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

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

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...

Palaeoglaciological Reconstructions

As an example of how these principles can be used to make a simple palaeoglacio-logical reconstruction we start with a map of the distribution of landforms on a hypothetical ice-sheet bed (Figure 12.9A) and outline how these landforms can be used to infer the basic shape and dimensions of a former ice sheet (Figure 12.9B). 1. Terminal moraines. Large terminal moraines mark the outer limit of the ice sheet. In some places the terminal moraine is a single ridge in other places it may be a series...

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....

Suggested Reading

Subglacial observations of glacial erosional processes in basal cavities and tunnels are recorded in several papers Boulton (1974), Vivian (1980), Anderson et al. (1982), Rea and Whalley (1994) and Cohen et al. (2005, 2006). Boulton (1974, 1979) discusses the fundamental principles of glacial erosion and introduces a model of glacial abrasion. Alternative abrasion models are outlined by Hallet (1979, 1981) and Hindmarsh (1996a). The supply of basal debris to the base of a glacier and its role...

Box 93 Formation Of Stacked Till Sheets And Icemarginal Moraines

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...

Glaciofluvial Icemarginal Landforms

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...

Box 122 The Origin And Significance Of Streamlined Glacial Landforms

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 22 The Patagonian Icefields

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...

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...

Box 96 Drumlins And Subglacial Deformation

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...

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...

Tropical Glaciers The Cordillera Blanca 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...

The Principles Of Basal Thermal Regime

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...

Icemarginal Moraines

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...

Quarrying

Like glacial abrasion, the direct observation of glacial quarrying is extremely difficult because it involves digging tunnels through a glacier to access basal cavities. However, a 2 km-long artificial tunnel under the glacier Engabreen in Norway, originally built as part of a hydroelectric scheme, provides direct access to the bed of this temperate glacier some 200 m beneath the glacier surface. A number of important experiments have been conducted here on subglacial processes, including...

Box 94 Interpreting Landform Assemblages Examples From The Scottish Highlands

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...

Box 85 Sediment Transport In Glacial Meltwater Streams

Linguoid Bar

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 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...

Mechanism

Glacial Erosion Diagram

Most ablation will occur at the snout or terminus of a glacier, which is usually its point of lowest elevation, where air temperatures will be highest and where iceberg calving may occur. The rate of mass loss (ablation) will decrease with elevation, as atmospheric air temperature falls with altitude. Accumulation may be more uniform over the surface of a glacier, but will tend to increase with elevation. A glacier or ice sheet can, therefore, be divided into two areas (i) an accumulation zone,...

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...

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...

Subglacial Landforms Formed By Ice Or Sediment Flow

Ice Gouges

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...

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...

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...