Glacial Erosion

Box 114 Moraine Banks And Cyclic Glacier Fluctuations

Flute Ice Glacier

The behaviour of individual Alaskan tidewater or fjord glaciers has often been observed to be asynchronous with other tidewater glaciers and with those on land. It has been suggested that tidewater glaciers may behave in a cyclic fashion due to the inherent instability of a calving glacier margin. The rate of glacier calving is strongly influenced by water depth. As a tidewater glacier advances down a fjord in equilibrium with its mass balance a moraine shoal or moraine bank will form along its...

Glacier Hydrological Systems

Erosao Glacial

Glacial meltwater finds its way through the glacier from its point of origin along a variety of different flow paths (Figure 4.2). Meltwater derived from surface melting and from basal melting will tend to follow different paths through a glacier, although both will invariably involve channel flow either within or on the glacier. The type of channel system within a particular glacier depends primarily on its thermal regime. On glaciers consisting of cold ice, meltwater is unable to penetrate...

Microscale Features Of Glacial Erosion

Glacier Uny

Microscale landforms of glacial erosion are generally less than 1 m in size and are often found superimposed on larger landforms. There are four types of microscale landform (i) striations (ii) micro crag and tails (iii) friction cracks and (iv) p-forms and microchannels (Table 6.1). Glacial abrasion produces lines or scratches on a rock surface as debris is dragged over it (Figures 6.2 and 6.3). The scratches created by glacial abrasion are known as striations, which in combination produce...

Fluvial Sedimentation

Glacier Erosion

Glacial meltwater entrains and transports sediment, which is subsequently deposited on, within, beneath or beyond the glacier. Sedimentation on or within the glacier may occur in surface channels and in either englacial or subglacial tunnels. Sedimentation in surface (supraglacial) channels is inhibited by the steep channel gradients and the smooth ice walls, which provide little frictional drag on the flow and its sediment load (Figure 4.1A). Deposition does, however, occur due to changes in...

Glaciofluvial Icemarginal Landforms

Outwash Fan Landform

There are two main sources of glacial meltwater at a glacier snout water emerging from subglacial meltwater portals or from supraglacial channels fed by surface melt and the emergence of englacial conduits. As glacial meltwater emerges at the ice margin its velocity typically falls due to changes in confining pressure, gradient and increases in bed or channel friction. As a consequence, meltwater streams deposit sediment rapidly to form a variety of outwash-related landforms. The morphology and...

Sedimentation In Lacustrine Environments

Ole Railway Earth Wire Clamp Pole

Lacustrine ice margins develop in a variety of different situations. Glaciers may dam lakes, lakes may develop in front of a glacier due to the melting of stagnant ice beneath the proglacial surface, lakes may be dammed between moraines and an ice front, or a glacier may simply drain into a rock-confined basin. During the last glacial cycle both the Laurentide and Fennoscandinavian ice sheets terminated in large proglacial lakes formed in part by the glacio-isostatic depression of the crust in...

Box 122 The Origin And Significance Of Streamlined Glacial Landforms

Komatiites Glacial Till Flow

Some of the most commonly used landforms in ice-sheet reconstructions are a family of ice-moulded and streamlined landforms comprising glacial lineations, drumlins, megaflutes and megascale glacial lineations (MSGL). These landforms are visible on satellite images as large-scale patterns of topographic streamlining, sometimes with distinctive cross-cutting relationships. The image below shows a Landsat subscene (left panel) and interpretation (right panel) of the glacial geomorphology of the...

Box 121 Glacial Geological Inversion Models

Glacial Cycle Erosion

In a series of papers, Johan Kleman and his co-workers have outlined the procedures used in glacial geological inversion models (i.e. a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets) to reconstruct past ice sheets from the glacial geomorpholo-gical record (Kleman and Borgstrom, 1996 Kleman et al., 1997, 2006). Their inversion model comprises a classification system for glacial landform assemblages and uses the following 'rules' 1. The...

Box 91 Megascale Thrustfault Complex In Northwest Russia

Glaciolacustrine Erosion

Larsen et al. (2006) describe a large glaciotectonic moraine system on the northern coast of the Kanin Peninsula in northwest Russia. Exposed on the coast is a 20 km long section up to 80 m high, which is composed of alternating layers of diamict, glaciofluvial, glaciolacustrine and marine sediment. These sediments have been dated to the first part of the last glacial cycle and were initially deposited in marine embayments and ice-dammed lakes between ice of the Barents Sea Ice Sheet and the...

Box 69 Landforms Of Erosion And Deposition Associated With Coldbased Glaciers

Hills Created Clacier

Cold-based glaciers do not produce large quantities of subglacial meltwater, so it is commonly assumed that they neither slide nor abrade their beds. As a result, it is commonly assumed that they do not achieve much glacial erosion. Atkins et al. (2002) challenged this assumption, describing glacial geomorpho-logical features associated with erosion and deposition by a cold glacier in the Allan Hills, Antarctica. They mapped and described a range of landforms, bedrock features and sediments...

Tropical Glaciers The Cordillera Blanca Peru

Valley Glaciers Peru

We normally think of glaciers as occupying only cold places so it may come as a bit of a surprise to learn that tropical glaciers occur near the Equator on the mountains of East Africa, Indonesia and in the Andes. On a global scale, the total area of these tropical glaciers is small ( 2500 km2) and they make up only 4 of the area of all 2.6 Tropical Glaciers The Cordillera Blanca, Peru 27 Earth's mountain glaciers which is equivalent to about 0.15 of the Earth's total glacier area. More than 99...

Box 34 Observations Of Subglacial Deformation

Subglacial Soft Bed Deformation

Direct observations of deforming sedimentary layers beneath glaciers come from tunnels and cavities. Boulton and Hindmarsh (1987) excavated a series of tunnels within the basal ice of Brei9amerkurjokull in Iceland, from which a series of strain markers (small cylinders) and other instruments were inserted through drill holes into the subglacial till. The experiment lasted for 136 hours, during which time glacier velocity and the subglacial water pressure were monitored. At the end of the...

Box 94 Interpreting Landform Assemblages Examples From The Scottish Highlands

Glaciers Erosion Examples Project

The mountains of the British Isles were last occupied by glacier ice during a short sharp return to cold conditions at the end of the last glacial cycle, which is known as the Younger Dryas. Glaciers left a distinctive topography of mounds and ridges that became known as Scottish Hummocky Moraine. These deposits were interpreted in the late 1960s and 1970s as the product of ice stagnation a theory consistent with emerging evidence of the rapidity of climate warming after the Younger Dryas. This...

Glacial Erosion

Images Glacial Erosion

Although we can easily identify individual landforms of glacial erosion it is often much more difficult to quantify the extent to which an entire landscape has been modified or affected by glacial erosion. Haynes (1977) and Riedel et al. (2007) quantified the effects of glacial erosion on the Scottish and the USA Canadian Cordilleran landscapes respectively by examining the degree of valley connectivity in these areas. Fluvial landscapes tend to produce dendritic drainage patterns with low...

Debris Entrainment

Glacier Erosion

The evacuation and entrainment of rock fragments at the glacier bed were briefly outlined in Section 5.2.2 but before we consider low-level debris transport in depth we need to consider the main processes of basal debris entrainment. Figure 7.5 Supraglacial debris stripes emerging from longitudinal foliation on the surface of Midre Lovenbreen, Svalbard. The glacier has receded in recent years and the debris stripes can be traced onto the glacier forefield. Photograph N.F. Glasser Figure 7.5...

Box 85 Sediment Transport In Glacial Meltwater Streams

Subsidence Deposition

Reliable estimates of bedload transport within meltwater streams are difficult to obtain, due to the high flow magnitudes and large sediment volume involved. One of the first studies to provide reliable estimates of bedload transport was that of 0strem (1975). Two techniques were used to measure the bedload of a proglacial stream in front of the glacier Nigardsbreen in Norway. The first method involved the construction of a 50 m steel fence across the main meltwater stream during the summer of...

Box 22 The Patagonian Icefields

Patagonian Icefield Image

There are many glaciers in the Andes south of 46 S in Patagonia. Two major ice masses exist in the region. First is the 4200 km2 North Patagonian Icefield (47 00'S, 73 39'W), which is some 120 km long and 40-60 km wide, capping the Andes between altitudes of 700-2500 m a.s.l. Annual precipitation on the western side of the icefield increases from 3700 mm at sea level to an estimated maximum of 6700 mm at 700 m a.s.l. Precipitation decreases sharply on the eastern side of the icefield. The North...

Box 71 The Structure Of Glaciers

Glacial Structions And Formations

Glaciers contain a number of distinct structures. These can be divided into (i) primary structures that result from accumulation, and (ii) secondary structures that develop due to ice deformation during flow. The principal primary structure is ice stratification, which results from the accumulation of snow each year. Summer surfaces are usually indicated by a refrozen melt-layer of bluish ice and by a concentration of debris that has fallen onto the glacier surface. Regelation layers are also...

Arctic Glaciers

Net surface mass balance is the sum of winter accumulation and summer losses of mass from glaciers and ice sheets. Where this is positive, glaciers grow and where it is negative, they recede. Net surface mass balance is determined primarily by changes in climate. Dowdeswell et al. (1997) examined records of surface mass balance for more than 40 Arctic ice caps and glaciers going back in some cases as far as the 1940s. They deliberately excluded the large Greenland Ice Sheet because its surface...

Icemarginal Moraines

The Quaternary Enhanced Glacial Erosion

Ice-marginal landforms produced directly by the action of a glacier are known as ice-marginal moraines. They may form by the action of four main processes, none of which need to be mutually exclusive (i) ice-marginal or submarginal glaciotec-tonics (ii) ice-marginal dumping of debris via a range of processes including rockfall and debris flow (iii) ice surface marginal meltout and (iv) subglacial transport and meltout of debris. As a consequence, a practical and universally accepted taxonomy...

Mesoscale Features Of Glacial Erosion

Global Warming The Earth Then And Now

Mesoscale features of glacial erosion are those between 1 m and 1 km in size and comprise a family of landforms which includes (i) streamlined bedrock features (ii) stoss and lee forms (iii) rock grooves and basins and (iv) meltwater channels (Table 6.2). At a mesoscale the most common effect of glacial erosion is to streamline bedrock protrusions to produce positive and upstanding landforms. These streamlined landforms, whalebacks, are also referred to by a wide variety of different terms...

Box 63 Glacial Meltwater Channels In The Labyrinth Evidence For Huge Subglacial Floods From The Antarctic Ice Sheet

Overstroming Smeltwater

Perhaps the most famous meltwater channel system in the world is the Labyrinth, a > 50 km long network of bedrock channels and scoured terrain in front of Wright Upper Glacier in Antarctica. Lewis et al. (2006) described in detail the geomorphology of the channels that make up the Labyrinth. It consists of a number of different levels. On the upper and intermediate levels are signs of glacial erosion in the form of striations and ice-moulding. On the lower level the channels and canyons are...

Sources Of Glacial Meltwater

Images Glacial Erosion

Glacial meltwater is derived from the melting of ice in one of three positions supraglacial, which means on the ice surface, subglacial, at the bed, and englacial, which means within the glacier (Figure 4.1). Melting occurs whenever there is sufficient heat to turn the ice back into water, and this heat can be supplied by (i) solar radiation (ii) friction generated by ice flow and (iii) heat derived from the Earth's crust beneath the glacier (geothermal heat). Melting on the ice surface is the...

The Principles Of Basal Thermal Regime

Velocity Profile Glacier

The principal control on which combination of flow processes operate beneath a given glacier - creep or basal sliding - is the temperature of the basal ice. Some glaciers are frozen to their beds and no meltwater is present at the ice-bed interface and basal sliding does not occur (Figure 3.9). Such glaciers are composed of cold ice. In contrast, other glaciers are composed of warm ice, where basal ice is constantly melting and the ice-bed interface is therefore lubricated with meltwater. In...

Macroscale Features Of Glacial Erosion

Glacial Erosion Features

Macroscale features of glacial erosion are those features that are 1 km or greater in dimension. They are large enough to form significant landscape elements and may contain many of the smaller landforms already considered in this chapter. Five main landforms are recognised at this scale (i) regions of areal scour (ii) troughs (iii) cirques (iv) giant stoss and lee forms and (v) tunnel valleys (Table 6.4). The most commonly encountered landscape of glacial erosion is one of areal scour. It...

Box 62 Using Landforms Of Glacial Erosion To Reconstruct Glacier Dynamics

Hubbard Glacier Depth

Landforms of glacial erosion are often used by glacial geologists to infer basic glaciological parameters such as ice-movement direction and change over time, but are seldom used for anything more complex. Sharp et al. (1989) demonstrated for the first time how glacial erosional landforms might be used to reconstruct parameters that affect the operation of basal processes and glacier dynamics. Their field study area is Snowdon, North Wales an area that has been subjected to multiple glaciations...

Suggested Reading

The literature on glacial deposition is considerable. The books by Evans and Benn (2004) and Hubbard and Glasser (2005) provide a good insight into a range of sedimentary methods associated with dealing with glacial sediments, as does the paper by Kriiger and Kj r (1999). Eyles et al. (1983) cover the use of lithofacies analysis, and the paper and book by Brodzikowski and Van Loon (1987, 1991) provide useful overviews of glacial sedimentary facies. The two volumes edited by Menzies (1995a,b)...

Box 98 Formation Of Braided Eskers Some Examples From The Scottish Highlands

Eskers Drawing Figure

There are a number of very good examples of braided eskers within the Scottish Highlands. In particular several good examples exist within a corridor of eskers, kames and outwash sediments that runs from Lanark in the southwest towards Edinburgh in the northeast. This corridor of glaciofluvial sediment appears to have been deposited in an interlobe environment created by the progressive decoupling, during deglaciation at the end of the last glacial cycle, of two confluent ice sheets within the...

Arctic Polythermal Glaciers

Glaciers are common on many of the landmasses surrounding the Arctic Ocean. Excluding the Greenland Ice Sheet, glaciers and ice caps cover about 275 000 km2 of the archipelagos of the Canadian, Norwegian and Russian High Arctic and the area north of about 60 N in Alaska, Iceland and Scandinavia. Although there is of course huge variability in their behaviour it appears that since the 1940s most Arctic glaciers have experienced a predominantly negative net surface mass balance. Indeed, there is...

Mechanisms Of Ice Flow

A glacier flows because the ice within it deforms in response to gravity. This gravitational force is derived from the fact that glaciers slope towards their termini as a result of the spatial imbalance between accumulation and ablation discussed in the previous section. If there is no surface slope - no imbalance between accumulation and ablation - the glacier would not flow. The force per unit area set up within a mass of ice by gravity which causes it to deform is known as the shear stress....

Subglacial Water Pressure

4.6.1 Subglacial Water Pressure and Effective Normal Pressure Subglacial water pressure has an important role in many subglacial processes, through its control on the effective normal pressure beneath a glacier. Effective normal pressure is the force per unit area imposed vertically by a glacier on its bed. For a cold-based glacier it is effectively equal to the weight of the overlying ice thick ice imposes a greater pressure than thin ice. This is summarised by where N is the normal effective...

Box 82 Till Fabric A Genetic Fingerprint

Glacial Till Diagram

Till fabric analysis involves recording the compass orientation and dip of elongated clasts within a till. Generally only clasts with a pronounced long axis relative to the short and intermediate axes are analysed. Suitable clasts are carefully excavated from a cleared face of undisturbed till and the dip or inclination of each particle, along its long axis, is measured using a compass clinometer. The orientation of each particle, in the direction in which the long axis dips, is also recorded...

Box 83 Architectural Components Of A Subglacial Till

Bands Glacial Deposits

Boyce and Eyles (2000) used an architectural element analysis to unpick the components present within a subglacial till (Northern Till) in Ontario, Canada. The method works by breaking down the individual components present (elements or lithosomes), while also focusing on the characteristics of the boundaries between these elements. The approach is to establish the individual building blocks that make up a complete facies and recognise a hierarchy of bounding surfaces that separate these...

Glaciofluvial Subglacial Landforms

The principal landform formed by meltwater flow beneath a glacier is the esker. Eskers are the deposits of former subglacial, englacial or supraglacial channels. They are usually slightly sinuous ridges of glaciofluvial sediment that undulate in height along their length. Their orientation is controlled by glacier slope and the pattern of water pressure potential within the glacier they may therefore show little respect for subglacial topography and need not trend downslope. Two broad types of...

Box 33 Estimates Of Basal Shear Stress And The Accuracy Of Glacial Reconstructions

Erosion Accumulation Obstacles

During the last glacial period the Yellowstone National Park in North America was covered by a number of small mountain icefields, one of which is referred to as the Pinedale ice mass. From a careful field study of the landforms and sediments within this area Pierce (1979) was able to reconstruct the morphology of this ice mass. Any reconstruction of a former icefield, on the basis of geomorphological evidence, is likely to be speculative in some respect. Pierce (1979) argued that glaciology...

Box 46 Predicting The Magnitude Of Jokulhlaups

Attempts have been made to predict the peak flow magnitude of jokulhlaups, both for theoretical purposes and for use in design of bridges or structures that may have to withstand jokulhlaup flows. Developing theoretical relationships to predict discharge from ice-dammed lakes has proved difficult, not least because the exact mechanisms of drainage are poorly understood in many cases. However, Clague and Mathews (1973) developed an empirical equation based on data from the jokulhlaups for which...

Glaciolacustrine Landforms

There is a wide variety of different types of glacial lake, although two broad types can be identified those that form along an ice margin and those that occur in a supraglacial setting. Ice-marginal lakes may form in front of glaciers or when ice dams water in a valley or against a hill side (see Figure 2.7). Supraglacial lakes can develop either where an ice-dammed lake expands over an ice margin (Figure 11.2) or in areas of complex ice-cored topography. The geomorphological products of these...

Southern Hemisphere Temperate Glaciers Patagonia And New Zealand

Numerous ice caps, outlet glaciers and independent valley glaciers occur in the mountainous areas of southern South America (Patagonia) and in New Zealand. In Patagonia there are many glaciers in the Andes south of 46 S, fed by high precipitation from the Southern Westerlies (Box 2.2). Annual precipitation on the large icefields here is estimated to be as high as 10000 mm water equivalent - one of the highest in the world. As a result, the western glaciers have high mass-balance gradients and...

Box 51 Direct Observations And Measurements Of Glacial Abrasion

The direct observation of abrasion in action is extremely difficult because it involves digging tunnels through a glacier to access basal cavities. Some of the first observations are those of Boulton (1974) who described the movement of a basalt fragment over a large basalt roche moutonnee 20 m below the surface of Brei9amerkurjokull in southeast Iceland. The basalt fragment was removed from the base of the glacier and the surface that had been in contact with the bed was inspected. The...

Estimating Rates Of Glacial Erosion

In the previous sections we examined the principal processes of glacial erosion. The efficiency of these processes coupled with the length of time over which they operate determines the amount of glacial erosion that can be achieved by a glacier. In this section we examine the methods available with which to calculate the depths and rates of glacial erosion. There are four ways in which estimates of the rates of glacial erosion can be obtained. 1. Direct observation beneath modern glaciers....

Patterns Of Glacial Erosion

The primary or regional pattern of glacial erosion within a glacier or ice sheet is controlled by basal thermal regime (Figure 5.9). Only when an ice sheet or glacier is warm-based is meltwater produced in large quantities, and only when meltwater is abundant can widespread basal sliding and therefore glacial abrasion occur. As we have seen, glacial quarrying is also facilitated by the presence of meltwater. It is possible to predict where erosion is likely to occur within an ice sheet on the...

Box 93 Formation Of Stacked Till Sheets And Icemarginal Moraines

Gletschererosion

Evans and Hiemstra (2005) reported observations made at a number of Icelandic glaciers. They identify the presence of stacked sequences of till layers within ice-marginal settings forming moraine systems. Study of both microscopic and macroscopic processes indicate that the tills were deposited by a range of processes and not just subglacial deformation. They suggest that each layer of sediment was deposited by a distinctive cycle of events 1. During the late summer subglacial lodgement, and...

Box 103 Onset Of Northern Hemisphere Glaciation

Marine sediments provide a better record of Earth's glacial history than terrestrial sediments because their rate of sedimentation is more continuous and they are subjected to fewer episodes of erosion. A glacial signature can be found in these sediments via the presence of ice-rafted material such as dropstones or pods of coarse-grained debris within very fine-grained marine silts. This type of evidence has been used recently to challenge the accepted date for the onset of northern hemisphere...

Patterns And Rates Of Ice Flow

Brett Whiteley Wave Painting

Within a glacier flow usually follows the direction of the surface slope. Figure 3.3 shows a cross-section through both an ice sheet and a glacier. In an ice sheet the ice flows in two opposite directions from the summit or ice divide (Figure 3.3). In the accumulation area, flow takes place downwards into the ice, counteracting the upward growth of the surface through accumulation. In the ablation zone the surface is lowered by ablation, which causes ice to effectively rise towards the surface....

Box 31 The Mass Balance Of The Polar Ice Sheets And Global Sealevel Rise

One of the most important advances in glaciology in the past two decades has been the ability to monitor how the Earth's two polar ice sheets are changing using satellite measurements. As global temperatures have risen, so have rates of snow accumulation, ice melting and glacier flow in the Polar Regions. Although Glacial Geology Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser 2009 John Wiley & Sons, Ltd the balance between these opposing processes has varied...

Box 96 Drumlins And Subglacial Deformation

Subglacial Erosion

Evidence for the theory of drumlin formation by subglacial deformation is provided by Boyce and Eyles (1991). These authors studied the Peterborough drumlin field in central Canada, which was formed beneath a lobe of ice at the margin of the former Laurentide Ice Sheet during the last glacial cycle. They examined the morphology and internal composition of the drumlins along a line parallel to the direction of glacier flow (a flow line). Along the flow line the drumlins change from elongate to...

Box 101 Glacial Till Or Icerafted Diamicts

Along the shore of Lake Ontario to the east of Toronto, in Canada, a sequence of glacial sediments is exposed at Scarborough Bluffs. This sequence is dominated by large units of fine-grained diamict separated by bedded and laminated sand and mud units. Traditionally, this sequence has been interpreted as the product of multiple ice advances by a grounded ice sheet. Between each advance lakes formed to give the sandier sediments. Eyles and Eyles (1983) challenged this interpretation on the basis...

Direct Glacial Sedimentation

The Quaternary Enhanced Glacial Erosion

Debris deposited directly by a glacier is known as till. A till is defined as sediment deposited by glacier ice, but one which has not been disaggregated, although it may have suffered glacially induced flow either in the subglacial or supraglacial environment. It normally consists of large pebbles, cobbles or boulders, referred to generally as clasts, set within a fine-grained matrix of silt and clay. Its characteristics are, however, highly variable, as eloquently expressed in the following...

Northern Hemisphere Temperate Glaciers Alaska And Iceland

Temperate glaciers are found in the maritime mountains of Alaska and in Iceland. The high rates of snowfall and ablation in Alaska give rise to some of the steepest mass balance gradients in the world (see Section 3.2) and consequently some of the fastest glaciers, with ice velocities in some cases up to 500 m per year. The steep mountains surrounding many Alaskan glaciers give these glaciers large supragla-cial debris components and complex ice margins with extensive spreads of supra-glacial...

Subglacial Landforms Formed By Ice Or Sediment Flow

Glacial Erosion

This category of landforms is divided into those which have been ice-moulded and those that have not. Ice-moulded landforms are significant because they provide information about the direction and velocity of glacier flow. 9.2.1 Ice-Moulded Subglacial Landforms Three broad families of ice-moulded subglacial landforms (bedforms) have been identified on the basis of size (Figure 9.15). Although each may be genetically distinct, these are as follows. Figure 9.15 Schematic representation of...

Storage Of Water In Glaciers

It is important to note from the outset that meltwater not only flows through glaciers but that it can also be stored within a glacier in a number of ways. Storage occurs as ice, snow and water at a number of different spatial and temporal scales. For example, in subglacial settings, meltwater can be stored in cavities, in the pore space of subglacial sediments and in subglacial lakes. It can also be stored within the ice in englacial water pockets, tunnels and cavities, and on the glacier...

Glacial Meltwater Erosion

The rate of fluvial abrasion increases with the flow velocity. Similarly, the more turbulent the water flow the greater the rate of abrasion because sediment particles are brought into contact with the bed and channel walls more frequently than when the level of turbulence is low. 3. Properties of the channel. The roughness and orientation of facets within a channel as well as its planform all affect the rate of fluvial abrasion. Erosion is greatest where sediment-charged water...

Lowlevel Debris Transport

Low-level debris transport is the transport of debris at or close to the base of the glacier. Debris may be derived directly from the bed by subglacial entrainment or indirectly from debris that falls onto the glacier surface and finds its way to the bed via crevasses and the downward movement associated with extending flow or basal melting (Figure 7.1). This debris remains at the base of the glacier until it is either deposited subglacially or released at the glacier snout or margin....

Box 81 Were Deforming Glacier Beds Widespread

During the early 1990s there was a paradigm shift within glacial geology which involved the increasing acceptance of the idea of subglacial deformation as a key glacial process responsible not only for many subglacial tills but also for associated landforms such as drumlins. Not all researchers have accepted the idea that glacier bed deformation was extensive beneath former mid-latitude ice sheets. At the heart of this question lies a debate over the interpretation of the glacial evidence and...

Distinguishing Glaciolacustrine And Glaciomarine Diamicts From Glacial Tills

Distinguishing the diamicts produced in glaciolacustrine and glaciomarine environments from those formed in other settings, particularly from glacial tills, is difficult on the basis of internal sediment properties alone (see Section 8.1.3). In general, waterlain diamicts may be recognised by the presence of stratification and graded bedding, although this is not always present. Waterlain diamicts tend to have weak particle fabrics and these fabrics, where present, usually reflect either local...

Box 95 Drumlins Formed By Subglacial Meltwater

Landforms From Meltwater Erosion

Over the past 20 years John Shaw has championed the controversial idea that drumlins are formed by subglacial floods. The idea stems from the similarity in morphology of drumlins with erosional marks at the base of turbdites It rests on the premise that inverted erosional marks at the ice bed are subsequently infilled to form drumlins, as illustrated below. The recognition in the 1980s that many drumlins have a core of sand and gravel gave support to this hypothesis. Significantly, the theory...

Greenland In The Greenhouse

The Greenland Ice Sheet is the second largest ice sheet on Earth and is considered to be especially vulnerable to global warming. There are two reasons for this first it spans a range of latitudes from 60 to 83 N and second it is surrounded by ocean waters, which unlike Antarctica do not possess ocean currents that isolate the ice sheet from mid-latitude heat transfer. Studies show that the ice sheet has experienced record amounts of surface melting in recent years (Figure 2.5). Many of the...