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A review of the creation of glacial erosional striae and their significance in Shropshire
Geraint T. H. Jenkins1 and Tom Holt2
JENKINS, G.T.H. & HOLT, T. (2012). A review of the creation of glacial erosional striae and their significance in
Shropshire. Proceedings of the Shropshire Geological Society, 17, 17–21. Glacial erosional striae are used in
conjunction with palaeoglaciological and sedimentological studies to determine former environmental conditions
and ice dynamics in regards to the Devensian glaciation of Shropshire. Internal morphological conditions of
glacial scour marks have been used to determine eroding mechanisms and particulate trajectories across substrates.
1
Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB,
Wales. E-mail: [email protected]
2
Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB.
< 1m and are regularly located superimposed
upon larger erosional landforms (Rastas &
Seppala, 1981). The creation of striations
through alteration in abrasion rates is seen to be
dependent upon the rate of basal sliding (Budd
et al., 1979) and concentration of basal debris
(Hallet, 1979); bed material induced frictional
drag can been seen to retard abrasion efficiency
and in turn influence striae creation (Hallet,
1981; Figure 2).
INTRODUCTION
The formation of striations occurs as a result of
abrasive processes whereby asperities within
basal particles move across erodible surfaces,
leaving remnant scours (Iverson, 1990; Iverson,
1991; Benn & Evans, 2010). The creation of
striae occurs through discontinuous movements
of particulate matter resulting from a build-up of
elastic strain within asperities. This results in
repeated failure events that coincide to produce
a scour lineation (Drewry, 1986).
The differentiated morphology of striae has
led Chamberlin (1888) and Iverson (1991) to
allocate specific types that illustrate altered
creation conditions due to changing clast
incisions. The creation of striae is dependent
upon basal abrasion (Atkinson, 1984;
Hindmarsh, 1996) and therefore the abrasion
models of both Boulton (1974) and Hallet
(1979) are relevant regarding striation creation.
It has also been observed that basal debris
supply (Iverson, 1993), concentration (Hallet,
1979) and lithology (Boulton, 1979) also
contribute to striation formation.
CREATION
Striations are remnant scour forms that are
produced by the incision of particulate matter
into rock surfaces (Figure 1). The creation of
these is related to the extent of glacial abrasion
(Iverson, 1990; Iverson, 1991). Striae are
spatially classified as microscale erosional
features (Bennett & Glasser, 2009; Benn &
Evans, 2010). These have typical dimensions of
Proceedings of the Shropshire Geological Society, 17, 17−21
Figure 1. Continuously striated bedrock illustrating the extent
of incision during former ice coverage (Bennett & Glasser,
2009).
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G.T.H. JENKINS & T. HOLT
attributed type numbers to specific striation
morphologies (Figure 4).
Type A striae become both deeper and wider
with distance down-glacier until ending
immediately, inferring creation whereby the
abrading clast ploughed forward until reaching a
threshold where the clast no longer has the
potential to striate (Bennett & Glasser, 2009).
Type B striae exhibit maximum depth and
width at their centre-most position with scour
becoming progressively deeper and wider
nearing the centre point, and inverse following
this. This type of striae indicates creation
conditions of clast deceleration until reaching
the maximum depth whereby the particle then
accelerates (Glasser & Bennett, 2004).
Type C striae start abruptly becoming
gradually narrower and shallower down-glacier
and are created through low ploughing angles of
the abrading clast (Glasser & Bennett, 2004). It
is apparent therefore that the morphology of
striations indicates altered basal clast conditions
upon creation.
Figure 2. Schematic illustration of basal debris concentration
and ice sliding velocity illustrating a sliding velocity decrease
with high debris loads (Bennett & Glasser, 2009).
Figure 3. Schematic diagram of striation creation through
protruding particle asperities moving across erodible surfaces
(Benn & Evans, 2010).
The creation of striations results from abrasive
processes (Iverson, 1990) whereby asperities
projecting from abrading particles are moved
across erodible surfaces thereby creating areas
of concentrated scour (Figure 3). Drewry (1986)
examined striation creation and established that
the striating process consists of discontinuous
movements resulting from a build-up of elastic
strain within the asperity leading edge resulting
in repeated failure events and brittle fracture
(Benn & Evans, 2010).
Drewry (1986) also established that the depth
and width of striae are determined by asperity
form and particle loading against the erodible
surface. The creation of striations as illustrated
by Benn & Evans (2010) is also seen to reflect
the rotational nature of the abrading bed clast
whereby asperities are cleared by rotation from
the abraded substrate and are replaced by a new
asperity on which comminution has not yet
occurred (Benn & Evans, 2010). Differentiated
striation morphologies illustrate altered creation
conditions and have been considered by
Chamberlin (1888) and Iverson (1991) who
Proceedings of the Shropshire Geological Society, 17, 17−21
Figure 4. Type A, Type B and Type C glacial striations as
described by Chamberlin (1888) and Iverson (1991) in both
plan and longitudinal cross-section view.
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© 2012 Shropshire Geological Society
GLACIAL STRIAE
A number of theories concerning the creation of
striae by glacial abrasion have been proposed
(Atkinson, 1984; Cuffey & Alley, 1996;
Hindmarsh, 1996). There are two principal
theories for the mechanics of basal abrasion,
these being proposed by Boulton (1974) and by
Hallet (1979). Both have predicted abrasion
parameters from relationships of wear, whereby:
The creation of glacial striae has been observed
to be influenced by the lithology of both the
abrading clast and the material which abrasion
occurs across (Drewry, 1986). Igneous and
metamorphic lithologies abrading sedimentary
facies have been observed to be the most
efficient process regarding striae creation and
abrasion (Boulton, 1979).
For continuous abrasion and striae creation to
occur, basal debris renewal is required. This
involves the replacing of asperities where
comminution has occurred by more angular
material (Gomez & Small, 1985; Iverson, 1993).
The creation and location of striae can be
thus be used to infer former ice extent and basal
thermal regimes, and can be used together with
other geomorphological evidence (Edwards,
1975; Sharp et al., 1989).
Ȧ = α* Cd vp N
The abrasion rate (Ȧ) is calculated from an
empirical factor relating to rock hardness (α*),
the concentration of basal particles (Cd), the
velocity at which the particles travel (vp), and
the normal stress pushing the particulate
material into the erodible bed (N) (Paterson,
1999; Benn & Evans, 2010).
Boulton (1974) proposed that contact
pressure between basal particles and the bed
over which they lie is related to effective normal
pressure whereby normal pressure from the
overlying ice weight and basal water pressure,
acting in opposition through buoyancy, both
contribute (Krabbendam & Glasser, 2011).
Hallet (1979) proposed that the contact
pressure between the basal particulate matter
and the bed it overlies is related to the rate at
which ice flows towards its bed, thereby forcing
the particle into contact with the bed (Glasser &
Bennett, 2004).
Aforementioned abrasion models and
therefore striae creation will be dependent upon
the basal thermal regime whereby increased
basal temperatures result in enhanced potential
for abrasion (Hall & Glasser, 2003; Sugden,
1978; Figure 5).
STRIATIONS IN SHROPSHIRE
Palaeoglaciological investigations of Shropshire
have suggested that additional research is
required to understand both the extent and
dynamics of Devensian ice masses within the
county (Cook, 2012), for which the recording of
striae distribution and overall geomorphology
will be necessary prerequisites.
The distribution of glacial erosional features
including glacially smoothed landforms was
established by Brown (1971) for parts of southwest Shropshire (Figure 6). This was used to
infer direction of ice movement. Similar
research was undertaken further north by Shaw
(1972) whereby glacial striae were incorporated
into environmental reconstructions for the
northern half of Shropshire.
The presence of glacial erosional striae may
indicate evidence towards a previous basal
thermal regime (Glasser & Hambrey, 2001),
thus affecting process glaciological parameters
such as the likelihood of englacial thrusting
(Cook, 2012) and hence basal till interpretation.
The presence of erosional striae however
need to be considered in relation to bedrock
lithology with respect to erodibility (Boulton,
1979).
Figure 5. Schematic cross-section of an ice sheet illustrating
locations where abrasion and therefore striae creation occur
(Bennett & Glasser, 2009).
Proceedings of the Shropshire Geological Society, 17, 17−21
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G.T.H. JENKINS & T. HOLT
will provide greater insight into former ice
dynamics for Shropshire.
ACKNOWLEDGEMENTS
The authors would like to thank Jennifer Geary and Dr
Duncan Quincey for their constructive comments throughout
the compilation of this study.
REFERENCES
Atkinson, B.K. (1984). Subcritical crack growth
in geological materials. Journal of
Geophysical Research, 89, 4077–4114.
Benn, D.I. and Evans, D.J.A. (2010). Glaciers
and Glaciation. London: Hodder Education.
Bennet, M.R. and Glasser, N.F. (2009). Glacial
Geology: Ice Sheets and Landforms. WestSussex, Wiley-Blackwell.
Boulton, G.S. (1974). Processes and patterns of
glacial erosion. In: Glacial Geomorphology
(ed. D.R. Coates) Proceedings of the Fifth
Annual
Geomorphology
Symposia,
Binghampton. London: Allen & Unwin.
Boulton, G.S. (1979). Processes of glacier
erosion on different sub-strata. Journal of
Glaciology, 23, 15–38.
Brown,
M.J.F.
(1971).
The
glacial
geomorphology of parts of south west
Shropshire and Montgomeryshire. Doctoral
thesis, University of London (unpublished).
Budd, W.F., Keage, P.L. and Blundy, N.A.
(1979). Empirical studies of ice sliding.
Journal of Glaciology, 89, 157–170.
Chamberlin, T.C. (1888). The rock scourings of
the great ice invasion. U.S. Geological
Survey 7th Annual Report, 155–248.
Cook, S.J. (2010). The glacial geology of
Shropshire: insights from modern glaciology.
Proceedings of the Shropshire Geological
Society, 15, 20–27.
Cuffey, K. and Alley, R.B. (1996). Is erosion by
deforming subglacial sediments significant?
(Toward till continuity). Annals of
Glaciology, 22, 17–24.
Drewry, D.J. (1986). Glacial Geologic
Processes. London: Edward Arnold.
Edwards, M.B. (1975). Glacial retreat
sedimentation in the Smalfjord Formation,
Late
Precambrian,
North
Norway.
Sedimentology, 22, 75–94.
Glasser, N.F. and Bennett, M.R. (2004). Glacial
erosional landforms: origins and significance
Figure 6. Glacial erosional features for south-west Shropshire
including the localities of Minsterley and Westbury (Brown,
1971).
CONCLUSIONS
It is apparent that the creation of striae occurs
through abrasive processes (Iverson, 1990)
whereby asperities in abrading clasts are moved
across an erodible surface (Benn & Evans,
2010). Elastic strain within the tip of the
abrading material results in repeated failure
along a single plain as observed by Drewry
(1986). The creation of striae occurs through
rotational movement of the abrading clast
whereby the asperity of the abrading particle is
continuously renewed, ensuring consistent
abrasion (Drewry, 1986). Altered basal particle
incision results in differentiated striae
morphology (Bennett & Glasser, 2009; Iverson,
1991; Chamberlin, 1888) and can be used to
infer preceding ice extent and basal thermal
conditions (Sharp et al., 1989).
Striation creation is related to the presence of
basal abrasion. Models to quantify the extent of
this have been introduced by both Boulton
(1974) and Hallet (1979). However, the thermal
basal regime preconditions the location of
abrasion occurrence (Sugden, 1978).
It has also been observed that the creation of
glacial striae is influenced by bedrock lithology
(Boulton, 1979) with igneous and metamorphic
lithologies being less susceptible to abrasion.
It is therefore considered that the
investigation of striae distribution in
conjunction with palaeoglaciological studies
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GLACIAL STRIAE
evidence: Snowdon, North Wales. Journal of
Quaternary Science, 4, 115–130.
Shaw, J. (1972). The Irish Sea glaciation of
north Shropshire – some environmental
reconstructions. Field Studies Council, 4,
603–631.
Sugden, D.E. (1978). Glacial erosion by the
Laurentide Ice Sheet. Journal of Glaciology,
83, 367–392.
for palaeoglaciology. Progress in Physical
Geography, 28, 43–75.
Glasser, N.F. and Hambrey, M.J. (2001). Styles
of sedimentation beneath Svalbard valley
glaciers under changing dynamic and thermal
regimes. Journal of the Geological Society,
158, 697–707.
Gomez, B. and Small, R.J. (1985). Medial
moraines of the Haut Glacier D’Arolla,
Valais, Switzerland: Debris supply and
implications for moraine formation. Journal
of Glaciology, 31, 303–307.
Hall, A.M. and Glasser, N.F. (2003).
Reconstructing the basal thermal regime of
an ice stream in a landscape of selective
linear erosion: Glen Avon, Cairngorm
Mountains, Scotland. Boreas, 32, 191–207.
Hallet, B. (1979). A theoretical model of glacial
abrasion. Journal of Glaciology, 23, 39–49.
Hallet, B. (1981). Glacial Abrasion and Sliding:
Their
Dependence
on
the
Debris
Concentration in Basal Ice. Annals of
Glaciology, 2, 23–26.
Hindmarsh, R.C.A. (1996). Sliding of till over
bedrock: scratching, polishing, comminution
and kinematic-wave theory. Annals of
Glaciology, 22, 41–47.
Iverson, N.R. (1990). Laboratory simulations of
glacial abrasion: comparison with theory.
Journal of Glaciology, 36, 304–314.
Iverson, N.R. (1991). Morphology of glacial
striae: Implications for abrasion of glacier
beds and fault surfaces. Geological Society of
America Bulletin, 103, 1308–1316.
Iverson, N.R. (1993). Regelation of ice through
debris at glacier beds: Implications for
sediment transport. Geology, 21, 559–562.
Krabbendam, M. and Glasser, N.F. (2011).
Glacial erosion and bedrock properties in
NW Scotland: abrasion and plucking,
hardness and joint spacing. Geomorphology,
130, 374–383.
Patterson, W.S.B. (1999). The Physics of
Glaciers. Oxford: Butterworth-Heinemann.
Rastas, J. and Seppala, M. (1981). Rock
Jointing and Abrasion Forms on Roches
Moutonnees, SW Finland. Annals of
Glaciology, 2, 159–165.
Sharp, M., Dowdeswell, J.A. and Gemmell, J.C.
(1989). Reconstructing past glacier dynamics
and erosion from glacial geomorphic
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