20
the most part melt-out ti11 is less affected by basal or
internal abrasion and therefore tends to be coarser
grained than lodgement till. Melt-out till mayaiso
contain frozen blocks of unlithified substrate which
are incorporated into the basal zone of a glacier.
Clast orientation is preferentially parallel to the direction of ice flow, and a considerable number of
elasts dip up glacier. Due to running melt-water, this
type of till also contains locally minor areas of stratified sorted beds. If these are deposited on top of ti11
still holding some ice, they later become contorted or
intensely faulted.
Melt-out till accumulating on top of the ice or at
the slopes of moraines is frequently redeposited by
mass flows (Fig. 2.1). Such flowti11s, flowed tills or
flow (flowed) diamicts are difficult to discriminate
from debris flow deposits of other environments
(Sect. 5.4.1) unless they contain striated elasts and/or
ice-cemented blocks, or unless they show deformation characteristics of ice contact. As intercalations
of a glacial sedimentary sequence, flow tills as we11
as grain flows originating from glaciofluvial outwash
are easier to identify. At standing ice margins, meltout ti11 and supraglacial outwash may form end moraines of considerable height and length. These moraines are frequently overridden and deformed by
advancing ice (Fig. 2.lc).
Special morphologie al features developed in the
subglacial environment are drumlins, eskers, and tunnel va11eys. Eskers and drumlins form narrow ridges
or elongate hills on the land surface after the ice has
melted, tunnel valleys may cut deeply into the underlying bedrock. Tunnel valleys probably result from
subglacial meltwater erosion, wherc the channels are
completely filled with water and therefore do not
need a continuous gradient.
Deep-cut tunnel valleys are known from North America
and Europe. In northem Germany such valleys locally
reach depths of 400 to 500 m below the present sea level.
They are cut into weakly indurated Neogene sandstones,
have steep side walls, and are infilled by glacial outwash
diarniet and finer grained water-deposited sediments. They
were investigated by geophysical methods and drillholes.
Such deep tunnel valleys have a very high preservation
potential in the geologie al record, although it may be difficult to identify them as such when only small portions of
the former tunnel system are exposed or preserved by erosIOn.
Transitional facies between ti11 and glaciofluvial sediments are glacial outwash, primarily deposited either
supraglacially on top of the ice (e.g., kame terraces),
or englacially in large ice cracks or tunnels at the
base of the ice (esker). These sediments may show
sharp lateral facies changes into other glacial deposits, because the meltwater streams were originally
bounded by ice.
Chapter 2 Continental Sediments
Glaciofluvial Sediments
. Fluvial sediments in the proglacial region usua11y
show the same characteristics as deposits of alluvial
fans and braided rivers (Sect. 2.2). Large va11ey glaciers and some continental ice sheets release enormous quantities of coarse- and fine-grained debris
which is transported by meltwater into the proglacial
lowlands. Along the ice margin, some of the material
is deposited in outwash fans, and further away in
outwash plains (sandur, Fig. 2.la). Outwash deposits
of alpine glaciers are mostly rich in gravel, whereas
outwash plains of continental ice sheets (for exampIe, those of the Scandinavian Pleistocene ice sheet)
consist predominantly of sand. This results not only
from the general downstream decrease in grain size,
but also from the lower proportion of coarse-grained
material delivered by the continental ice mass. In any
case, meltwater streams, with their high bedload,
tend to accumulate the coarser grained portion of
their load and may therefore generate comparatively
thick glaciofluvial sequences. Distinctive features of
the glacial environment, in contrast to normal fluvial
conditions, are deformation structures formed by
post-depositional melting of buried ice (Fig. 2.1 b
and d), and the occurrence of "frozen blocks" of ti11
or sand, wh ich were carried and finally deposited in
this state (Fig. 2.le and f).
A special feature of proglacial meltwater sediments are the
deposits of catastrophic floods, the so-called jökulhlaups,
which were described from Iceland and other presentiy or
formerly glaciated regions (e.g., Maizels 1989; Smith
1993; cf. Sect. 7.9.6). These floods result form sudden
drainage of ice-damrned lakes, from landslides on to glaciers, or from subglacial volcanic eruptions. Jökulhlaup
flows may contain very high suspended sediment loads
(hyperconcentrated flows with up to 35% sediment concentration) consisting predominantiy of silt, sand, and gravel.
These flows predominantly form thick, more or less homogeneous, poorly structured massive beds.
As long as the outwash plains are devoid of vegetation, sand and silt sized partieles are easily blown
away by strong winds. The sand can be deposited
nearby in the form of dunes; the silt is carried farther
away and accumulates as loess, often in areas where
tundra vegetation is present trapping the wind-blown
dust (Sect. 2.3.5). On surfaces where neither ac cumulation nor erosion takes place for some time, patterned ground, ice wedges, and other phenomena
typical of the periglacial zone can develop (Fig.
2.lg). The accumulations of flood plains may provide good evidence of alternating colder and warmer
elimates (Fig. 2.2). Cold periods are indicated by patterned ground (cryoturbation), ice wedges, and the
deposition of loess; warmer periods enable the
growth of dense and varied vegetation and thus the
formation of soil.
the most part melt-out ti11 is less affected by basal or
internal abrasion and therefore tends to be coarser
grained than lodgement till. Melt-out till mayaiso
contain frozen blocks of unlithified substrate which
are incorporated into the basal zone of a glacier.
Clast orientation is preferentially parallel to the direction of ice flow, and a considerable number of
elasts dip up glacier. Due to running melt-water, this
type of till also contains locally minor areas of stratified sorted beds. If these are deposited on top of ti11
still holding some ice, they later become contorted or
intensely faulted.
Melt-out till accumulating on top of the ice or at
the slopes of moraines is frequently redeposited by
mass flows (Fig. 2.1). Such flowti11s, flowed tills or
flow (flowed) diamicts are difficult to discriminate
from debris flow deposits of other environments
(Sect. 5.4.1) unless they contain striated elasts and/or
ice-cemented blocks, or unless they show deformation characteristics of ice contact. As intercalations
of a glacial sedimentary sequence, flow tills as we11
as grain flows originating from glaciofluvial outwash
are easier to identify. At standing ice margins, meltout ti11 and supraglacial outwash may form end moraines of considerable height and length. These moraines are frequently overridden and deformed by
advancing ice (Fig. 2.lc).
Special morphologie al features developed in the
subglacial environment are drumlins, eskers, and tunnel va11eys. Eskers and drumlins form narrow ridges
or elongate hills on the land surface after the ice has
melted, tunnel valleys may cut deeply into the underlying bedrock. Tunnel valleys probably result from
subglacial meltwater erosion, wherc the channels are
completely filled with water and therefore do not
need a continuous gradient.
Deep-cut tunnel valleys are known from North America
and Europe. In northem Germany such valleys locally
reach depths of 400 to 500 m below the present sea level.
They are cut into weakly indurated Neogene sandstones,
have steep side walls, and are infilled by glacial outwash
diarniet and finer grained water-deposited sediments. They
were investigated by geophysical methods and drillholes.
Such deep tunnel valleys have a very high preservation
potential in the geologie al record, although it may be difficult to identify them as such when only small portions of
the former tunnel system are exposed or preserved by erosIOn.
Transitional facies between ti11 and glaciofluvial sediments are glacial outwash, primarily deposited either
supraglacially on top of the ice (e.g., kame terraces),
or englacially in large ice cracks or tunnels at the
base of the ice (esker). These sediments may show
sharp lateral facies changes into other glacial deposits, because the meltwater streams were originally
bounded by ice.
Chapter 2 Continental Sediments
Glaciofluvial Sediments
. Fluvial sediments in the proglacial region usua11y
show the same characteristics as deposits of alluvial
fans and braided rivers (Sect. 2.2). Large va11ey glaciers and some continental ice sheets release enormous quantities of coarse- and fine-grained debris
which is transported by meltwater into the proglacial
lowlands. Along the ice margin, some of the material
is deposited in outwash fans, and further away in
outwash plains (sandur, Fig. 2.la). Outwash deposits
of alpine glaciers are mostly rich in gravel, whereas
outwash plains of continental ice sheets (for exampIe, those of the Scandinavian Pleistocene ice sheet)
consist predominantly of sand. This results not only
from the general downstream decrease in grain size,
but also from the lower proportion of coarse-grained
material delivered by the continental ice mass. In any
case, meltwater streams, with their high bedload,
tend to accumulate the coarser grained portion of
their load and may therefore generate comparatively
thick glaciofluvial sequences. Distinctive features of
the glacial environment, in contrast to normal fluvial
conditions, are deformation structures formed by
post-depositional melting of buried ice (Fig. 2.1 b
and d), and the occurrence of "frozen blocks" of ti11
or sand, wh ich were carried and finally deposited in
this state (Fig. 2.le and f).
A special feature of proglacial meltwater sediments are the
deposits of catastrophic floods, the so-called jökulhlaups,
which were described from Iceland and other presentiy or
formerly glaciated regions (e.g., Maizels 1989; Smith
1993; cf. Sect. 7.9.6). These floods result form sudden
drainage of ice-damrned lakes, from landslides on to glaciers, or from subglacial volcanic eruptions. Jökulhlaup
flows may contain very high suspended sediment loads
(hyperconcentrated flows with up to 35% sediment concentration) consisting predominantiy of silt, sand, and gravel.
These flows predominantly form thick, more or less homogeneous, poorly structured massive beds.
As long as the outwash plains are devoid of vegetation, sand and silt sized partieles are easily blown
away by strong winds. The sand can be deposited
nearby in the form of dunes; the silt is carried farther
away and accumulates as loess, often in areas where
tundra vegetation is present trapping the wind-blown
dust (Sect. 2.3.5). On surfaces where neither ac cumulation nor erosion takes place for some time, patterned ground, ice wedges, and other phenomena
typical of the periglacial zone can develop (Fig.
2.lg). The accumulations of flood plains may provide good evidence of alternating colder and warmer
elimates (Fig. 2.2). Cold periods are indicated by patterned ground (cryoturbation), ice wedges, and the
deposition of loess; warmer periods enable the
growth of dense and varied vegetation and thus the
formation of soil.