Microscale Features Of Glacial Erosion

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).

6.1.1 Striations

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 polished surfaces. Striations are orientated parallel to local ice movement and can be used to make inferences about the pattern of ice flow both in space and time. It is important to note, however, that striations only provide the orientation and not the direction of ice flow. For example, a striation orientated north to south may have been cut by ice flowing either from the north or from the south. On their own striae are seldom sufficient to give the direction of ice flow. To determine this, reference must be made to other criteria such as: (i) the morphology

Glacial Geology: Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser © 2009 John Wiley & Sons, Ltd u>

Figure 6.1 Classification of landforms of glacial erosion.

Table 6.1 Microscale landforms of glacial erosion and their significance for the reconstruction of former ice masses.

Landform

Morphology

Glaciological significance

Striae

Micro crag and tails

Bedrock gouges and cracks

Small grooves or scratches on bedrock surfaces, often continuous over several metres.

Commonly associated with polished bedrock surfaces. Striae can be assigned to one of three morphological classes.

Type 1 striae become progressively wider and deeper down-glacier until they end abruptly, often as deep steep-walled gouges.

Type 2 striae start and terminate as faint, thin traces. They steadily broaden and deepen until they reach a maximum width and depth near their centre point.

Type 3 striae begin abruptly as deep gouges, then become progressively narrower and shallower down-glacier.

Small tails of rock formed in the lee of resistant crystals, grains or nodules.

Gouges and cracks, often crescentic in outline, cut into bedrock surfaces. Often occur as a series of gouges or cracks.

Warm-based ice carrying a basal debris load. Striae orientated in the direction of local ice flow.

Individual scratches created by large (>1 mm) clasts.

Polishing caused by fine (silt-sized?) fraction. High clast-bed contact pressures (>1 MPa) inferred from clast penetration into bedrock and from intimate ice-bedrock contact.

Type 1 striae indicate that the striating clast ploughed forward and downward, before either the striator point broke off the clast or the torque on the clast was sufficiently large to rotate it out of the groove.

Type 2 striae indicate that a clast slowed down until the maximum striation depth was reached and then steadily accelerated until at the striation terminus the clast had the same velocity as the ice.

Type 3 striae indicate that a striator point contacted and indented the bed. The clast rotated with little displacement along the bed producing a low ploughing angle, so that a gradual reduction in indentation depth occurred as sliding proceeded.

Warm-based ice carrying a basal debris load.

Crag and tail indicates orientation and direction of ice flow.

High clast-bed contact pressures

(>1 MPa) inferred from intimate ice-bedrock contact.

Warm-based ice carrying a basal debris load. Direction of forward dip of bedrock fracture indicates the direction of local ice flow.

High clast-bed contact pressures (>1 MPa) inferred from evidence of bedrock fracturing. Temporal fluctuations in clast-bed contact pressures are implied because gouges and cracks often occur in series. Some evidence that the length of individual crescentic fractures and gouges increases linearly with ice thickness, allowing estimates of former ice thickness.

p-forms and Smooth-walled sculpted s-forms depressions and channels cut in bedrock. Encompasses a variety of morphological expressions including sichelwannen, hairpin erosion marks, potholes, bowls, channels and grooves.

Landforms carrying striae indicate warm-based ice carrying a basal debris load and high clast-bed contact pressures (>1 MPa) inferred from intimate ice-bedrock contact. Landforms where striae are absent indicate presence of abundant basal meltwater, possibly concentrated by catastrophic discharge. Low effective normal pressures inferred for these landforms.

[Modified from: Glasser, N.F. and Bennett, M.R. (2004). Progress in Physical Geography, 28, 43-75.1

Figure 6.2 Examples of microscale landforms of glacial erosion. (A) Smoothed and abraded bedrock outcrop with two sets of cross-cutting striations in front of a glacier in Svalbard. (B) Striations on the bedrock outcrop in front of a glacier in the Khumbu Himalaya. Note that the bedrock is also fractured along joints and that it is also stained by subglacial precipitates. (C) Striations on a facetted boulder in front of the Tasman Glacier, New Zealand. (D) Micro crag and tail on a bedrock surface in Canada. Former ice flow was right to left. (E) Long, continuous striations on a bedrock surface in western Ireland. (F) Crescentic fractures on a bedrock surface in western Ireland. [Photographs: N.F. Glasser]

Figure 6.2 continued
Figure 6.2 continued
Figure 6.2 continued

of the bedrock surface on which they are located, in particular the presence of stoss and lee forms (see Section 6.2.2); (ii) the presence of micro crags and tails; (iii) the presence of friction cracks; or (iv) relationship to other larger landforms.

Striations are usually not more than a few millimetres in depth, but may be over several metres long. Their continuity is broken by small gaps or breaks where contact between the bed and clast was temporarily broken during their formation (Box 6.1). This may occur due to the formation of small subglacial cavities or alternatively where a clast rides up over a cushion of debris. The depth and

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