Box 61 The Morphology Of Glacial Striae And Inferences For Glacial Abrasion

Iverson (1991) used glacial striae, recently exposed on carbonate bedrock adjacent to Saskatchewan Glacier, Canada, to make inferences about the mechanics of glacial abrasion. Iverson measured the width, depth and length of individual striae, as well as making observations about a range of morphological criteria. The shapes of striae indicate that abrading fragments commonly rotate. Iverson defined three types of striae. Type 1 striae become progressively wider and deeper down-glacier until they end abruptly, often as deep steep-walled gouges. They are inferred to form as a striating clast ploughs forward and downward, before either the striator point breaks off the clast or the torque on the clast is sufficiently large so that it rotates out of the groove. 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. They are probably formed by sharp striator points that are rotating as they slide. The point initially has a large ploughing angle, causing progressive incision of the striation. Deeper ploughing causes more rapid clast rotation as the torque on the clast increases. Rotation, together with comminution of the clast point, reduces the ploughing angle so that there is a steady reduction in striation depth. Consequently Type 2 striae indicate clasts that slow down until the maximum striation depth is reached and then steadily accelerate, until at the striation terminus, the clast has the same velocity as the ice. Type 3 striae begin abruptly as deep gouges and then become progressively narrower and shallower down-glacier. They are inferred to form where a striator point contacts and indents the bed. Clast rotation with little displacement along the bed produces a low ploughing angle, so that a gradual reduction in indentation depth occurs as sliding proceeds. Overall, the conclusion is that clasts with steep leading edges will abrade progressively deeper into the bed with sliding, whereas those with more gently inclined leading edges will climb out of their grooves. The paper shows how, therefore, glacial striae can be used to make inferences about former subglacial processes.

Source: Iverson, N.R. (1991) Morphology of glacial striae - implications for abrasion of glacier beds and fault surfaces. Geological Society of America Bulletin, 103,1308-16.

continuity of striations is a balance between the effective normal pressure, which keeps the base of the glacier in contact with its bed, and changes in basal water pressure, which allow small cavities to form (see Section 4.6). Striations formed by different ice-flow directions may be superimposed in a cross-cut pattern (cross-cut striations; Figure 6.2A). This occurs when the ice-flow direction changes, either due to a readvance of ice over a deglaciated area or due to changes in ice-flow direction within a glacier. Cross-cut striations record the fact that the second ice flow was unable to erode all the evidence of the earlier flow, either because of a lower efficiency of glacial abrasion or due to insufficient time. The occurrence of crosscut striations can be used, therefore, to make inferences about former glacier dynamics (Box 6.2).

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