17
The genesis of tills from Åstadalen, southeastern Norway
SYLVI HALDORSEN
Haldorsen, S., The genesis of tills from Åstadalen, southeastern Norway.
Vol. 62, pp. 17-38. Oslo 1982. ISSN 0029-196X.
Norsk Geologisk Tidsskrift,
lee movements in Åstadalen originated from the north and later from the northwest. Most of the area is
covered by a 1-5 m thick till which does not create independent landforms. In the mountainous areas, this
lill is local and coarse-grained and iriterpreted as a subglacial melt-out til!. Here the glacier was probably
rather passive when the till was formed. A complex till occurs along the eastern valley side. At typical lee­
side localities, the local coarse-grained till is most probably of a subglacial melt-out origin. Along the
western valley side and western upland area, the till is everywhere rather fine-grained and compact, and
cobbles and boulders are abraded. :Jost parts of this till were formed by a lodgement process. Transverse
moraine ridges, mainly occurring along the western part of the valley, are composed of till which is
interpreted as subglacial melt-out til!. Some probably supraglacial material occurs at the top. Hummocks
consisting of diamicton are found, particularly along the eastern side of the valley. These are composed
mainly of earlier deposited subglacial till, which was subsequently transported on to the surface of the
valley ice from ice-free valley sides. Most of the till material in Åstadalen is therefore believed to
originale from basally transported glacial debris.
S. Haldorsen, Department of Geology, Agricultural University of Norway, N-1432 Ås-NLH, Norway.
The classification of till related to its genesis has tion proposed by Shaw (1979a) for the INQUA­
been a subject of broad interest for glacial geolo­ Comrnission on Genesis and Lithology of Qua­
gists during the last ten years, greatly stimulated ternary Deposits.
Subglacial till is formed by subglacial deposi­
by the work of INQUA-Commission on Genesis
and Lithology of Quatemary deposits (see Drei­ tion of basal debris (Fig. 1). Lodgement till is
manis 1976, 1979, 1980). The commission has deposited by lodging of material from basal slid­
proposed different detailed tentative classifica­ ing ice (e. g. Boulton 1976, 1978). Friction
tion systems for tills (see Dreimanis 1976, Fig. against the bed forces the clasts to be lodged one
. by one, and by a plastering on they are gradually
5).
In Norway, tills have for a long time been embedded in more fine-grained till matrix. Sub­
divided into two genetical groups; basal till and glacial melt-out till (Fig. l) is deposited by a
ablation till (Holtedahl 1953, pp. 754-58, G. melt-out process. Obviously also the lodgement
Holmsen 1956). The first type refers to deposi­ process involves subglacial melting (Goldthwait
tion of material from the basal part of ice and the 1971) and should logically form one part of the
second to deposition of supraglacial material. subglacial melt-out tills. However, the processes
The same division is still used today (e. g. Søren­ of lodgement are assumed to form tills with char­
sen 1979a, Sveian 1979, Vorren 1979). In many acteristics different from tills deposited by a pure
cases it is difficult to distinguish between these melt-out (see Boulton 1976, 1978). In this paper
two till types in Scandinavia, because varieties of subglacial melt-out till therefore refers to tills
basal tills may be so similar to varieties of abla­ which were deposited where there was no lodg­
tion till (see descriptions and discussions in e.g. ing from sliding ice, i.e. from zones where the
glacier was totally or basally stagnated (Shaw
Hoppe 1963, J. Lundqvist 1969a).
The study described here was carried out in 1977a, 1979a). Subglacial melt-out till may form
Åstadalen, southeastern Norway. The aim has beneath frontal parts of mainly cold-based gla­
been to try a detailed genetical interpretation eiers (Shaw 1977a), beneath thick temperate gla­
based on morainic morphology, till texture, and eiers in zones of basal stagnation because of fric­
till structure. Since there is no single generally tion against the substratum (G. Lundqvist 1940),
accepted genetical classification system of tills, it or by melting of debris-rich basal ice remnants
is necessary to define the terms used in this paper during the final part of a deglaciation phase
(Fig. 1). They are mainly based on Boulton (Mickelson 1971).
Supraglacial till is formed by debris which oc(1976) and Dreimanis (1976), and on a classifica2 - Geologisk Tidsskr. 1182
18
NORSK GEOLOGISK TIDSSKRIFT l (1982)
S. Haldorsen
Genetic classification of tills
Position of
transport
l
l
Basal
l
l
Process of
deposition
•
•
Lodgement
Mett- ou_t
Genetic term
Lodgement
till
Subglaci
melt-out till
l
l
Fig.
1
J
Subglacial till
l
�
l
Englacial
l
1_
Mett -out
J
l
Supraglacial
l
l
_l
Lowerin g
Flow
Supraglacial
melt-out till
L
J
l
J
Flow till
J
_l
J
Supraglacial till
l. Classification system for tills. Based on Dreimanis (1976), Boulton (1976) and Shaw (1979a).
curred at the surface of the ice and was deposited
as till by a lowering when the underlying ice
melted away (Fig. 1). Supraglacial melt-out till is
formed by debris which was mainly transported
englacially before it occurred in supraglacial po­
sition. In some cases debris has been transported
totally in supraglacial position without any incor­
poration in the glacial ice and has then been
deposited by a pure lowering (Fig. 1).
Before and during deposition the supraglacial
debris has in most cases slid or slumped and been
retransported as sediment flows (Lawson 1979),
and the sediment may then be classified as flow
till (Fig. l) (e.g. Boulton 1971).
In cases when till melted out from debris-rich,
basal ice remnants, the melting may have oc­
curred both from above and below (e.g. Lawson
1979) and the distinction between sub- and su­
praglacial melt-out is without significance. The
till may then be referred to as basal melt-out til!.
It was realized long ago that the properties of
tills in Scandinavia are clearly related to the to­
pography of the substratum (G. Lundqvist 1940).
There is further a clear relationship between till
genesis and the related morainic morphology
(see general discussion in Sudgen & John 1976,
pp. 235-257). The following descriptions are re­
lated to the main topographical position of the
tills and to the main morainic morphology. Till
genesis is discussed for each topographical or
morphological group separate ly.
Methods
The content of boulders and cobbles in till sec­
tions was usually estimated visually. A few sec­
tions were thoroughly cleaned and the areas oc­
cupied by boulders and cobbles were measured.
Both visual and areal methods give rough estima­
tions only.
Material finer than 64 mm was analysed by the
sieving and hydrometer method.
Standard samples were applied for field deter­
minations of roundness, and roundness index
was calculated according to Krumbein (1941).
Long-axis orientation was measured for parti­
eies with a-axis > 4 cm > 2b-axis. Fabric data
were evaluated statistically by eigenvalue meth­
od (Mark 1973, Lawson 1979, Haldorsen & Shaw
in press). Computer programming was per­
formed at the University of Alberta, Canada.
Regional position
The study was carried out in the Åstadalen area,
which Iies between the eastern Norwegian main
Key map showing the position of the investigated area
(framed). Bedrock geology after J 0 Englund (1973, pers.
comm. 1980) and J. P. Nystuen (pers. comm. 1980). Direction
of ice movements after G. Holmsen (1960, Fig. 2). Numbers
give height above sea leve! in metres. The stippled line on inset
map shows the maximum extension of the Weichselian ice
sheet, and the black square the position of the main map.
Fig. 2.
-
.
.
Genesis of ti/Is
NORSK GEOLOGISK TIDSSKRIFT l (1982)
�
D
o
5
10
15 km
1 1 1111111111
Brøttum Formation
Mainly other Late Precambrian
sedimentary rocks
Position of ice devide
ll
11
19
20
S. Haldorsen
NORSK GEOLOGISK TIDSSKRIFT l (1982)
5km
Contour 1nterval 1OOm
LEGEND
[2J
�::::::J
�
�
D
Exposed bedrock
Boulder fields
Thin and discontinuous
cover of till
}
Continuous
Cover
moraine
cover of till
Glacial striae, movement
towards observatiOn point
Glacial striae, different ages
older' short , younger, long
nn
Hummocky terrain
�IN
v
Moraine
•
ridges
Sinall bedrock exposure
V
v
V-"-
v_
Fig. 3. Map of the moraine types, distribution of till and glacial striations in Åstadalen. Partly based on Østeraas (1978, 1982).
Sediments other than till cover only a minor area and are omitted.
NORSK GEOLOGISK TIDSSKRIFT l
(1982)
Genesis
of tills
21
Faults and prominent joints
\�
\\
\
Bedrock structures
in Astadalen < Englund 1979 l
------- lithological boundary
l.U.L.L.LI.l -----
-'-
50-90
�
Sjusjøen
..,...
Cleavage
.....t...
- - --
Fig.
Strike and dip of bedding
90·1009
-+-
O
}
Minor thrust plane
0-50g
g
..L
1
Prominent joint
2
3
4
5km
'(.
l photo-geological data l
l
-....
5%
\
\
l
-
4. Bedrock geology in Å stadalen. Area A comprises sandstones with
T \/
;\
\
l
\
shale horizons, area B comprises sandstones with
some conglomeratic beds. Boundary between A and B is shown by thick line.
valleys of Gudbrandsdalen and Østerdalen (Fig.
2).
The river Åsta drains into the river Glomma
in Østerdalen. The investigation was concentrat­
ed in the catchment area of the Åsta north of
Bjørnåsbrua (Fig.
about
down
2).
The valley bottom Iies at
800 m a.s.l. in its upper part
to 620 m a.s.l. at Bjørnåsbrua,
Bedrock
The bedrock consists of Late Precambrian sedi­
mentary rocks belonging to the Brøttum Forrna­
tion (Englund
1972, Fig. 17) which in the south is
and slopes
composed of rather homogeneous sandstones,
and is thus
and in the north of sandstone alternating with
considerably higher than the floor of Gudbrands­
thin beds of sil ty shale (Figs.
dalen and Østerdalen (see numbers in Fig.
Brøttum
2).
Åstadalen is surrounded by mountain plateaus
to the east and northeast with an average eleva­
tion of
1100
800--1000
m a.s.l. and peaks up to about
m, to the north by a mountain range with
heights from
1000
to
1200
m a.s.l., and to the
Formation
other
2 & 4). North of the
Late
sedimentary rocks occur (Fig.
2),
Precambrian
predominantly
with a composition similar to the Brøttum For­
mation
(J.-0.
Englund, pers. comm.
1980).
The
strike of the bedrock is generally E-W (Fig.
4).
The bedding and the main regional fractures in
west by upland areas with mountain peaks up to
NW-SE and NNE-SSW direction strongly con­
about
tro! the morphology.
1050
m. The valley is fairly wide with
gentle side slopes (Fig.
3),
the northeastern val­
Above
11-1200
m a.s.l., the bedrock is inten­
ley side being somewhat steeper than the south­
sively fractured by postglacial frost cracking, and
western one.
the surface is here partly covered by boulder
22
S. Haldorsen
NORSK GEOLOGISK TIDSSKRIFT l (1982)
and the same direction was observed at some
other places in the Østerdalen region (Fig. 2) (G.
Holmsen 1960, Fig. 2). The dominating striation
in the area reflects a glacial movement from the
northwest (Fig. 3), which is also dominant other
places in the Østerdalen region south of the ice
divide (Fig. 2) (G. Holmsen 1960, Fig. 2). Cross­
ing striations in the mountains (Fig. 3) show that
this movement is younger than the one from the
north.
Fig. 5.
Typical boulder field at the mountain Hirnmelkampen.
fields (Fig. 5). Such fields are common in the
central part of Scandinavia and have been de­
scribed by G. Holmsen (1960), G. Lundqvist
(1951, pp. 81-82), and J. Lundqvist (1969a, p.
119). In many places the boulders Iie in situ and
each separate sandstone bed can be traced across
the boulder fields.
lee divides and ice rnovernents
Data of G. Holmsen (1960), P. Holmsen (1951,
1964), and Vorren (1977) indicate that the Åsta­
dalen area lay only a few tens of kilometres south
of the ice divide during parts of the Weichselian
(Fig. 2). and for some time even beneath the ice
divide (P. Holmsen 1951, Vorren 1977, Fig. 3).
The directions of ice movements are interpret­
ed from striations. A north-south direction was
found at several places in the mountains (Fig. 3)
Morainic rnorphology and related till
types
Cover moraine
Most of the area is covered by till of thickness 15 m. No morainic landforms parallel with the
older glacial movement from the north were ob­
served. It is difficult to determine if some of the
small-scale topography reflects independent mo­
raine forms, but an impression from air photos
and field interpretations is that the northwest southeast trending morphology mainly reflects
the bedrock structures (Fig. 4).
Thin till covers like the ones in Åstadalen are
common in Norway. The related morphology is
classified as ground moraine (see Holtedahl
1960, pp. 389-403, G. Holmsen 1954, p. 50,
1960, p. 62) as, for example, in Finland (Aario
1977, p. 88). However, if the definition given by
Flint (1971, p. 199) is applied, where a moraine is
defined as 'possessing initial constructional form
independent of the floor beneath it', then such
forms are usually lacking in Åstadalen. The till
cover found here does not properly create mo­
raines but is merely a part of a drift sheet. Aario
(1977, p. 88) proposed the term 'cover moraine'
for the morphology within areas where the im­
mediately underlying bedrock is reflected. Al­
though this term is not in accordance with the
definition of moraine used by Flint (1971, p.199),
it applies very well to Åstadalen and is therefore
used in this paper.
Related to its topographical position, the cover
moraine can be divided regionally into three
areas: (l) the mountainous areas in the north and
east, (2) the northeastern vally side, and (3) the
southwestern valley side and the western upland
area (Fig. 3).
Mountainous areas.
6 Cover moraine at Øyungenfjell with characteristic coarse
and angular surface material.
Fig.
- The mountain areas in
the north and east are generally characterized by a
thin till cover and frequently exposed bedrock
Genesis of tills
NORSK GEOLOGISKTIDSSKRIFT l (1982)
23
Boul ders • cobbles
(>64nvn)
•
Mounta!n plat&au
+
Eastern vo lley stde
o
Western volley side
and uptand area
50
Cio y
Grave! L_----"----"Sand•Silt+Qoy
(64-2mm)
50
l
<
2mm )
�
� 20
�
�
Eastern volley side
Eastern valtey side, total
Lee side l ocalities
N:
32
10.W=�
Cloy
Northwestern volley side
-4
16
-3 -2
8
Gravol
4
-1
2
o
1
1 2 3
0.5 0.25
Sand
5
�
6
7
6
9
125 64 32 16
8
4
2t-�m
mm
4
'
l
'
Silt
-4
16
l
Cloy
Groin·size
-3 -2 -1
o
4
1
8
Gravet
2
1
2 3 4 5
'
Q5 Q25mm '
125 64 32
Sand
l
�
6
7
6
9
16
8
4
2 �m
Silt
Grain·size
l
Clay
Fig. 7. Grain-size distributions of different till types from the cover moraine area·s. Shaded: one standard deviation from the mean
values. LettersA-E in the triangular diagram have the same meaning as the lettersA-E in the histogram.
(Fig. 3) with continuous cover moraine found in
depressions only. The bedrock topography is
mainly irregular on both stoss and lee sides. Such
an irregular topography probably existed even
when the ice melted away, but in many places the
present surface may also be the result of postgla­
cial frost cracking.
Frost and slope activities have obviously al­
tered the original till characteristics in several
places. Boulders from boulder fields may by ta­
lus activity have moved onto the moraine surface
and may even have been mixed into the till mate­
rial, as described from Sweden by J. Lundqvist
(1969a, p. 119). Frost heave and solifluction have
also destroyed the original till structures, degree
of compaction, till fabric, and granulometric
composition. However, by excluding all localities
where such supposed secondary influences have
occurred, it is possible to recognize some typical
original characteristics of the mountain plateau
tills. Both on a regional and vertical scale the till
is rich in boulders, cobbles, and grave! (Figs. 6 &
7). The grain-size distribution of tills covering
the sandstone/shale bedrock in the north is simi­
lar to that of tills covering the more homogene­
ous sandstone bedrock in the south. This is partly
because the content of shale in the bedrock is
small compared with sandstones and partly be­
cause neither the shale nor the sandstone have
been intensively comminuted.
The significant content of silt ( - 20 %) relative
to sand ( - 80 %) and the general Jack of sand
lenses indicate that the low matrix content
(sand+ silt+clay) is mainly a primary property
and not the result of removal of fine-grained
material by melt-water. The coarse-grained tex­
ture therefore shows that glacial comminution
beyond grave! size has been small and that the
relative supply of coarse clasts was always great.
An important factor here is the deeply and inten­
sively fractured bedrock which certainly also ex­
isted prior to the last glaciation. Such material
was easily incorporated into the glacier and pro­
vided a great supply of coarse material.
24
S. Haldorsen
NORSK GEOLOGISK TIDSSKRIFf l (1982)
.,.
B
60
Eostern
vo lley side,
lee-side
locoliti es
Mountoin
area
n =13
Eostern
volley side,totol
p =0.16
n= 5
p= 0.15
p
5 = 0.04
s = 002
s = 0.07
02
06
n =15
Q2
06
0.4
= 0.18
0.6
.,. r------,
O
60
Western
volley side ond
BOULDERS
...�...
...
uplond area
·.
n =12
p =020
A
20
: \
c ' ,-:- .... .
,
�
'\...,
.
,,
:
0.1
Roundness
\
-·-·�.: '
s :002
02
0.3
0.4
P
0.5
0.6
0.7
0.8
Roundness
P
Fig. 8. The degree of roundness for til! clasts from the cover moraine areas. Histograms A-D: gravel fraction 16--32 mm. Stippled:
one standard deviation from the mean values. n number of samples, each sample consisted of minimum 100 particles. p=mean
roundness after Krumbein (1941), s=standard deviation of P.
The boulder data are based on about 500 boulders from each lill type, letters A-D have the same meaning as in the histograms.
=
Some rounded and striated clasts occur, but
ation in it. Finally, there is general lack of melt­
generally angular material is quite dominant
water channels, glaciofluvial material, and dead­
(Fig. 8). The general lack of abraded clasts even
in the deepest part of the til!, and the very low
ice topography with hummocks and ridges
(Østeraas 1978, 1982). Thus there is no indica­
roundness index are most certainly primary char­
tion of a lowering by melting of underlying dean
acteristics of the till material.
ice or of the presence of great amounts of melt­
The high contents of coarse and angular mate­
rial (Figs.
water.
8) indicate a melt-out origin. The
The till may have been formed in the following
clasts could hardly have avoided abrasion if they
way (Fig. 9): Boulders and cobbles from the
were overridden by an active sliding glacier from
intensively fractured bedrock were incorporated
the time they were Jodged until they were em­
into the glacier and transported a short distance
7&
bedded in till material.
in basal position. Some internal movement in the
Several characteristics indicate that the glacial
ice, because of shearing or plastic deformation,
debris was carried mainly in a basal position.
may have caused the clasts to collide or be
First, it is of a very Jocal origin. This is, for
pressed against the substratum. Matrix material
instance, clearly visible near the boundary be­
was formed by the grinding of small clasts
tween the sandstone with shale in the north and
between bigger ones or between bigger particles
the pure sandstone in the south, where the con­
and the bedrock. Fine-grained material was also
tent of shale ends abruptly at the bedrock bound­
formed by abrasion, but this did not significantly
ary. Furthermore, the described till rests directly
change the degree of roundness. It should be
on the bedrock and except for the effects from
emphasized that the till is rather thin. lts forma·
frost heave there is no systematic vertical vari-
tion may well be related to the period of decreas·
NORSK GEOLOGISK TIDSSKRIFT l (1982)
MOUNTAIN AREA
Genesis of tills
NORTHEASTERN VALLEY SIDE
25
SOUTHWESTERN VALLEY SIDE
ACTIVE ICE
STAGNANT ICE
(DEGLACIATION)
\
Subglaclal
mett-out til!
Oebris
rich
ba .. l ice
Lodgement
til! 1
Melt-out Ull
Supraglacial
sediment•
Y
SITUATION
AFTER THE
Lodgement
tiH?
DEGLACIA TION
.Y�
-·�···�
/;<
lee-side
melt-out lill
Local
.
;o.��
/
/
Subglaclal
mett-out tlll
>
1
Mainly
lodgement
lill
Fig. 9. Models of lill formatiDn in Åstadalen.
ing glacial activity, a short time before the final
deglaciation.
Zones with more abraded clasts occur mainly
along same of the marked depressions, for in­
stance at Lyngen and along the upper part of the
river Skalla near Skollfjellet (Fig. 3). In such
cases, the abraded particles occur together with
quite angular material. One component has been
transported along the base of active sliding ice
for a relatively lang time. Befare or during the
deposition it has been mixed with very local and
angular material.
Northeastern valley side. The topography along
the northeastern valley side is variable. It is rela­
tively steep along the Øyungenfjellet and Hynnlia
(Fig. 3), and much more gentle where the tribu­
tary valleys join Åstadalen. The exposed bed­
rock surfaces are in same places irregular and
show signs of glacial plucking, in other places
there are more abraded surfaces (e.g. the great
bedrock exposure SW of Øyungen (Fig. 3 ) ).
Along the upper part of the valley side the bed­
rock exposures are most frequent (Fig. 3), and
the till thicknesses, obtained by drilling, do not
-
usually exceed 3m. The lower parts of the valley
side have a more continuous till cover of thick­
ness 2-5m.
A characteristic till occurs at localities which
were in lee-side positions of both the older and
younger ice movements (Fig. 3). An excavation
down to l.S m west of Øyungenfjellet (Fig. 3)
exposed a loose and nearly structureless till. Two
fabric measurements carried out at Øyungen
(Fig. 3, locality l) and Bjørnåsen (Fig. 3, locality
2) showed no strong preferred a-axis orientation
(Fig. lOA & B). The frequency of surface boul­
ders is usually high at lee-side localities, and
sections and numerous drillings indicate that
boulders and cobbles are also abundant deep in
the till (Fig. 7). The content of gravel is usually
comparable with that of sand, and the content of
silt + clay is very low (Fig. 7B). The total clast
material has a high degree of angularity (Fig. 8).
The lee-side till shows clear evidence of a local
source; in particular the angular clasts can in
most cases be traced directly to the nearest bed­
rock exposure.
At one locality southeast of Øyungenfjellet (Fig.
3, locality 1), this till type was studied in more
S. Haldorsen
26
NORSK GEOLOGISK TIDSSKRIFT l (1982)
Cover moraine, eastern valle y side
A. Loe. 1
N
Øyungen
C. Loc.4
B. Loc.2 Bjørnåsen
N
270
90
N
.
.
·'
270
Øyungsåa
270
90
+
.
.
90
+
..
180
"
•
324
51• o.s8
180
p
•
s
" : 324
n = 80
51• O.S6
180
p
•
6
" • 322
n = 80
p
51• 0.68
•
4
n = 50
Fig. 10. Long-ruds fabric of clasts from the eastern valley side. The site of localities l, 2 and 4 is shown in Fig. 3. n = number of
particles, A and P give the azimuth and dip of the eigenvector V1 in degrees, S1 gives the strength of clustering around the mean
ruds. Projection: Schmidt's net, lower hemisphere.
detail (Fig. 11) . Close to a steep and irregular
bedrock wall there was an abundance of big an­
gular boulders of which most had been glacially
transported. About SG-- 100m further south, the
material was gravelly and sandy and appeared
A
structureless. Some big angular boulders oc­
curred at the surface (Fig. 12) . At several other
lee-side localities there was a similar distribution
of material. In some cases the till was very grav­
elly for some distance from the headwall, where-
Direction
of
ice movement
2
B
32mm
%
75
50
25
'
'
'
1
2mm
63pm
c
%
....
.... ...?
....
�
...
40
....
....
....
..........
20
....
Silt
Grain-size
o
1
...,
Gra vel
16-32mm
1\
:\
1
:\2
1\.
.2
.3
.4
Roundness P
Fig. li. A. Sketch of the lee-side locality l south of Øyungenfjellet (Fig. 3). 1: just south of the headwall, 2: 50 m further towards
south. B. Grain-size distribution, l m depth. C. Roundness of grave!, l m depth.
Genesis of tills
NORSK GEOLOGISKTIDSSKRIFr l (1982)
as at other places, as for instance near Skvaldra
(Fig. 3, locality 3), it was more sandy. The sandy
or gravelly till is not believed to be the result of
removal of silt + clay by melt-water. The sam­
ples were taken some distance from the visible
melt-water channels, and the till was homogene­
ous without lenses of sorted material.
The till texture at the lee-side localities is con­
cluded to be the result of local glacial erosion,
transport, and deposition. The coarse material
has been eroded by the steep parts of hill sides,
where the fractured bedrock was exposed to gla­
cia! plucking. Afterwards, when the big boulders
were transported a short distance away from
their source area, they might have been pressed
against bedrock or underlying drift material to
form effective grinding tools. In most cases this
results in a gravelly material, but in some cases
also in a sandy one.
In this way the lee-side till has the same char­
acteristics as the mountain plateau till. lts coarse­
ness and its angular clasts indicate that it is a
melt-out till. The Jack of melt-water sorting and a
local provenance indicate a basal transport.
It is difficult to determine whether a comminu­
tion like that described above occurred before,
during, or after deposition. The pressure from
big lee-side boulders incorporated in the ice may
have gradually crushed the already deposited un­
derlying material. Such a crushing may explain
why the particles in the till have a rather broad
scatter of a-axis orientation. The big boulders on
the top then represent the last, uncrushed mate­
rial which was finally deposited when the ice
melted away.
The typical lee-side till is regarded as
representative for the initial stage in till forma­
tion along the northeastem valley side (Fig. 9).
Along most parts of the valley side, particularly
along its lower parts, a much more silty, compact
till type occurs with abraded boulders and cob­
bles. At some localities there is a marked a-axis
orientation parallel with the last direction of ice
movement (Fig. lOC). These characteristics indi­
cate a significant transport along the base of an
active glacier and the deposition has probably
mainly been by a lodgement. In areas where such
material dominates there are local occurrences of
coarse till with angular clasts (Fig. 13). In many
cases there is also a mixture of quite angular and
abraded clasts. It is very difficult to find the
boundary between the different types in the field
and to classify them genetically.
27
Fig. 12. Local lee-side boulders south of Øyungenfjellet, at
locality 2, Fig. llA.
Southwestern va/ley side and western up/and area.
The topography along the southwestem valley
side and the western upland area is rather
smooth, and the valley side is gently sloping
towards the northeast. There are no marked tri­
butary valleys or any steep mountain sides which
have a lee-side position during either of the two
ice movements (Fig. 3). The few exposed bed­
rock surfaces are smooth and striated. Based on
drilling, the till thickness is estimated to be on
average 3-5 m.
The road cuts show a compact till with a dis­
tinet fissility. The till is massive except for some
thin silt-rich horizons, which occur in particular
at the distal sides of boulders.
At most localities there is a strong a-axis orien­
tation parallel to the last direction of ice move­
ments (see locality 5-8, Figs. 3 & 14). The dip is
towards northwest (Fig. 14 A-D) and is easily
-
Fig. 13. Local, coarse tillwith angular clasts from an areawhere
much more abraded material dominates. Photo taken south­
east ofØyungenfjellet.
28
NORSK GEOLOGISK TIDSSKRIFT l (1982)
S. Haldorsen
Cover moraine, western valley side
1 -2m depth
A. Loc.S Hamarseter
8. Loc.6 Olshølen
N
N
90
270
+
270
90
.
180
A
•
s1.
�
·
180
329
p •
7
/(
0.72
n •
60
s1=
•
314
p •
6
o.73
n
60
•
C. Loc.7 Steinstilen
D. Loc.8 Svartåsen
1 .2- 1 .4m depth
1 .3- 1 .5m depth
N
N
.
.
..
.
.
.
.
90
270
+
270
90
.
.
180
A
•
s ·
1
180
p •
322
n
0.66
.
.
329
8
A
81
s1"' o.11
E. Loc.7 Steinstilen
> 2m depth
p •
6
n
60
•
F. Loc.8 Svartåsen
2.0-2.2m depth
2, 1 -'2.3m depth
N
N
'
.
.
..
+
270
180
A
•
s1·
.
.
·
.
.
+
270
90
Fig. 14. Long-axis fabric of clasts from the
180
7.5
P = 10
0.61
n =
80
A =
345
s, = 0.64
recognized in sections (Fig. 15). At two localities
(locality 7 & 8, Fig. 3) the clasts in the lowermost
part of the till show an a-axis orientation in N-S
direction with a dip towards north (Fig. 14 E &
F) . This probably reflects the early direction of
_
P= 13
n
= 80
till along the western valley side ( site: see
Fig. 3). For further explanation: see Fig. 10.
ice movement. The upper part of these tills con­
tains particles with the regular NW-SE orienta­
tion.
On average the content of silt is considerably
higher in the till along the western valley side
Genesis of tills
NORSK GEOLOGISK TIDSSKRifT l (1982)
Fig. 15. Imbrication of clasts in subglacial till from Olshølen. lee
movement from the right.
than in other till types
(Fig. 7).
The gravet is
29
Fig. 16. Typical bullet-nosed boulder from subglacial lill, Kvar­
stadseter.
typical for a lodgement till. In the present case
more rounded than in the two other cover mo­
they reflect a transport along the base of a sliding
raine types
glacier, during which it is reasonable that lodge­
( Fig. 8),
and the boulders and cob­
bles are generally more abraded and striated.
ment was the dominating way of deposition.
Many of the boulders have a typical bullet-nosed
There is, however, a question of whether these
shape ( Fig.
characteristics are diagnostic of the lodgement
16).
The till characteristics prove that the material
process, as it was defined in the introduction.
has been transported in a zone with intensive
The bullet-nosed boulders may have been
glacial abrasion and crushing and indicate that
shaped and orientated before they were deposit­
the ice was debris-rich. The particles have been
ed. Boulders in the area commonly have a trian­
in frequent contact during the transport. As the
gular shape with a distinct long axis even in their
till seems in all places to rest directly on the
primary stage. During the glacial transport such
bedrock, the transport has been interpreted as a
boulders were probably rapidly orientated so
basal one.
that they afforded the smallest possible resist­
A marked fissility, strong ·a-axis orientation,
ance to glacial movement: that is, with their
abraded clasts, a high proportion of fine-grained
snout pointing upstream. The proximal part may
material, and bullet-nosed boulders with sharp
truncated distal end (Fig. 16) as described above,
are characteristics which for instance Boulton
(1976, 1978)
and Dreimanis
(1976)
Direction of ice movement
regarded as
have been abraded and become bullet-nosed dur­
ing transport. The distal, angular end is then not
necessarily the result of a lee-side excavation as
described by Boulton
8
125
'lb
.5
2
l
.25mm
(1978),
63
32
but rather the
16
8
4
2}Jm
"' 100
O>
"'
c
"'
o
:;;
60
40
·;;
:;:
--
2 ------
,;:O>
c.
1
80
3 .........
4---
20
-1
o
2
Sand
3
4
j
5
6
Silt
7
8
9
l
�
Cl;�y
Grain-size
Fig. 17. A. Sketch of an abraded boulder in lodgement till at Hamarseter. B. Grain-size distribution of the material finer than 2 mm
around the boulder.
S. Haldorsen
30
NORSK GEOLOGISK TIDSSKRIFT l (1982)
B
A
%
60 ,-------�
40
20
- ...
'
'
'
'
0.2
0.4
0.6
Roundness P
ca
100m
Fig. 18. A. Sketch of the transverse moraine ridges at Åkerseter. B. The degree of roundness of grave( fraction 16--32 mm from site
l in the lateral part of the valley and site 2 in the middle of the vaUey.
original surface of the boulder which pointed
downstream and therefore avoided abrasion.
During the final part of the deglaciation a melt­
out till may have been deposited from remnants
The fine-grained till texture does not prove
of debris-rich basal ice. This may form the upper­
that lodgement took place. Most of the matrix
most part of the till. In that case it has been
material (sand+ silt+ clay) was probably formed
strongly influenced by secondary soil processes
by intensive glacial comminution of the clasts
and is no longer readily identifiable. Therefore,
during the transport. Boulton
some characteristics which are diagnostic of the
as the till in the west is so uniform, the total till
has here been interpreted as a lodgement till
lodgement till alone. One of these was that the
(Haldorsen
(1978)
described
1981).
lodged boulders have an upper surface which is
Commonly also the upper surface is more uni­
comparison between the northeastern and the
southwestern valley side. - The till is generally
formly striated, in a manner which clearly re­
thicker and much more continuous along the
flects the last direction of ice movement. Similar
southwestem than along the northeastem valley
characteristics are found several places along the
side (Fig.
southwestem valley side.
gentle parts of the northeastem valley side. The
significantly more abraded than other surfaces.
A
3).
This applies even for the rather
In some cases, the till just above, and at the
difference may be explained in the following
lee-side of boulders, is particularly rich in silt
way: During the early glacial phase with mave­
(Fig.
This may be explained as a deposition
ment from the north, the northeastern valley side
of fine-grained material formed by abrasion of
was a lee-side area while the southwestern side
17).
the boulders. Such a concentration of silt could
was a stoss-side area (Fig.
hardly occur during the transport.
topography along parts of the northeastern valley
As
the
3).
The step-wise local
diagnostic characteristics described
side promoted entrainment of material by glacial
above are only observed in special cases, they do
plucking and regelation. Part of this material was
not prove that the entire till was deposited by a
transported across the valley and upwards along
lodgement. However, as pointed out above and
the southwestem stoss side, where a basal melt­
also in the introduction, deposition of lodgement
ing dominated. Material originating from the
till must have predominated as long as the glacier
northeastem valley side was in time lowered to
in the valley was active. A pure melt-out process
the zone of traction, and parts of it may have
was then probably only of more local occurrence.
been deposited as a lodgement till. Net erosion
Genesis of ti/Is
NORSK GEOLOGISKTIDSSKRIFT l ( 1982)
.,.
40
A Glaciofluvial
,.--...,
l
l
l
l
short transport
.,.
40
B Glaciofluvial
long transport
22
0.47
= 0.05
n = 18
n=
0.32
s = nos
p=
p=
20
Roundness
5
20
p
31
Roundness
p
Fig. 19. Roundness of gravel16-32 mm from glaciofluvial sediments inÅ stadalen. A: Kames and local eskers, B: main eskers and
terraces along the central part of the valley.
probably occurred along the northeastem valley
side and less till was deposited there than to the
west.
During the late glacial phase, material was no
longer transported from the northeastem to the
southwestem valley side because the flow was
then parallel with the valley. New till material
was certainly formed and deposited along both
valley sides. However, the initial difference be­
tween the two valley sides may explain why there·
is more till in the west today.
Melt-water activity is another important factor
in this connection. Much more melt-water oc­
cured along the northeastem than along the
southwestem valley side. A great amount of de­
posited till and glacial drift was eroded and rede­
posited as glaciofluvial material in the vicinity of
the northeastem slope.
The bedrock structures have strongly controlled
the behaviour of the glacier in Åstadalen. The
strike of the bedrock is mainly east-west and the
dip is towards the north along the central part of
the valley (Fig. 4). In combination with the main
fracture zones (Fig. 4) this is responsible for the
steep and irregular topography along the north­
eastem valley side. When the glacier flowed
from the north and down into the valley it bad to
pass from one fractured sandstone bed to an­
other along the valley side. A uniform basal
sliding was therefore not established. Along the
southwestem valley side the ice could more or
less follow the surface of one sandstone bed for a
long distance. This allowed a more uniform flow.
In combination with the lee-side plucking and
stoss-side abrasion (Boulton 1970, 1971) it gave
favourable conditions for formation of lodge­
ment till in some places and subglacial melt-out
till in others.
Ridges transverse to the direction of ice
movement
Several places along the valley bottom, particu­
larly west of Åsta, there are moraine ridges
which are oriented more or less transverse to the
last direction of ice movement (Fig. 3). The
ridges are partly curved with the convex side
downstream. The length varies from 20 to 60 m
and height from 2 to 10 m. The distal slopes are
generally steeper than the proximal. The ridges
are arranged in groups in a fish-scale-like pattem
(Fig. 18A).
The till is everywhere loose and sandy with
lenses and beds of sorted sediments, indicating
influence from melt-water. The roundness of
grave! is some places similar to that of the lodge­
ment till (Figs. 8, 18A, site l & 18B, curve 1).
Towards the centre of the valley the roundness
index is higher (Figs. 18A, site 2 & 18B, curve 2)
indicating a fluvial prehistory for some of the
material (cf. Fig. 19).
The long-axis orientation is mainly parallel to the
last direction of ice movement, indicating a basal
formation (Haldorsen & Shaw in press). The
undisturbed internat structures and absence of
drumlinization show that the ridges were not
overridden by active ice after the formation.
The total situation, structure, and texture im­
ply a formation just before the ice became stag­
nant, and a deposition by a subglacial melt-out
(Fig. 9). The history is believed to be similar to
that of the Rogen moraines in Sweden ( G.
Lundqvist 1943, p. 21, J. Lundqvist 1969b and
Shaw 1979b).
The uppermost parts of the ridges - above the
zone of proper melt-out till - Jocally consist of
sorted stratified material and sandy diamicton
(Fig. 20). The layering is irregular, and the fabric
32
S. Haldorsen
NORSK GEOLOGISK TIDSSKRIFT l (1982)
N
180
'
A'= 298
n
P'= 19
R*= 19
50
N
90
270
180
A' =323
P' = 9
n
R*=38
=
71
Fig. 20. Section in a transverse moraine ridge at Åkerseter (site l, Fig. 18), 1-2 m below the surface. To the right, fabric diagrams.
Maximum of a-axis orientations is given by the asimuth (A') and by the average dip (P'). Magnitude of the resultant vector in
relation to the number of observations (n) gives the strength of orientation (values R). A', P', and R are calculated by the method
described by Steinmetz (1962).
*significant orientation at a 5 % leve! of significanee.
A. Subglacial melt-out till with a preferred long-axis orientation of clasts parallel with the valley. B. Coarse-grained diamicton or
supposed supraglacial origin. A broad scatter of long-axis orientation of clasts is found. The boundary between A and B is defined
by a layer of fine sand and silt.
of the diamicton is roughly parallel to the local
surface slope. The sediments may have a supra­
glacial origin or may be the uppermost part of
the subglacial till which flowed during or after
deposition (Fig. 9).
Hummocks
Hummocks containing diamictons are mainly
found along the valley floor. They are most fre­
quent in the east (Fig. 3). The hummocks are
from 2 to 8 m high. Irregular ridges also occur.
There are several small gravet pits and road
cuts in the hummocks, which show an abundance
of sorted zones, mainly as irregular bands and
lenses. The diamictons are commonly rich in
gravet and have a sandy and loose matrix. They
are several places overlain or underlain by sort­
ed, stratified material. There is in most cases no
correlation between the directions of ice move­
ment and the long-axis orientation of clasts.
The hummocks are mainly found in areas
where eskers and kames are frequent. Drilling
showed that they occasionally rest on a homo­
geneous silty diamicton, probably a till of the
same type as that of the cover moraine. In some
places the hummocks are found on eskers. This,
and the fact that they are clearly influenced by
melt-water and have disturbed internat structure,
indicate a supraglacial origin.
NORSK GEOLOGISK TIDSSKRIFT l (1982)
Genesis of ti/Is
33
A
Hummocks at Kattugletjern
E
N
0
..
270
.
.
;
..
.
.· .
. ·-:.··
90
180
Local
surface dip
n
=
45
A=212
P=2
s, =0.66
8
o
o
<:>
CJ
oo
o
o
<:>
Gravelly
diamicton
Sandy gravelly
diamicton
with lenses of
fine sand
0.5m
c
o
··
·
·
·
%
...
·
1
40
75
\
50
r-------,
.
3
30
2
.20
25
10
32
16
8
Gra vel
4
2
l
1.0
.5
.25
.1
16pm
Sand
.2
.3
.4
.5
.6
.7
Roundness P·
Silt
Fig. 21. A. Sketch of the hummocky terrain at Kattugletjern (Fig. 3). B. Part of the section shown in A. C. Grain-size distribution
of the three samples marked in B. D. Roundness of grave! 16-32 mm, from samples in B. E. Long-axis fabric of clasts from the
lowermost part of the section shown in B. Further explanation: see Fig. 10.
A section through the upper part of a hum­
mock at Kattugletjern is shown in Fig. 21 B. The
lower part of this consists of a sandy, gravelly
diamicton. (Fig. 21 B & C: l) with irregular
lenses of fine sand (Fig. 21 B & C: 2). The upper
part consists of a diamicton with a low content of
3-
Geologisk Tidsskr.
1/82
sand and silt (Fig. 21 B & C: 3). Part l is rather
massive while part 3 has a diffuse stratification.
There is no sharp boundary between part l and
part 3. The gravel has the same degree of round­
ness in both parts (Fig. 21 D).
The long-axis is roughly parallel with the local
S.
34
NORSK GEOLOGISK TIDSSKRIFT l (1982)
Haldorsen
A
Stratified sand
and grave!
Sandy gravelly
diamicton with lenses
of coarse sand
Sand and grave!
Disturbed
stratification
8
c
1
%
75
3 ------
,,...·.
'.·.
25
2
1.0
l
'� �
.5
1
40
2--
�
�
t:
�·.
\'.
50
···········
%
2
············
--
3 -----·
30
20
···· ·
·
>
·
�
10
.... __
32
Sand
.2
.3
.4
.5
.6
.7
Roundness p
Silt
N
D
.1
16JJm
N
E
90
270
180
n
=
60
180
n
=
60
Fig. 22. A.
The internal part of an irregular ridge at Myrbakken, 1-2.5 m depth. B. Grain-size distribution of different parts of the
sediments. C. Roundness of gravel16-32 mm. D. Fabric of clasts from the whole diamicton shown in A. E. The fabric in D, related
to the surface of sedimentation. Values calculated by the method of Steinmetz (1962) ( further explanation, see Fig. 20) .
surface slope (Fig. 21 E) but with slightly less dip
than the surface gradient.
The presence of sorted material (Fig. 21 B: 2)
and the fabric in the diamicton indicate a depo­
sition by flow. The cap of coarse material accord­
ing to Nemec et al. (1980) is typical for subaerial
debris flows. The lower part of the section (Fig.
20 B: l) could represent a true debris flow, with
a high concentration of solids. The upward
movement of pore water may have given an in­
creased concentration of water towards the top
of the flow. The material here has been totally
liquified and a syndepositional removal of fines
thereby occurred. This facies may be represented
by the upper, gravelly part (Fig. 21 B: 3).
At Myrbakken (Fig. 3) a section in an irregular
ridge shows a lowermost part (Fig. 22 A: l)
consisting of stratified sediments with a variable
gravel content. The material finer than 2 mm
contains <lO% silt + clay (Fig. 22 B). The
stratification is partly disturbed.
Above, there is a sandy, gravelly diamicton
Genesis of tills
NORSK GEOLOGISK TIDSSKRIFT l (1982)
(Fig. 22 A: 2) with a silt content of about lO%
(Fig. 22 B), poorly developed stratification and
irregular lenses of coarse sand (Fig. 22 A). There
is no sharp boundary between the diamicton and
the underlying stratified sediments or between
the diamicton and the sand lenses.
At the top of the sequence there are sorted
and stratified sand and grave! beds (Fig. 22 A: 3)
with a silt content below 5% (Fig. 22 B) and with
a well defined and probably erosional contact to
the underlying diamicton.
The grave! has similar roundness through the
whole section (Fig. 22 C) and is considerably
more rounded than the grave! in most of the
subglacial tills (Fig. 8). This indicates a fluvial
transport even for the grave! of the diamicton
(see also Fig. 19).
The long axes of clasts in the diamicton Iie in
the whole section dose to the horizontal plane
(Fig. 22 D), but compared with the bedding
pfanes there is a distinct dip towards northeast
(Fig. 22 E). If the long-axis orientation was par­
alle! with the movement of the diamicton during
its deposition, this implies a transport from
northeast.
The low silt content and the roundness of grav­
el indicate that all the material has undergone a
short glaciofluvial transport. Part l was probably
deposited in a crevasse with a northeast-south­
west direction. Later part of these deposits - or
other sediments with the same composition flowed by gravity along the crevasse, in a south­
western direction, forming the diamicton. The
top of the diamicton was probably eroded by
melt-water before the glaciofluvial sediment unit
3 was deposited.
The deformation of the sediment l may result
from the melting of underlying ice. It is reason­
able that the processes which caused the defor­
mation also caused the sediments to flow. The
undisturbed stratified sediments at the top indi­
cate on the other hand that no significant lower­
ing occurred after the deposition of part 3.
Alternative sources of the supraglacial sedi­
ments are shown in Fig. 23. The eastern valley
side and the eastern mountainous areas were
probably nearly deglaciated while the valley bot­
tom was still filled with ice, and may be the
source for much of the supraglacial material (Fig.
23 C). This is indicated by the clast abrasion
which is similar to that of the lodgement till and
by the concentration of hummocks in areas
where melt-water channels are frequent along
the upper part of the valley side.
35
For mation of supraglacial sediments
A
B.
E.
Fig. 23. Alternative sources of supraglacial sediments in Åsta­
dalen.
Basal drift may have been lifted to an englacial
position along the eastern valley side where the
topography is variable (Fig. 23 B). However,
during the last part of the glaciation there was
certainly an overall net melting along the base of
the glacier and most of the basal drift was de­
posited at subglacial till.
The supraglacial sediments may have bad a
complex history also after their introduction to a
supraglacial position.
This is well demonstrated by the two studied
sections and also illustrated in Fig. 23 D-E-F. All
the supraglacial diamictons which have been
studied in sections bear evidence of flow. None
of them are, therefore, supraglacial melt-out tills
or lowered supraglacial tills (Fig. 1). In fact, the
active processes in the supraglacial position were
either flow or running water. The only till type
which was formed from original tills in this envi­
ronment was consequently a flow till (Fig. 23).
The debris-flow sediments at Kattugletjern (Fig.
21), is of this kind. The diamicton at Myrbakken
(Fig. 22), on the other hand, should not be classi­
fied as a till. At other localities an original glacial
till may have been mixed with glaciofluvial sedi­
ments during flow. This is indicated along Skolla
(Fig. 3) where the diamictons include grave! with
a bimodal roundness distribution (Fig. 24), and a
rapid variation in grain-size distribution. Zones
with silty material occur frequently and are inter-
S.
36
NORSK GEOLOGISK TIDSSKRIFT l (1982)
Haldorsen
%
n=6
40
P=0.32
30
s�O.O?
r----·
west. The surface of the ice may for a time have
sloped towards northeast. Possible supraglacial
material along the western valley side could have
been transported towards the east and there con­
tributed to the supraglacial sediments.
20
Till genesis - concluding
10
Fig. 9 shows the supposed formation of different
till types in Åstadalen. Most of the material is
interpreted as basally transported and subglacial­
ly deposited.
Lodgement till is probably the predominant till
type in the valley. However, this conclusion is
based more on general glaciological consider­
ations than on diagnostic characteristics of the till
itself.
Local lee-side till and a significant part of the
sparse till cover in the mountain areas have been
classified as melt-out till. This interpretation is
based on the conclusion that the material could
not be identified as lodgement till. In addition
the melt-out till may locally constitute parts of
the regular till sheet in the valley (Fig. 9). The
studies indicate that it is often easier to identify a
melt-out till than a lodgement till.
It has been difficult to distinguish between
subglacial and supraglacial melt-out till as both
are formed from melt-out of basal drift and as
the melt-out process was probably mainly related
to the final part of the deglaciation phase (Fig.
9).
The relation between tills formed from basal
drift and diamictons formed from supraglacial
material is in many cases complex. The trans­
verse moraine ridges in Åstadalen (Rogen type
moraines) were formed mainly by melt-out of
subglacial debris. As shown in Figs. 9 & 20, this
material may be covered by a cap of supraglacial
sediments. The hummocky moraines, on the
other hand, were probably mainly formed from
supraglacial sediments. The source of such sedi­
ments may in most cases have been subglacial till
from the valley slope. The study illustrates that it
is often difficult to define what is a "basal till"
and what is an "ablation till", and even to deter­
mine what is a till.
0.2
0.6
0.4
Roundness P
Fig. 24.
Degree of roundness of grave) material 16-32 mm from
diamictons forming the hummocks along Skolla. Each sample
consists of 100 particles. Further explanation: see Fig. 8.
preted as fine-grained material from the original
till. Diamictons of this kind are not true tills.
There may be several reasons why the hum­
mocks are more common along the eastern than
along the western valley side. The eastern valley
side is steeper and the mountain area east of the
valley is generally higher. These areas were
probably free of ice before the western upland
area (Fig. 3). In addition, the eastern side is the
sunny side of the valley (Fig. 3), and therefore a
greater melting rate occurred here than in the
ØVRE ASTBRUA GRAVEL PIT
A
B
Q
o
o
Q
o
i]
·0.37
<::>
a <::>
o
"
o
Oiamicton
.2
o
�ffl
�����
,...
._-,...,
,...,,.
... -....
. -::..�--
Gravel·and
cobble-rich
stratified
sediments
.4
Roundness p
c
N
Oisturbed
compact silt
and sand with
lumps of ela y 270
remarks
180
A •352
51• 0.85
P• 15
n
.
80
Stratigraphy of the grave) pit at Øvre Åstbrua.
B. Degree of roundness of gravel material 16-32 mm from the
base of the diamicton. C. Long-axis orientation and dip of
clasts from the base of the diamicton.
Fig. 25. A.
Chronology of till formation
Åstadalen was not swept completely free of sedi­
ments before the present tills were formed. At
NORSK GEOLOGISK TIDSSKRIFT
l (1982)
Genesis of ti/Is
37
Øvre Åstbrua (Fig. 3), sorted sand and silt with
Late
Preboreal
Weichselian
lumps of clay are found beneath a till from �he
(after 25000 B.P.I
early glacial phase (Fig. 25). The clay contams
remnants of turf and mosses from an ice-free
Hummocky
period, and the till contains significant amounts of
terrain
�
subrounded gravels which indicate an incorpora­
tion of water transported material (Fig. 25).
Morai ne
l Melt·out l
The organic matter in the sub-til! sediments at
ridges
0
Øvre Åstbrua has a sparse pollen content, indi­
Q)
cating an interstadial origin (R. Sørensen pers.
,s; Upper
l Melt-out :
comm. 1980). The material has not been dated,
",
l Lodgement l
ti li
'o
but may for instance be of the same age as the
E ------ ------------------------------,
: Melt-out,
sub-til! sediments in Gudbrandsdalen (Bergersen
Lower
-·
�
& Garnes 1981) or the organic material in Bru­
l Lodgement )
ti li
o
(.)
munddal (Helle et al. 1981). These are related to
Early Weichselian interstadials, and may repre­ Fig. 26. Chronology of till formation and deposition in Asta­
sent the last time in the Weichselian that the dalen. F fonnation of moraine ridges.
Åstadalen area was deglaciated.
No defined boundary between the lower til!
transported from the north and the upper til!
transported from the northwest has been recog­ dominant during an early part of the last glacial
nized. The difference between them is only re­ phase (Fig. 26). When the ice became more pas­
flected in the fabric. There is therefore no indica­ sive the most important processes became stack­
tion of an ice-free period after the deposition of ing �f basal ice and deposition of melt-out til!.
the lower til!, which is obviously younger than Such deposition may have continued until the
the organic material at Øvre Åstbrua.
very last of the ice melted (Fig. 26).
The early glacial phase may have started short­
It has been postulated that a regional glacial
ly after the last interstadial and thus a lo�g time stagnation
occurred over many inland parts of
before the Weichselian maximum. There ts, how­ southern Norway (Reusch 1901, p. 88, Strøm
ever no indication of a gap in time between the 1943 Mannerfelt 1945, Holtedahl 1953, p. 753).
forn:ations of the lower till and the upper til!. On The �ountain area in Åstadalen was not ice free
the contrary, sedimentation seems to have been before the end of the Preboreal (Sørensen
continuous. The early glacial movement from 1979b). The valley bottom Iies about 200-300 m
the north could therefore be younger than the below the general leve! of the mountain plateau,
Weichselian maximum and represent part of the and there was probably some ice movement
Late Weichselian (the time after 25000 B.P., along the valley until the glacier distintegrated
Fig. 26). The change in ice mo�ement was pr?b­ into ice remnants along the valley floor. Howev­
ably caused in a shift of the tce accumulatton er, melt-out may have dominated compared with
centre, and the last movement lasted until all lodgement long before that phase. �e Prebor­
glacial activity stopped, as the topography also eal, and particularly the later part of tt, .·� here­
�
favours a southeastern direction (Figs. 2 & 4).
fore believed to be dominated by deposttton of
If both tills were deposited after the Weichse­ subglacial melt-out till and supraglacial sedi­
lian maximum, there is a gap of many thousand ments (Fig. 26).
years between the deposition of �he organic �a­
terial at Øvre Åstbrua and the ttll. A long ttme Acknow/edgements. - Dave M. Mickelson and John Shaw vis­
.
was thus available for the removal of old sedi­ ited the field area and initiated inspiring discussions. The fmal
of the manuscript was commented upon by Aleksis Drei­
ments. After the Weichselian maximum there draft
manis, Per Jørgensen, Jan Mangerud, and John Sha . Gr te
�
:-"
were periods with a great net melting and thus a Bloch and Aslaug Borgan assisted in the field. Mane-Loutse
net deposition. Such periods are therefo�e .not Falch typed the manuscript and Aslaug Borgan drafted the
representative for all parts of the l�st glactatto�. figures. Data analysis of fabric measurements was perfo�ed at
University of Alberta. Financial support was provtde� �y
The Weichselian might well have mcluded pen­ the
the Norwegian Council for Science and the Humamttes
ods with significantly more erosion than during (NAVF). To all these persons and institutions I tender my
its last parts.
sincere thanks.
Manuscript received September 1981.
Deposition of lodgement til! was probably
w
·-· � ... -
-
=
------
38
S.
NORSK GEOLOGISK TIDSSKRIFT l (1982)
Haldorsen
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