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

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 Central Lowlands of Scotland. To the north was the Highland Ice Sheet and to the south was the ice dome of the Central Uplands. As these ice sheets decoupled progressively from the northeast the area between them became a focus for glaciofluvial sedimentation. Good braided esker systems are to be found at Newbiggin and at Carstairs. The esker system at Newbiggin is described by Bennett et al. (2007) and consists of several morphological components, including a single linear esker ridge, interrupted and terminated by shallow fans along its length; a series of multiple subparallel ridges and shallow fans; and a complex multiridge structure. The sedimentolgy of the system is consistent with the progressive infilling of a large lake basin by a subglacial conduit, which becomes progressively emergent at the ice surface as a series of ice-walled channels feeding a system of subaqueous fans, as illustrated below. The model demonstrates the complex evolution of glaciofluvial systems along an active ice margin receiving a pronouced meltwater discharge.

Quarry C

Quarry C

Formation Eskers

Source: Bennett, M.R., Huddart, D. and Thomas, G.S.P. (2007) in: The Newbiggin Esker System, Lanarkshire, Southern Scotland: A Model for Composite Tunnel, Subaqueous Fan and Supraglacial Esker Sedimentation (eds M.J. Hambrey, P. Christoffersen, N.F. Glasser and B. Hubbard) Glacial Sedimentary Processes & Products, International Association of Sedimentologists Special Publication, 39, 177-202. [Modified from: Bennett et al. (2007) in: Glacial Sedimentary Processes & Products (eds Hambrey et al.), International Association of Sedimentologists Special Publication, 39, figure 14, p. 196]

More sophisticated models of bedform formation have also been proposed. These are based on the idea that instabilities within a deforming layer may itself be sufficient for bedform creation (Box 9.7). Bedforms are created by instabilities within many other natural systems (e.g., sand ripples within a river) where there is a mechanism that amplifies small disturbances within a system leading to a regular wavelength of perturbation. The idea is that natural variations in till rheology may lead to formation of similar instabilities within a deforming layer, creating bedforms. Although the computation behind these theoretical models is complex, the basic premise is that bedforms can be generated from instability of flow within a deforming layer. This theory has in recent years found a particular place in explaining the formation of transverse bedforms such as rogen moraines (Box 9.7) and is an area of ongoing research.

9.2.2 Non-Ice Moulded Subglacial Landforms

Subglacial landforms can form in situations where subsequent ice flow is minimal and consequently no ice-moulding occurs. The most important of these landforms are geometrical ridge networks or crevasse-fill ridges (Figure 9.25). They are low (1-3 m) ridges, symmetrical in cross-section, with a distinct geometrical pattern forming networks when viewed in plan. This geometrical pattern often appears similar to the pattern of crevasses visible at the adjacent ice margin. The ridges are normally composed of basal till. They are believed to form by the squeezing of basal till into subglacial crevasses. If crevasses penetrate to the base of a glacier then basal debris may be squeezed into them and reach an englacial position. Squeezing may also occur into subglacial tunnels. Survival of these ridges is only possible if the ice is stagnant or becomes cold-based immediately after ridge formation. At modern glaciers, geometrical ridge networks are commonly associated with glacier surges. Rapid ice velocities during the surge open up basal crevasses and cause steep thrusts to form, which later become filled with basal sediment as the surge ends. Exceptionally good examples of crevasse-squeeze ridges are to be found on the island of Coraholmen in the Ekman Fjorden fjord, Svalbard. As shown in Figure 9.6 the Sefstrombreen glacier surged forward into the Ekman Fjorden fjord and part of the ice margin became grounded on the island of Coraholmen. As the heavily

Crevasse Squeeze Ridges
Figure 9.25 Crevasse-squeeze ridges formed by a surge of Hagafellsjokull-Eystri (Iceland) in 1999. [Photograph: M.R. Bennett]

crevassed ice-margin settled into the marine mud that it had transported from the floor of Ekman Fjordan onto the limestone island, sediment was squeezed out in front of the ice margin to form a moraine and sediment was squeezed into basal crevasses. Upon deglaciation, a series of crevasse-squeeze ridges was uncovered behind a distinct moraine ridge.

Extensive areas of hummocky moraine around the southern lobes of the former Laurentide Ice Sheet in North America have also been attributed to the upward movement of subglacial sediment (Figure 9.26). Traditionally these were explained by widespread stagnation of ice under a thick supraglacial debris cover melting out

Eskers Drawing Figure
Figure 9.26 Sedimentological model for the formation of hummocky moraine via subglacial deformation. [Reproduced from: Eyles etal. (1999) Sedimentary Geology, 123, figures 1, 4 and 5, pp. 164, 168 and 170, Copyright © 1999, Elsevier Ltd]

from debris-rich ice. Amongst this landform assemblage are some distinctive components such as doughnut-like rim ridges, flat-topped moraine plateaux and more linear ridges. Within the Lethbridge Lobe in southern Alberta (Canada) chaotic non-oriented hummocky moraine occurs around the margins of the former ice lobe, which was confined to lower lying terrain. This chaotic terrain passes downslope into weakly oriented hummocks that are transitional to drumlins located in the lobe centres. All these landforms are composed of fine-grained tills with rafts of soft glaciotectonised bedrock derived from the Bearpaw Shales, which underlie the area. A model has recently been put forward which explains this morphological assemblage as a consequence of subglacial 'pressing' into a soft deforming glacier bed during deglaciation (Figure 9.26). The drumlins within the lobe centre record continued active ice flow whereas the hummocky moraine was produced below the outer stagnant margins of the ice lobe by gravitational loading or 'pressing' of dead-ice blocks into wet, plastic till (Figure 9.26). This model has been extended to explain other areas of hummocky moraine below the Laurentide

Ice Sheet and used to support the idea of an extensive deforming subglacial layer. It also provides a challenge to more conventional ideas that hummocky moraine is simply the product of supraglacial sedimentation.

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