and
grain-size analysis (Table 2). The result of the
grain-size analysis of six samples is summarized in
Fig. 13.
Fortunebay (site 1, Fig. 10). About 2.5 m of the
gneiss is exposed along a minor stream. The gneiss
is overlain by sub-aerially deposited basalt. The
gneiss is weathered to a saprolite, 30–40 cm thick,
and it is also present along fractures and joints in the
gneiss section. The clay fraction is dominated by
smectite, with some kaolinite and traces of illite
(Table 2). Spherodial boulders occur in situ in the
saprolite. The size of boulders is highly dependent
on the spacing between the extensively weathered
joints. The contact between fresh unweathered basement and the saprolite, the weathering front, is sharp
(Fig. 14a).
Sand
2-0.06 mm
100
0
7b
8
7a
6
50
2
50
5
0
100
Gravel
20-2 mm 0
Silt and clay
50
100 < 0.06 mm
Fig. 13. Grain size diagram of the six measured saprolites on
southern Disko.
Fortunebay (site 2, Fig. 10, Table 2). A few
metres thick section of the gneiss basement profile
and its contact to the basalt is exposed. Beneath the
contact of sub-aerially deposited basalt, basement is
weathered to a sandy-gravelly saprolite, up to 3 m
thick. In the sample, taken about 2.5 m below the
basalt/gneiss contact, the fine fraction is dominated
by smectite and low-charge vermiculite but also
contain small amounts of kaolinite. The weathering
is most intense along fractures and joints. Boulders
of unweathered rock, rounded by spheroidal weathering (corestones) are embedded in the saprolite and
their extent is limited by the spacing of joints
(Fig. 14b).
Tuuapassuit valley (site 5, Fig. 10, Table 2). The
valley sides of the incised Tuuapassuit valley are
covered by talus and many landslides have reached
the valley floor. Basement is partly exposed due to
recent fluvial erosion. Close to the valley floor a
gravelly sandy saprolite, a few cm thick, was encountered. Smectite/vermiculite is abundant in the
fine fraction of the sample, which also contain
some kaolinite. No weathering forms could be
identified.
Akuarut valley (site 6, Fig. 10, Table 2). A deeply
incised valley has cut through the basalt and about 20
m into the gneiss. At the entrance of the valley,
basement is weathered to saprolite (Fig. 15a). The
fine fraction of the sandy-gravelly saprolite contains
abundant kaolinite, some smectite/vermiculite and
small amounts of illite (Fig. 16). About 50 m higher
up in the valley, fresh basement is in contact with
basalt. The gneiss has vertical and horizontal joints,
which are somewhat weathered and the section gives
the impression of locked corestones in a weathering
profile (Fig. 15b). Although no saprolite in situ is
present here, the joints and edges are weathered to
rounded convex forms (Fig. 15c).
South Lyngmarksfjeld (site 7a and 7b, Fig. 10,
Table 2). Along a minor valley a 20 m wide and
8 m high section of deeply weathered basement is
exposed. The gneiss structures are still visible in
the saprolite. About 50 m west of the site fresh
gneiss emerges from the basalt. The saprolite is
sandy and was sampled at two locations 10 m
apart from each other. The weathered section is
exposed beneath fresh basalt which is underlain
by a red-coloured weathered basalt horizon imme-
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
13
Fig. 14. a) Weathered basement below basalt at Fortunebay (sample site 2). Photo is towards southwest. The saprolite is approximately 2.5 m
thick, containing corestones. b) Recently exposed weathered basement at Fortunebay (sample site 1). Note that the rounded gneiss boulders
build tors.
diately on top of the saprolite. The fine fraction in
7a contains abundant smectite/vermiculite while
sample 7b contains small amounts. Kaolinite is
abundant in sample 7a, while some kaolinite is
present in sample 7b. Small amounts of gibbsite
were also identified.
Lyngmark water well (site 8, Fig. 10, Table 2). In
an amphitheatre-shaped location the weathering front
is exposed by a minor stream. The saprolite here is of
gravelly sandy type and appear similar to the saprolite
in site 5 described above. On the opposite side of the
amphitheatre, fresh basalt overlays a sandy saprolite,
which was sampled. Vermiculite dominates the clay
fraction and also smectite is present. Traces of a
poorly defined kaolin mineral and gibbsite were identified (Fig. 17).
4.5. Glacial features on the gneiss surface
Striae and chatter marks were only found on
Qeqertarsuaq peninsula (Fig. 18; Table 3). The
post-glacial weathering has generally removed one
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14
J.M. Bonow / Geomorphology xx (2005) xxx–xxx
Fig. 15. a) At the entrance of the deeply incised Akuarut valley, a thick section of basalt overlay the gneiss. The incision ends abrupt slightly
below the basalt/gneiss contact. The contact is indicated by the dashed line. The gneiss is deeply weathered to saprolite to the left (marked by
arrow, sample 6, Table 2). To the right some talus covers the gneiss. b) Contact between basalt and gneiss. The gneiss is dissected by weathered
horizontal and vertical joints. c) The gneiss is rounded and has exfoliation sheets, even in contact with the basalt. This may suggest that it is a
pre-basalt form.
to three cm of rock destroying the glacial surface, if
upstanding quartzite veins with preserved glacially
polished surfaces are regarded as the glacial reference surface. It should be noted that all identified
striae but one are situated in low lying areas, which
are recently raised above sea level. Striae show a
main ice-flow direction from NE. On one locality
two directions and their relative age could be established (Table 3: striae 6 and 7). The older direction
showed ice flow from the north and the younger ice
flow from the NE. These ice-flows directions apply
for all the study area.
Air dried
Etylen-glycol solvated
Glycerol solvated after
magnesium saturation
O
Heated at 550 C
14 Å
10 Å
7Å
Fig. 16. X-ray diffractogram of the clay fraction at Akuarat valley (site 6, Fig. 10). The kaolinite spike is at 7Å.
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
15
Air dried
Etylen-glycol solvated
Glycerol solvated after
magnesium saturation
14 Å
4.85 Å
Fig. 17. X-ray diffractogram of the clay fraction at Lyngmarken water well (site 8, Fig. 10). Smectite/vermiculate is present shown by the 14Å
spike, but the non-swelling by glycerol treatment indicates that vermiculite dominate the clay fraction.
side of the hill (Figs. 18, 19a). The hills in Fortunebay are also plucked on the SW side, but the
form of the hills here is remarkably different as the
7683500
The hills on the top surface of the Qeqertarsuaq
peninsula are mostly asymmetrical with a gentle
stoss-side and a steep lee-side, plucked on the SW
Indicated ice flow by
striae and chattermarks
0
2
4
N
100 m
Cleft
n=15
399000
Ice flow indicated by
striae and chattermarks
W
E
S
Contour 5 m
Fig. 18. Interpreted ice flow direction from striae and chattermarks at the Qeqertarsuaq peninsula and location of clefts. Note that all but one
striated outcrops is in low elevation areas.
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16
J.M. Bonow / Geomorphology xx (2005) xxx–xxx
Table 3
Striae and chattermarks on Qeqertarsuaq peninsula
No
Sample type
Ice flow towards
1
2
3
4
5
Stiae
Chattermark
Striae
Striae
Striae
2348
2708
2448
2338
2308
6
7
8
9
10
11
Striae
Striae
Chattermark
Chattermark
Chattermark
Striae
1808
2308
2308
2708
1908
2308
12
Striae
2508
13
14
15
Large striae
Striae
Striae
2308
2508
2408
5. Analysis and discussion of formation of
basement relief
Remark
5.1. Saprolites and their origin
Microstructures in
same direction
Older than number 7
Younger than number 6
Relative age to number
12 unknown
Relative age to number
11 unknown
Maybe P-form?
stoss-sides are steep and the lee-sides are gentle
(Fig 19b, c). Thus, the appearance of the hills and
their form is mainly controlled by bedrock structures and the orientation of joints, and not from the
dominant ice flow direction.
4.6. Summary: basement landforms in the study
area
The general landscape in the basement area on
southern Disko is characterised by distinct hills of
varying height (5–100 m) and aerial extent (50–250
m), defined by the structural lineaments. This hilly
relief continues offshore towards the south and
across the Disko Bugt. Several landforms are associated with the weathering processes, which were
active prior to the volcanic flows in the Paleocene.
The gneiss beneath the basalt cover rock is weathered both along joints and occasionally weathered to
saprolite, up to 8 m thick. In detail the gneiss has
rounded weathering forms. Away from the basalt, 5
to 10 m deep clefts have similarly rounded forms,
but here no saprolites are found, probably due to that
clefts only occur below the highest post-glacial marine limit and therefore the saprolites are washed
away. The upper part of clefts and the top surface
above them has been somewhat reshaped by glacial
and postglacial processes.
The saprolites on southern Disko are sandy to
gravelly with low clay content. The clay minerals
are dominated by smectite/vermiculite and contain
some kaolininte (Table 2). They are probably thin as
the maximum measured thickness is 8 m. A minimum
age for the Disko saprolites is given by the overlaying
basalt, dated to 61–59 Ma (Henriksen et al., 2000).
Characteristics and age determinations of saprolite
remnants in central and northern Europe have been
summarized by Lidmar-Bergström et al. (1999) and
Migón and Lidmar-Bergström (2001). They concluded that the formation of kaolinitic saprolites mainly
occurred in the Mesozoic and in the Paleogene, while
grus (gravelly) saprolites were formed during the
Neogene. The global climate in the Paleocene was
warm (Frakes et al., 1992). In Europe saprolites
formed during this period are clay rich and kaolinitic.
The Disko saprolites are not in agreement with the
saprolites in Europe, and are also different from deep
kaolinitic saprolites on Nuussuaq (Pulvertaft, 1979)
and in the Kangâmiut well, West Greenland (Bate,
1997; Fig. 1). The saprolite from the borehole onshore
central Disko in the Kuugannguaq valley seems to
have similar characteristics to the saprolites in the
study area (Gregers Dam, pers. comm. 2003). So
when did the saprolites form? There are two possibilities. First they may be the lower section of a more
clayey kaolinitic saprolite profile (Table 2). It is
known that the basement ridge rose to a high position
prior to the basalt flows, maybe triggered by early
rifting in the Labrador Sea and along the Ungava fault
line (Chalmers et al., 1999). Major uplift and erosion
occurred southeast of the study area in the early
Danian, and the latest Cretaceous was removed in
that area, followed by sedimentation coevally with
the major volcanism in West Greenland (Dalhoff et
al., 2003). Thus it is possible that this uplift also
caused a major stripping of a thick kaolinitic clayey
saprolite. The second possibility is that the old saprolite was completely stripped and a new saprolite
formed shortly before the basalt eruptions. If so, it is
possible that the saprolites only represent a short timespan of weathering immediately before the onset of
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
17
a)
Ice flow
b)
Ice flow
c) Ice flow
after Ljungner (1930)
Fig. 19. a) Gentle stoss-side and steep lee side topography (roche moutonée like forms) at the Qeqertarsuaq peninsula. Ice flow from right to left
(south). b) Steep stoss-side and gentle lee side topography at Fortunebay. Ice flow from right to left (south) towards Disko Bugt. These
photographs illustrate that the bedrock structures and the dipping joints control the development of detailed topography, not the direction of ice
flow. c) Ljungner (1930) also stressed the importance of structures for development of roche moutonées. Drawing after Ljungner (1930, Fig. 86)
(mirrored).
volcanism in a climate cooler than in the late Mesozoic and Early Paleocene (Frakes et al., 1992), thus
favouring sandy-gravelly weathering (cf. Migón and
Lidmar-Bergström, 2001).
5.2. Formation of clefts
References to clefts and their origin are sparse in
the literature. Clefts, very similar to those on Disko
(Fig. 12), are described from a granite dominated area
in southwest Sweden (Olvmo et al., 1999; Johansson
et al., 2001a). These clefts occur in an area originally
re-exposed from below a Mesozoic cover, and might
also have experienced a later weathering phase, indicated by sparse saprolite remnants of different character. In this heavily glaciated area weathering forms
in the clefts (weathered joints, tor pillars, interlocked
corestones) could be distinguished from glaciated
forms, which occurred in the upper parts, and signs
of wave polishing in the lower and at specific locations glacifluvial erosion.
The clefts in the southern Disko area are jointcontrolled. The detailed forms in the clefts, rounded
edges, pillars of interlocked corestones and weathered
joints are analogous to those weathering forms encountered at the gneiss/basalt contacts (cf. (Figs 11,
12, 14, 15)). Therefore, it seems probable that weathering occurred before the onset of volcanic flows, is
the main process for initiation of the clefts along
vertical joints. Glacial erosional forms are only encountered in their upper parts. All clefts occur below
the highest shoreline (~120 m a.s.l.) since the last
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
glaciation and polished quartz grains in the wall are
present in the lowermost parts of the clefts (Fig. 11). It
is possible that the polish on grains could have been
achieved by subglacial meltwater, but seems less likely as no indicative p-forms are present here. Sorted
sand is present in small hollows in the cleft walls.
Thus, the ultimate stripping of the saprolite that once
filled the clefts, most likely occurred by wave wash in
late glacial and post glacial time.
5.3. Hills, structural influence and origin
Weathering processes are known to exploit structural weaknesses and thereby enhance differences in
the bedrock (e.g., Thomas, 1994; Lidmar-Bergström,
1995; Johansson et al., 2001a). The abundant saprolite remnants in the area and clefts with many weathering forms clearly point to deep weathering as an
important factor for accentuating bedrock structures
and thus for formation of the hills. The extension of
hills is controlled by lineaments in certain directions.
In Fortunebay, the strike of the schistocity (ENE–
WSW) is the most important limiting direction and
in the Qeqertarsuaq area the NW–SE direction is of
major importance. Because these directions limit the
hills, they must have been present at the time for
deep weathering, before the basalt eruptions, and are
therefore old. The N–S lineament system has not
been important for the formation of hills and is
thus probably younger. The interpretation of the N–
S system here agrees with the interpretation by Klint
(in Japsen et al., 2002). Klint also interpreted the
NW–SE lineament system to be younger than the
basalts as both systems cut through them. However
based on the weathered structures it is concluded
here that the NW–SE system is older than the basalts
but must later has been reactivated, after the basalt
eruptions, creating the new, young fractures in the
basalt (Figs 8, 9).
5.4. Hills in a landscape context
Hilly relief terrains have been recognized in various part of the world in connection with deep weathering (Ollier, 1984; Lidmar-Bergström, 1988b;
Thomas, 1994; Ollier and Pain, 1996; Migón and
Lidmar-Bergström, 2001). In Scandinavia landscapes with hilly relief have developed in basement
rock from a primary sub-Cambrian peneplain,
which was re-exposed in the Mesozoic and deeply
weathered (Lidmar-Bergström, 1982, 1988a, 1989,
1995; Lidmar-Bergström et al., 1999). This hilly
relief is locally associated with deep clayey, kaolinitic weathering profiles along weakened structures despite heavy glacial erosion. The saprolites
and the Mesozoic covers are usually stripped, but
remnants occur in certain areas.
The hilly relief on southern Disko is a re-exposed
etch surface, as indicated by the following: 1) The
basement surface emerges from protective Paleocene
basalt, occasionally of sub-aerial type. It must therefore have been at the surface previous to the volcanic
flows, and have been protected since that time until
re-exposed by recent, mainly glacial erosion. 2)
Saprolites occur in connection with joints and structurally weakened zones. 3) Small-scale weathering
forms (tor-pillars, weathered joints, interlocked corestones) are present in clefts and have developed from
vertical and horizontal joints. 4) The extension of hills
is highly dependent on the direction and distribution
of lineaments, which indicates that weathering has
been of high importance for their formation. Twidale
and Bourne (1998) clearly demonstrated the connection between lineaments and extension of hills on an
etch surface in Australia. It is interesting to note that
in a detailed study of a medium sized hill in southern
Greenland, Glasser and Warren (1990 p.214) stated
bAt the scale of the entire hill, rock jointing does not
appear to be important in determining the effects of
glacial erosionQ. If this is true, glacial erosion does not
enhance the bedrock structures significantly, believed
to form the hill, and weathering processes might
instead be an explanation for the formation of the hill.
While the onshore hilly relief can be mapped with
accuracy, the extension of hilly relief offshore is more
uncertain (Fig. 1). Still, there are some arguments in
favour for a submerged etch surface, and at least
partly a re-exposed surface of pre-Cretaceous age in
the southern Disko Bugt. First, Cretaceous rock is
known to rest directly on the basement. At Kûk,
Nuussuaq (Fig. 1) the basement is deeply kaolinised
beneath a cover of Cretaceous strata (Pulvertaft,
1979), and the landscape north of Nuussuaq is characterised by exposed basement with a hilly relief
emerging from basalt (Pulvertaft and Larsen, 2002),
analogous to the hilly relief on southern Disko. Sec-
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ond, the amplitude of the hills offshore is similar to
the amplitude onshore southern Disko (Fig. 7). Third,
the geographical position of the basement ridge is a
direct continuation of the onshore area (Fig. 1), a ridge
which had been uplifted to a high position prior to the
basalt flows (Chalmers et al., 1999). Thus, there are
reasons to believe that the larger hills offshore in
southern Disko Bugt originated by deep weathering
mainly in the Mesozoic.
19
al., 1999 p. 158). The effect on the top basement
surface may include removal of basalt, saprolites,
corestones and maybe sheet slabs.
Offshore, glacial erosion has exploited fracture
zones and formed deep troughs in the southern
Disko Bugt (Long and Roberts, 2003), and has certainly contributed to the stripping of Cretaceous and/or
Palaeogene cover rock from basement areas. Despite
heavily glacial erosion the etch surface in-between the
troughs has not been obliterated (Fig. 1).
5.5. Effect of glacial erosion on the sub-basalt surface
The hills have occasionally been reshaped by glacial erosion, but the effect is highly variable, with
negligible erosion close the basalt, and with gradually
increasing erosion towards the coastline. Glacial erosion has not obliterated the initial form of individual
hills, or the general appearance of the hilly relief
landscape, but has resulted in asymmetric hills with
plucked cliffs, preferably facing towards SW. On the
bedrock surface, striae and stoss-side lee-side topography is common. The shape of these forms is mainly
guided by the joint systems (Fig. 19b, c), not the
direction of ice flow (cf. Ljungner, 1930 and Glasser
and Warren, 1990). These landforms with stoss-side
lee-side topography are therefore not necessarily a
mainly glacial erosional landform as often assumed
in formerly glaciated areas, but probably have been
inherited from previous weathered forms. Lindström
(1988) was of the opinion that the relation of joints
was decisive for local bedrock topography, confirming
the conclusions of Ljungner (1930). Within a glacially
scoured etch surface in western Sweden, joints were
found to be significant for the shape of stoss- and leeside topography and roche moutonnées (Olvmo et al.,
1999; Olvmo and Johansson, 2002). Similar conclusions on the relation between joints and landforms
have been made from studies in Greenland (Gordon,
1981; Glasser and Warren, 1990) and in Scotland
(Gordon, 1981; Sugden et al., 1992; Glasser, 2002).
Glacial erosion has been highly selective. The
weathered forms and joints at depth in the clefts
clearly show that glacial erosion has not affected the
bedrock in these protected positions. Along the upper
part of the clefts, pre-weathered joints have been
transformed by subglacial meltwater to P-forms
(e.g., Dahl, 1965; Fig. 11), a reshaping identical to
what is described from southwest Sweden (Olvmo et
5.6. Effect of post-glacial surface weathering and
wave action
Post-glacial weathering on fresh rock surfaces is
mainly minor, with a few centimetres as a common
value (Dahl, 1967; Lidmar-Bergström, 1997; André,
2002). On southern Disko this weathering has largely
destroyed the glacial polish on the top surface. However, remnants of associated weathering products are
sparse. Maybe removal of weathering products is a
result of wave action due to the position of most of the
area; below the highest shoreline. The sorted sand in
small hollows in the cleft walls, overturned pillars
within the clefts (Fig. 12) and the polished quartz
grains in the lowermost parts of the cleft walls, all
support the view of post-glacial wave action.
5.7. Implications for modelling the long-term evolution of large landforms
The landforms in the basement on southern Disko,
both in general and in many details, belong to an etch
surface, recently re-exposed from protective cover
rocks, with forms similar to other stripped etch surfaces in high latitude areas (e.g., Lidmar-Bergström,
1988a; Bouchard and Jolicoeur, 2000; Johansson,
2000; Johansson et al., 2001b). This is also the case
for the hilly relief identified offshore. However, the
etch surface of southern Disko has the advantage that
both the original forms and the glacially reshaped
details can be distinguished. The established connection between deep weathering and the hilly relief
landscape can be applied to other areas in West Greenland, where overlying geological sequences or weathering remnants are lacking. Hilly relief landscapes in
basement rocks seem to be indicative of long periods
with deep weathering exploiting the structures. It is
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
only in formerly glaciated areas that the saprolites are
almost entirely stripped and therefore these landscapes
are most conspicuous on the northern shields.
The identification of re-exposed sub-Paleocene
etch forms on Disko and also offshore with only
moderate glacial reshaping have implications for identification of exhumed etched relief to the south of
Disko Bugt and for the reconstruction of the general
landscape development in West Greenland. South of
Disko Bugt the denudation surface with hilly relief
merges from offshore Upper Cretaceous strata, and it
is slightly inclined (Bonow et al., in press). This
surface is cut off by a more horizontal summit planation surface, which thus is younger. The exhumation
of the hilly relief must have occurred after formation
of the planation surfaces, or else would the hilly relief
have been planated. This exhumation is probably
connected with tectonic uplift events during the Neogene, as documented in nearby areas to the south.
Thus, the identification of the hilly relief surface and
geological constraints of its minimum age provided a
valuable tool for deciphering the general landscape
development in West Greenland (Bonow et al., in
press). In this study we show that it is possible to
identify palaeosurfaces also in formerly glaciated terrain. Palaeosurfaces are a valuable tool for estimation
of glacial erosion and to reconstruct the geomorphological history over much longer time than generally
is thought (cf. Lidmar-Bergström, 1997).
representing the schistocity of the gneiss and NW–
SE fracture zones. These structures are thus interpreted to have been exploited by the deep weathering
while the frequent N–S lineaments have not and thus
are younger. Ice-flow from the NE has plucked the
SW sides of the hills and reshaped most of the summit
surfaces and eroded the bedrock knolls to moderate
depth. The resulting forms (stoss-side lee-side topography) are however more dependent on the sheeting
and joints than of the direction of the overriding ice.
Thus, these landforms, often assumed to be glacially
formed, may have been inherited from etch forms, as
they most probably are on southern Disko. The clefts
have mainly escaped glacial reshaping and show the
forms of the weathering front. The final stripping of
the saprolites is thought to have occurred by wave
action. The identification of re-exposed sub-Paleocene
etch forms on Disko and also offshore have implications for identification of exhumed etched relief to the
south of Disko Bugt and its relationship to younger
planation surfaces, and interpretation of tectonic
events. That it is possible to identify palaeosurfaces
also in formerly glaciated terrain makes it possible to
reconstruct longer geomorphological histories than
generally is thought. The re-exposed basement landforms on southern Disko are key features to understand pre-Paleogene landform development in West
Greenland.
Acknowledgement
6. Conclusion
Basement landforms on southern Disko are mainly
formed by deep weathering and subsequent stripping.
The weathering occurred prior to the volcanic flows,
which have protected the basement landforms and
remnants of saprolites until lately. The basement surface retains saprolites up to 8 m thick in close relation
to the cover rocks. Weathered joints, rounded
boulders, saprolite remnants, clefts and bedrock hills
clearly suggest an origin of the landforms at the
weathering front. The weathering has exploited weakened bedrock structures, thereby enhancing differences in the bedrock and forming the hilly relief
(etch surface). The hilly relief landscape continues
offshore in the Disko Bugt. The outline of hills is
governed by two lineament directions: ENE–WSW
I thank GEUS, Stiftelsen Margit Althins stipendiefond (KVA), John Söderbergs stiftelse, Andréefonden
(SSAG), Lagrelius fond and Department of Physical
Geography and Quaternary Geology for financial support. Siv Olsson, Lund and Vibeke Ernstsen, GEUS
made the XRD analysis and Hans Linderholm, Göteborg University, assisted the field work in 2003.
Arkisk Station provided logistic support. Peter Japsen,
Knud-Erik Klint, Fredrik Kromann Jensen (all GEUS)
contributed to the discussion during fieldwork in
2002. James Chalmers (GEUS) provided seismic profiles and knowledge of interpretation of seismic lines.
I also want to thank Karna Lidmar-Bergström, Clas
Hättestrand and Jens-Ove Näslund for helpful discussion and formulation of the paper. G.S.P. Thomas and
one anonymous referee provided valuable comments
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J.M. Bonow / Geomorphology xx (2005) xxx–xxx
and suggestions. The digital elevation model was
made available by KMS, Copenhagen.
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