Earth Science Conservation Review
| Introduction to the glacial Sand and Gravel Landscapes of Northern Ireland | Antrim, Armagh, Down, Fermanagh, Londonderry, Tyro |
| Site Type: | Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
|---|---|
| Rock Age: | Quaternary (Devensian, Late Midlandian) |
| Interest | |
| Other interest: | corrie, corrie glacier, cross-valley moraine, drumlin, glaciofluvial fan, hummocky moraine, ice lobe, moraine, moraine ridge, outwash, proglacial, raised beach, readvance moraine, recessional moraine |
Description:
SIGNIFICANCE AND CHARACTER OF GLACIOFLUVIAL LANDSCAPES IN N. IRELAND
Glaciofluvial sand and gravel landscapes occupy fairly discrete tracts of generally well-drained land formed by glacial meltwater sorting of glacial debris during melting of the last ice sheet in Northern Ireland between ~22 and ~14ka B.P. (McCabe, 1996). The sorted glacial debris forms non-renewable resources which often provide topographic diversity especially on a local scale that contrasts markedly with the monotonous, streamlined drumlin hills in the lowlands of Northern Ireland [e.g. Glarryford esker (AGE01), Kilrea esker (AKE01)]. An explanation of landform codes is provided at the end. Within mountainous terrain sand and gravel occurs in mountain valleys and slopes with a topographic detail of ridges and undulations which enhances landscape quality and catch the eye of both traveller and scientist. In many upland sectors, the irregularities and intricate patterns of this surficial drape provides the local basis for habitat development and form the backcloths to our wetlands and post-glacial peat mantles [e.g. Cumber delta kettle hole (SDH0204), Murnies delta (SDD0105), Teal Lough outwash (SLO01)]. Preservation and conservation is therefore a priority on aesthetic and ecological grounds because of accelerated degradation of sand and gravel landscapes in the last twenty years.
Scientifically, glaciofluvial deposits provide critical information on the nature, pattern and characteristics of ice wastage at the end of the last ice age. The different depositional settings and range of palaeoenvironments inferred from the sedimentological and geomorphic data provide evidence for inferences on the mechanisms and controls of ice-sheet decay. Without this information we are unlikely to understand how our landscape was formed or the possible climatic changes which occurred within the coupled ocean-atmosphere-cryosphere system in the amphi-North Atlantic region [e.g. Killard Point]. An understanding of these relationships is essential if we are to unravel the complex mechanisms that control ice-sheet activity and its relationship to climate change through time. The record of past climate change preserved in glacigenic sediments and landforms provides accessible data on the long term evolution of the earth's climate, which can be used to assess predictions of future global climatic change involving ice-atmosphere-ocean interrelationships.
THE PLEISTOCENE PERIOD IN N. IRELAND
GLACIAL HISTORY OF N. IRELAND
The Irish landmass, situated in the north-eastern sector of the North Atlantic basin has undergone repeated glaciations during the Pleistocene period (cf. Colhoun et al., 1972; McCabe et al, 1987). Successive glaciations, through the erosive capacity of the ice masses produced vast amounts of debris which form the glacigenic deposits covering >90% of Northern Ireland. Their present morphology was principally formed during the last glacial cycle, and modified throughout the Holocene Period. Reconstructions of previous glacial cycles are difficult because isolated pieces of evidence are poorly constrained. The last Irish ice-sheet reached a maximum extent at about 22ka B.P., during a period of global temperature lowering. The till and glaciofluvial deposits which provide evidence of the last glacial cycle were deposited mainly in the (deglacial) period, dating from the glacial maximum to the onset of the present warm stage (Holocene epoch). Since then, fluvial erosion and human activities have modified the appearance of many glaciofluvial landscapes, though pristine examples of original forms have survived almost intact [e.g. Murrins moraine (OMN01), Gortin delta (SGD01)].
THE EVIDENCE FOR GLACIATION
Popular opinion views contemporary ice-sheets as unique phenomena, typical of distant terrains far from present-day centres of population. These glaciers are active analogues to the large mid-latitude ice sheets which waxed and waned during the Pleistocene period. Yet only fifteen thousand years ago, the entire north of Ireland was covered by an ice sheet possibly a few kilometres in thickness. The legacy of ice sheet activity on Northern Ireland's landscape has been immense, creating topography, soil, ground water regimes, vegetation and local habitats. The imprints left by erosive and depositional events provide us with local topographic styles and complexity e.g. spectacular erosive gashes forming overdeepened mountain valleys [e.g. Glenelly], streamlined drumlin fields, steep-sided cross-valley ridges [e.g. Knockalerry ridge (SBG01), Black Hill ridge (AMR01), Deer Park moraine (FFR01)] and flat-topped deltas [e.g. Carey Valley delta (PCD01), Wood Hill delta (AMD01), Fruitfield delta (BLD01)]. The landscape mosaic is therefore complex both in aesthetic terms and scientific basis and should be managed as a valuable asset.
All forms of glacial geologic evidence are rare on a world scale in that they are only to be found in previously glaciated regions. Due both to the erosive and depositional action of glaciers, it is predominately the imprint or signature of ice sheet behaviour during the last glacial cycle which is observed in the form of distinctive landscapes. Sediments accumulated from previous glacial events are generally buried at depth or reworked into new forms. Northern Ireland enjoys a privileged position internationally as a microcosm of 'fresh' evidence for previous glacial activity and various glacial processes. The evidence for ice-sheet activity is in three principal forms. Data from all three categories are needed for a complete reconstruction of ice-sheet conditions during the glacial cycle.
1. Stratigraphic data Stratigraphic data form long term (~100,000 yr) climatic proxy data. It has been found at depth and records multiple glacial cycles in Northern Ireland. Deep exposures, especially at Aghnadarragh, Co. Antrim, on the margins of Lough Neagh, contain sediment piles which record two major ice sheet expansions separated by a long phase of Boreal and Arctic conditions (McCabe et al., 1987). Other sections in Co. Fermanagh at Derryvree and Hollymount indicate similar major environmental reversals of climate over the last 100kyr (Colhoun et al., 1972).
2. Ice flow patterns During the last (late Midlandian) glacial cycle, the last main phases of ice sheet flow occurred between 19ka and 14ka B.P. The imprint of these ice flows on the landscape is best seen in Northern Ireland in the patterns of flow transverse Rogen moraines and flow parallel drumlin swarms which are developed across the thick mantles of till which occur mainly below 150m O.D. Reconstructed early (~22-19ka B.P.) ice flow patterns show radial ice flow from ice dispersion centres in Lough Neagh, the Omagh basin and Lower Lough Erne/Donegal probably resulting from strongly positive ice-sheet mass budgets. Thick, temperate ice flows moved north across the Sperrins, along the Foyle valley and across north Antrim onto the continental shelf and generally southwestwards, southwards and southeastwards, across the Erne basin, Co. Armagh and Co. Down into the Irish Sea Basin.
3. Glaciofluvial landscapes Deglaciation (ice wastage) began around 19ka B.P. when the regional climate began to ameliorate. Ice thicknesses decreased and ice margins began to contract back, generally towards centres of ice dispersion in the lowland areas of north central Ireland. Large and often catastrophic meltwater releases washed and sorted glacially eroded debris occurring in, on and below the ice-sheet, creating the geologic base of Northern Ireland's sand and gravel landscapes. The accumulation of sand and gravel sediments occurred in a range of palaeoenvironmental settings beneath [e.g. Glarryford esker (AGE01)], beside [e.g. Lough Fea kame terrace (SLT01)] and in front of [e.g. Lough Fea outwash plain (SLO01)] the decaying ice masses. In broad terms and at variable spatial scales, these accumulations are time transgressive from the periphery of Northern Ireland inwards towards the core centres of ice dispersion and the centre of the province.
THE NATURE AND SPATIAL PATTERN OF GLACIAL IMPRINTS
However this simplistic pattern of ice decay and sediment deposition is conditioned and complicated by interaction between six factors which influence the final location and morphology of glaciofluvial landforms:
A. Rates of ice retreat The ice front remains relatively stationary when the rate of ice accumulation and subsequent forward movement is balanced by the melting flux (ablation) at the ice sheet margins. Continuous sediment delivery to the ice margin by ablated meltwater results in a morainic ridge or spread of glaciofluvial sand and gravel aggrading at or beyond the ice margin [e.g. Black Hill ridge (AMR01), Teal Lough moraine ridge (SLR01)]. Where climatic conditions result in rapid melting with a much smaller input of ice mass to the ice-sheet margin, rapid ice-marginal retreat rates may leave little evidence of former ice marginal positions in the form of sand and gravel landscapes. A reduction in the rate of ice supply to the margins may occur due to interactions between topography, direction of ice-sheet flow or changing climatic conditions. This may result in a suite of hummocky landforms formed by the deposition of sediment in and around stagnant proglacial ice blocks [e.g. Castlederg moraine (FFN02)].
B. Meltwater erosion Intensive meltwater erosion, which occurs as the ice margin vacates a site, may result in the destruction of ice marginal sediment accumulations. Many sand and gravel landforms therefore have a low preservation potential. The topography of those that survive is often as much the product of the post-depositional erosion (dissection) of the original depositional morphology. For example a moraine ridge may be eroded into a series of discontinuous ridges and outwash spreads [e.g. Deer Park moraine (FFR01) and Foyle valley outwash (FFO01)].
C. Ice sheet growth and decay cycles Ice sheet growth and localised frontal advances related to short term climatic deterioration can totally obliterate or modify previously formed frontal landforms [e.g. Muntober ridge (SBG02)]. Such oscillatory growth and decay of ice sheets is a common pattern of ice mass behaviour during a deglacial cycle (cf. McCabe, 1996).
D. Ice thinning Melting and downwasting of the ice-sheet results in ice thinning. Ice mass loss is usually concentrated in a relatively narrow zone around the margin, where a combination of lower elevation, higher temperatures and meltwater flux results in increased ablation rates. In this situation, small-scale topography can play an important role in controlling the location and shape of the ice margin. Emergent hills may split an ice sheet into independent sectors or lobes [e.g. Mullaghmore]. Sand and gravel deposition will tend to be concentrated in shallow ponded meltwater lakes along the sinuous, debris-rich margins. These margins may be banked against hillslopes [e.g. Lough Fea kame terrace (SLT01)], block valleys, or project through upland mountain cols [e.g. Murnies delta]. Subsequently the complex pattern of sand and gravel accumulation in spreads and ridges requires detailed field mapping before reconstruction of ice marginal positions can be attempted.
E. Extraneous (Scottish) ice In the northern coastal lowlands around Armoy [Armoy moraine ridge (AAG01)] and Ballykelly [Ballykelly moraine ridge (BBG01)], there is evidence of ice advance from western Scotland across the sea bed onto the northern coastal tract during the last deglacial cycle. Although only the relative age can be estimated, the extraneous ice along this coastal sector impounded large water bodies in front of the local ice which was decaying southwards from the coastal lowlands towards the Sperrin Mountains and across the Antrim Plateau.
F. Ice mass evolution and large scale topography Directional evidence from glacial striae, moulded bedrock landforms and erratic carriage suggests that the indigenous ice sheets which last covered the north of Ireland radiated outwards during the glacial maximum and after (~22 to 15ka B.P.) from centres of dispersion (ice domes) located over the lowland basins. Ice from the Omagh and Lough Neagh basins expanded and overtopped the Sperrin and Mourne mountains. The latter did not sustain an independent ice cap such as the one which radiated from the Donegal mountains.
INFLUENCES ON THE CHARACTERISTICS OF GLACIOFLUVIAL LANDFORMS
Erosion, entrainment, transport and deposition of sediment by ice sheets occurred over large portions of mid-latitude continents between 22-11ka B.P. Eroded rock fragments of all sizes were transported along complex pathways below, within and on ice masses. Usually, prolonged high-energy transport resulted in the reshaping and comminution of the eroded material by sustained mechanical crushing and fracture under large tractive forces and high local stresses. Glacigenic sediment transport is not a passive phenomenon but involves movements at different levels in the ice masses which may occur at different rates and times.
During the deglacial period, the amount of meltwater generated by glacial ablation increases rapidly and generally moves towards the ice sheet margins or limits under high hydrostatic pressure. Water movement may occur as both sheet and channelised flow. These flows may concentrate and sort the heterogeneous products of glacial erosion, stored mainly in the basal ice layers and along thrust planes. Subsequent deposition of stratified deposits occurs in a range of environments usually at, near or beyond the glacial margin when there is a reduction in the water's transport velocity and thus the sediment carrying capacity of the flow. If these deposits are not eroded by subsequent meltwater flows or frontal advances they give rise to glaciofluvial landscapes.
Glaciofluvial sediments and landforms differ from 'normal' fluvial landscapes in several respects. The principal differences result from the variable nature of glacial meltwater discharge and volume of debris being transported. Meltwater output is highly variable in space and time, reflecting diurnal, seasonal and longer-term climate cycles and events. Local factors such as the storage capacity of water in and under the ice, the influence of subglacial topography on the drainage network and the strength of ice dams will modify this pattern. For example, high discharge catastrophic floods (such as j”kulhlaups) associated with the failure of ice dams may rapidly erode and reshape the morphology and sedimentary record of an ice marginal landscape, which may have developed over centuries. Sediment yields at the ice margin may be controlled not only by water flow rates but also by the volume and type of debris available in the ice system for transport. The sorting of clast sizes and densities during the transport and settling processes usually results however in the aggradation of stratified sand and gravel deposits [e.g. Murnies delta exposure (SDD0101A)].
Resultant glaciofluvial landforms may therefore generally contain a wide range of clast sizes which may possess a distinctive facies architecture composed of a range of sand and gravel grades. The stratified deposits of sand and gravel are often deposited in contact with masses of ice resulting in distinctive, oversteepened (ice-contact) slopes [e.g. Sultan delta (SBD04)]. When masses of buried dead-ice melt, topographic inversion results in a distinctive pitted or hummocky topography [e.g. Kilrea kettle-holes (AKH01) and Castlederg Hummocks (FFN002)].
Glaciofluvial landforms are formed during periods of net overall deposition. Accumulation occurs in a range of depositional settings which are generally referred to as subglacial, ice-marginal, proglacial or extra-glacial. In general, subglacial features represent feeder channels (eskers) to accumulations within the marginal zone (e.g. moraines, kames, outwash trains and deltas). In many cases these landform associations (subglacial to ice marginal/proglacial) can be used to reconstruct the direction of ice sheet decay and the direction of sediment transport.
There are at least five other general factors which influence the location and characteristics of glaciofluvial deposits and landforms.
1. Meltwater conduits The regional gradient of the ice surface slope dictates the hydraulic head (hydrostatic pressure) pressure for ice-directed subglacial meltwater flow. Meltwater throughflow and thus sediment transport in different ice masses may depend on the degree of interconnection of subglacial conduits and their maintenance during flow, particularly during periods of reduced flow [e.g. Bell's pit exposure, Glarryford esker (AGE0150A)]. The size, shape and stability of conduits strongly influences final landform patterns, as they play an important part in the location of marginal deposits.
2. Local topography The wastage of ice inevitably occurs across an irregular substrate of bedrock and/or diamict. Topographic highs that emerge during ice mass downwasting form nunataks, which may produce supraglacial debris. Localised depositional environments are thus formed within and around the ice marginal zones where the ice sheet impinges on hillslopes or along troughs.
3. Drainage patterns in stagnant ice Large amounts of dead or dynamically inactive ice may develop an internal hydraulic system and channel network related to local hydrostatic pressures and different to that found in active ice. The complex, often discontinuous network becomes the focus for sand and gravel deposition and the cavities which are infilled result in hummocky landscapes when the ice support melts. This may occur when the ice supply is cut-off or reduced due to ice thinning across a topographic barrier [e.g. Castlederg moraine (FFN02)].
4. Ice marginal retreat rates The relative rates of ice wastage during frontal retreat (i.e. backwasting) will determine whether or not substantial glaciofluvial accumulation will result. Where there is an approximate equilibrium between rates of ice supply (accumulation) and wastage (ablation) the ice front position stabilises. Ice however continues to move through the ice mass towards the marginal zone continually supplying debris. This may be deposited at the ice front as morainic sand and gravel accumulations, building ice-contact ridges and spreads or couplets of ice-contact ridges grading distally into flat outwash plains or terraces [e.g. Knocknabrock ridge (SNR02) and Burn Dennett (upper) outwash (SNO01)]. In contrast rapid ice retreat rates will deposit less debris in the proglacial area. Depending on the local sediment supply rates, ice-marginal morainic accumulations from ice masses in retreating rapidly tend not to thicken into ice-margin subparallel ridges. However the higher flux of meltwater often washes the sediment into more ice-distal settings, thus lowering the landform preservation potential in these environments.
5. Glaciotectonic deformation Significant forward oscillations or surges of the ice margin often thrust or 'bulldoze' slices of proglacial debris into ridges subparallel to the former ice margin [Muntober ridge (SBG02)]. The ridges thus contain glaciotectonic internal structures [e.g. Muntober ridge exposure (SBG0201A)] which tend to duplicate vertical stratigraphies.
6. Ice-marginal waterbodies As a result of the local topography, the ice marginal zone may terminate or be grounded in fresh or marine waterbodies. Ice-contact lakes may be drained or be of variable extent due to a range of factors. The geometry and morphology of deposits formed where glaciofluvial streams or glacial effluxes enter waterbodies is significantly different to deposits formed under running water. Flat-topped deltas and subaqueous aprons are a common feature [e.g. Carey Valley delta (PCD01), Killard Point morainic apron (UKS01)].
(EXPLANTION OF THE CODES ACCOMPANYING FEATURES
The data model, or scheme used to organise the elements of the symbolic model, uses a hierarchical system of landscape subdivision. This data structure allows the codification of five hierarchical levels in an eight-character code. The linking concept in reality is the natural, nested, hierarchical subdivision or structuring of the landscape into spatially defined areas, i.e. sections, units, subunits and sites, and features of interest within those areas, i.e. exposures. A unique, composite code allows the automation of the data structure. The composite code allows for the storage of the information in a relational database. The codes are up to eight figures long, giving unique identification of each landscape component (at each of the five levels) as a discrete geospheric entity. An eight-figure size limit to the code length was enforced to allow the transfer of the data to GIS packages limited to DOS operating system naming limitations. In no landscape does field variability exceed the allotted number of locations for each level e.g. there being over ninety-nine separate deltas in one unit.)
Glaciofluvial sand and gravel landscapes occupy fairly discrete tracts of generally well-drained land formed by glacial meltwater sorting of glacial debris during melting of the last ice sheet in Northern Ireland between ~22 and ~14ka B.P. (McCabe, 1996). The sorted glacial debris forms non-renewable resources which often provide topographic diversity especially on a local scale that contrasts markedly with the monotonous, streamlined drumlin hills in the lowlands of Northern Ireland [e.g. Glarryford esker (AGE01), Kilrea esker (AKE01)]. An explanation of landform codes is provided at the end. Within mountainous terrain sand and gravel occurs in mountain valleys and slopes with a topographic detail of ridges and undulations which enhances landscape quality and catch the eye of both traveller and scientist. In many upland sectors, the irregularities and intricate patterns of this surficial drape provides the local basis for habitat development and form the backcloths to our wetlands and post-glacial peat mantles [e.g. Cumber delta kettle hole (SDH0204), Murnies delta (SDD0105), Teal Lough outwash (SLO01)]. Preservation and conservation is therefore a priority on aesthetic and ecological grounds because of accelerated degradation of sand and gravel landscapes in the last twenty years.
Scientifically, glaciofluvial deposits provide critical information on the nature, pattern and characteristics of ice wastage at the end of the last ice age. The different depositional settings and range of palaeoenvironments inferred from the sedimentological and geomorphic data provide evidence for inferences on the mechanisms and controls of ice-sheet decay. Without this information we are unlikely to understand how our landscape was formed or the possible climatic changes which occurred within the coupled ocean-atmosphere-cryosphere system in the amphi-North Atlantic region [e.g. Killard Point]. An understanding of these relationships is essential if we are to unravel the complex mechanisms that control ice-sheet activity and its relationship to climate change through time. The record of past climate change preserved in glacigenic sediments and landforms provides accessible data on the long term evolution of the earth's climate, which can be used to assess predictions of future global climatic change involving ice-atmosphere-ocean interrelationships.
THE PLEISTOCENE PERIOD IN N. IRELAND
GLACIAL HISTORY OF N. IRELAND
The Irish landmass, situated in the north-eastern sector of the North Atlantic basin has undergone repeated glaciations during the Pleistocene period (cf. Colhoun et al., 1972; McCabe et al, 1987). Successive glaciations, through the erosive capacity of the ice masses produced vast amounts of debris which form the glacigenic deposits covering >90% of Northern Ireland. Their present morphology was principally formed during the last glacial cycle, and modified throughout the Holocene Period. Reconstructions of previous glacial cycles are difficult because isolated pieces of evidence are poorly constrained. The last Irish ice-sheet reached a maximum extent at about 22ka B.P., during a period of global temperature lowering. The till and glaciofluvial deposits which provide evidence of the last glacial cycle were deposited mainly in the (deglacial) period, dating from the glacial maximum to the onset of the present warm stage (Holocene epoch). Since then, fluvial erosion and human activities have modified the appearance of many glaciofluvial landscapes, though pristine examples of original forms have survived almost intact [e.g. Murrins moraine (OMN01), Gortin delta (SGD01)].
THE EVIDENCE FOR GLACIATION
Popular opinion views contemporary ice-sheets as unique phenomena, typical of distant terrains far from present-day centres of population. These glaciers are active analogues to the large mid-latitude ice sheets which waxed and waned during the Pleistocene period. Yet only fifteen thousand years ago, the entire north of Ireland was covered by an ice sheet possibly a few kilometres in thickness. The legacy of ice sheet activity on Northern Ireland's landscape has been immense, creating topography, soil, ground water regimes, vegetation and local habitats. The imprints left by erosive and depositional events provide us with local topographic styles and complexity e.g. spectacular erosive gashes forming overdeepened mountain valleys [e.g. Glenelly], streamlined drumlin fields, steep-sided cross-valley ridges [e.g. Knockalerry ridge (SBG01), Black Hill ridge (AMR01), Deer Park moraine (FFR01)] and flat-topped deltas [e.g. Carey Valley delta (PCD01), Wood Hill delta (AMD01), Fruitfield delta (BLD01)]. The landscape mosaic is therefore complex both in aesthetic terms and scientific basis and should be managed as a valuable asset.
All forms of glacial geologic evidence are rare on a world scale in that they are only to be found in previously glaciated regions. Due both to the erosive and depositional action of glaciers, it is predominately the imprint or signature of ice sheet behaviour during the last glacial cycle which is observed in the form of distinctive landscapes. Sediments accumulated from previous glacial events are generally buried at depth or reworked into new forms. Northern Ireland enjoys a privileged position internationally as a microcosm of 'fresh' evidence for previous glacial activity and various glacial processes. The evidence for ice-sheet activity is in three principal forms. Data from all three categories are needed for a complete reconstruction of ice-sheet conditions during the glacial cycle.
1. Stratigraphic data Stratigraphic data form long term (~100,000 yr) climatic proxy data. It has been found at depth and records multiple glacial cycles in Northern Ireland. Deep exposures, especially at Aghnadarragh, Co. Antrim, on the margins of Lough Neagh, contain sediment piles which record two major ice sheet expansions separated by a long phase of Boreal and Arctic conditions (McCabe et al., 1987). Other sections in Co. Fermanagh at Derryvree and Hollymount indicate similar major environmental reversals of climate over the last 100kyr (Colhoun et al., 1972).
2. Ice flow patterns During the last (late Midlandian) glacial cycle, the last main phases of ice sheet flow occurred between 19ka and 14ka B.P. The imprint of these ice flows on the landscape is best seen in Northern Ireland in the patterns of flow transverse Rogen moraines and flow parallel drumlin swarms which are developed across the thick mantles of till which occur mainly below 150m O.D. Reconstructed early (~22-19ka B.P.) ice flow patterns show radial ice flow from ice dispersion centres in Lough Neagh, the Omagh basin and Lower Lough Erne/Donegal probably resulting from strongly positive ice-sheet mass budgets. Thick, temperate ice flows moved north across the Sperrins, along the Foyle valley and across north Antrim onto the continental shelf and generally southwestwards, southwards and southeastwards, across the Erne basin, Co. Armagh and Co. Down into the Irish Sea Basin.
3. Glaciofluvial landscapes Deglaciation (ice wastage) began around 19ka B.P. when the regional climate began to ameliorate. Ice thicknesses decreased and ice margins began to contract back, generally towards centres of ice dispersion in the lowland areas of north central Ireland. Large and often catastrophic meltwater releases washed and sorted glacially eroded debris occurring in, on and below the ice-sheet, creating the geologic base of Northern Ireland's sand and gravel landscapes. The accumulation of sand and gravel sediments occurred in a range of palaeoenvironmental settings beneath [e.g. Glarryford esker (AGE01)], beside [e.g. Lough Fea kame terrace (SLT01)] and in front of [e.g. Lough Fea outwash plain (SLO01)] the decaying ice masses. In broad terms and at variable spatial scales, these accumulations are time transgressive from the periphery of Northern Ireland inwards towards the core centres of ice dispersion and the centre of the province.
THE NATURE AND SPATIAL PATTERN OF GLACIAL IMPRINTS
However this simplistic pattern of ice decay and sediment deposition is conditioned and complicated by interaction between six factors which influence the final location and morphology of glaciofluvial landforms:
A. Rates of ice retreat The ice front remains relatively stationary when the rate of ice accumulation and subsequent forward movement is balanced by the melting flux (ablation) at the ice sheet margins. Continuous sediment delivery to the ice margin by ablated meltwater results in a morainic ridge or spread of glaciofluvial sand and gravel aggrading at or beyond the ice margin [e.g. Black Hill ridge (AMR01), Teal Lough moraine ridge (SLR01)]. Where climatic conditions result in rapid melting with a much smaller input of ice mass to the ice-sheet margin, rapid ice-marginal retreat rates may leave little evidence of former ice marginal positions in the form of sand and gravel landscapes. A reduction in the rate of ice supply to the margins may occur due to interactions between topography, direction of ice-sheet flow or changing climatic conditions. This may result in a suite of hummocky landforms formed by the deposition of sediment in and around stagnant proglacial ice blocks [e.g. Castlederg moraine (FFN02)].
B. Meltwater erosion Intensive meltwater erosion, which occurs as the ice margin vacates a site, may result in the destruction of ice marginal sediment accumulations. Many sand and gravel landforms therefore have a low preservation potential. The topography of those that survive is often as much the product of the post-depositional erosion (dissection) of the original depositional morphology. For example a moraine ridge may be eroded into a series of discontinuous ridges and outwash spreads [e.g. Deer Park moraine (FFR01) and Foyle valley outwash (FFO01)].
C. Ice sheet growth and decay cycles Ice sheet growth and localised frontal advances related to short term climatic deterioration can totally obliterate or modify previously formed frontal landforms [e.g. Muntober ridge (SBG02)]. Such oscillatory growth and decay of ice sheets is a common pattern of ice mass behaviour during a deglacial cycle (cf. McCabe, 1996).
D. Ice thinning Melting and downwasting of the ice-sheet results in ice thinning. Ice mass loss is usually concentrated in a relatively narrow zone around the margin, where a combination of lower elevation, higher temperatures and meltwater flux results in increased ablation rates. In this situation, small-scale topography can play an important role in controlling the location and shape of the ice margin. Emergent hills may split an ice sheet into independent sectors or lobes [e.g. Mullaghmore]. Sand and gravel deposition will tend to be concentrated in shallow ponded meltwater lakes along the sinuous, debris-rich margins. These margins may be banked against hillslopes [e.g. Lough Fea kame terrace (SLT01)], block valleys, or project through upland mountain cols [e.g. Murnies delta]. Subsequently the complex pattern of sand and gravel accumulation in spreads and ridges requires detailed field mapping before reconstruction of ice marginal positions can be attempted.
E. Extraneous (Scottish) ice In the northern coastal lowlands around Armoy [Armoy moraine ridge (AAG01)] and Ballykelly [Ballykelly moraine ridge (BBG01)], there is evidence of ice advance from western Scotland across the sea bed onto the northern coastal tract during the last deglacial cycle. Although only the relative age can be estimated, the extraneous ice along this coastal sector impounded large water bodies in front of the local ice which was decaying southwards from the coastal lowlands towards the Sperrin Mountains and across the Antrim Plateau.
F. Ice mass evolution and large scale topography Directional evidence from glacial striae, moulded bedrock landforms and erratic carriage suggests that the indigenous ice sheets which last covered the north of Ireland radiated outwards during the glacial maximum and after (~22 to 15ka B.P.) from centres of dispersion (ice domes) located over the lowland basins. Ice from the Omagh and Lough Neagh basins expanded and overtopped the Sperrin and Mourne mountains. The latter did not sustain an independent ice cap such as the one which radiated from the Donegal mountains.
INFLUENCES ON THE CHARACTERISTICS OF GLACIOFLUVIAL LANDFORMS
Erosion, entrainment, transport and deposition of sediment by ice sheets occurred over large portions of mid-latitude continents between 22-11ka B.P. Eroded rock fragments of all sizes were transported along complex pathways below, within and on ice masses. Usually, prolonged high-energy transport resulted in the reshaping and comminution of the eroded material by sustained mechanical crushing and fracture under large tractive forces and high local stresses. Glacigenic sediment transport is not a passive phenomenon but involves movements at different levels in the ice masses which may occur at different rates and times.
During the deglacial period, the amount of meltwater generated by glacial ablation increases rapidly and generally moves towards the ice sheet margins or limits under high hydrostatic pressure. Water movement may occur as both sheet and channelised flow. These flows may concentrate and sort the heterogeneous products of glacial erosion, stored mainly in the basal ice layers and along thrust planes. Subsequent deposition of stratified deposits occurs in a range of environments usually at, near or beyond the glacial margin when there is a reduction in the water's transport velocity and thus the sediment carrying capacity of the flow. If these deposits are not eroded by subsequent meltwater flows or frontal advances they give rise to glaciofluvial landscapes.
Glaciofluvial sediments and landforms differ from 'normal' fluvial landscapes in several respects. The principal differences result from the variable nature of glacial meltwater discharge and volume of debris being transported. Meltwater output is highly variable in space and time, reflecting diurnal, seasonal and longer-term climate cycles and events. Local factors such as the storage capacity of water in and under the ice, the influence of subglacial topography on the drainage network and the strength of ice dams will modify this pattern. For example, high discharge catastrophic floods (such as j”kulhlaups) associated with the failure of ice dams may rapidly erode and reshape the morphology and sedimentary record of an ice marginal landscape, which may have developed over centuries. Sediment yields at the ice margin may be controlled not only by water flow rates but also by the volume and type of debris available in the ice system for transport. The sorting of clast sizes and densities during the transport and settling processes usually results however in the aggradation of stratified sand and gravel deposits [e.g. Murnies delta exposure (SDD0101A)].
Resultant glaciofluvial landforms may therefore generally contain a wide range of clast sizes which may possess a distinctive facies architecture composed of a range of sand and gravel grades. The stratified deposits of sand and gravel are often deposited in contact with masses of ice resulting in distinctive, oversteepened (ice-contact) slopes [e.g. Sultan delta (SBD04)]. When masses of buried dead-ice melt, topographic inversion results in a distinctive pitted or hummocky topography [e.g. Kilrea kettle-holes (AKH01) and Castlederg Hummocks (FFN002)].
Glaciofluvial landforms are formed during periods of net overall deposition. Accumulation occurs in a range of depositional settings which are generally referred to as subglacial, ice-marginal, proglacial or extra-glacial. In general, subglacial features represent feeder channels (eskers) to accumulations within the marginal zone (e.g. moraines, kames, outwash trains and deltas). In many cases these landform associations (subglacial to ice marginal/proglacial) can be used to reconstruct the direction of ice sheet decay and the direction of sediment transport.
There are at least five other general factors which influence the location and characteristics of glaciofluvial deposits and landforms.
1. Meltwater conduits The regional gradient of the ice surface slope dictates the hydraulic head (hydrostatic pressure) pressure for ice-directed subglacial meltwater flow. Meltwater throughflow and thus sediment transport in different ice masses may depend on the degree of interconnection of subglacial conduits and their maintenance during flow, particularly during periods of reduced flow [e.g. Bell's pit exposure, Glarryford esker (AGE0150A)]. The size, shape and stability of conduits strongly influences final landform patterns, as they play an important part in the location of marginal deposits.
2. Local topography The wastage of ice inevitably occurs across an irregular substrate of bedrock and/or diamict. Topographic highs that emerge during ice mass downwasting form nunataks, which may produce supraglacial debris. Localised depositional environments are thus formed within and around the ice marginal zones where the ice sheet impinges on hillslopes or along troughs.
3. Drainage patterns in stagnant ice Large amounts of dead or dynamically inactive ice may develop an internal hydraulic system and channel network related to local hydrostatic pressures and different to that found in active ice. The complex, often discontinuous network becomes the focus for sand and gravel deposition and the cavities which are infilled result in hummocky landscapes when the ice support melts. This may occur when the ice supply is cut-off or reduced due to ice thinning across a topographic barrier [e.g. Castlederg moraine (FFN02)].
4. Ice marginal retreat rates The relative rates of ice wastage during frontal retreat (i.e. backwasting) will determine whether or not substantial glaciofluvial accumulation will result. Where there is an approximate equilibrium between rates of ice supply (accumulation) and wastage (ablation) the ice front position stabilises. Ice however continues to move through the ice mass towards the marginal zone continually supplying debris. This may be deposited at the ice front as morainic sand and gravel accumulations, building ice-contact ridges and spreads or couplets of ice-contact ridges grading distally into flat outwash plains or terraces [e.g. Knocknabrock ridge (SNR02) and Burn Dennett (upper) outwash (SNO01)]. In contrast rapid ice retreat rates will deposit less debris in the proglacial area. Depending on the local sediment supply rates, ice-marginal morainic accumulations from ice masses in retreating rapidly tend not to thicken into ice-margin subparallel ridges. However the higher flux of meltwater often washes the sediment into more ice-distal settings, thus lowering the landform preservation potential in these environments.
5. Glaciotectonic deformation Significant forward oscillations or surges of the ice margin often thrust or 'bulldoze' slices of proglacial debris into ridges subparallel to the former ice margin [Muntober ridge (SBG02)]. The ridges thus contain glaciotectonic internal structures [e.g. Muntober ridge exposure (SBG0201A)] which tend to duplicate vertical stratigraphies.
6. Ice-marginal waterbodies As a result of the local topography, the ice marginal zone may terminate or be grounded in fresh or marine waterbodies. Ice-contact lakes may be drained or be of variable extent due to a range of factors. The geometry and morphology of deposits formed where glaciofluvial streams or glacial effluxes enter waterbodies is significantly different to deposits formed under running water. Flat-topped deltas and subaqueous aprons are a common feature [e.g. Carey Valley delta (PCD01), Killard Point morainic apron (UKS01)].
(EXPLANTION OF THE CODES ACCOMPANYING FEATURES
The data model, or scheme used to organise the elements of the symbolic model, uses a hierarchical system of landscape subdivision. This data structure allows the codification of five hierarchical levels in an eight-character code. The linking concept in reality is the natural, nested, hierarchical subdivision or structuring of the landscape into spatially defined areas, i.e. sections, units, subunits and sites, and features of interest within those areas, i.e. exposures. A unique, composite code allows the automation of the data structure. The composite code allows for the storage of the information in a relational database. The codes are up to eight figures long, giving unique identification of each landscape component (at each of the five levels) as a discrete geospheric entity. An eight-figure size limit to the code length was enforced to allow the transfer of the data to GIS packages limited to DOS operating system naming limitations. In no landscape does field variability exceed the allotted number of locations for each level e.g. there being over ninety-nine separate deltas in one unit.)
| Enlander, I., Dempster, M. & Doughty, P., 2024. Introduction to the glacial Sand and Gravel Landscapes of Northern Ireland, County Antrim, Armagh, Down, Fermanagh, Londonderry, Tyro, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=439. Accessed on 2024-12-26 |
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