The quartile-deviation/median-diameter relationships of glacial deposits |
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| Authors: | Antony T Buller John McManus |
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| Institution: | 1. Tay Estuary Research Centre, University of Dundee, Newport-on-Tay, Fife Great Britain;2. Department of Geology, The University of Dundee, Dundee Great Britain |
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| Abstract: | Quartile-deviation (QDa) and median-diameter (Md) values have been calculated from the grain-size distributions of a variety of Pleistocene and modern glacial and para-glacial sediments. The QDa—Md pairs are plotted on double-log graph paper to determine whether their graphic distributions are distinctive, and to examine their sedimentological implications. Three distributions emerge. The first has a shallow gradient and represents coarse bi-modal tills consisting largely of rock fragments — distribution (a). The second, with an intermediate gradient, represents fine uni- or non-modal tills consisting mainly of monomineralic grains — distribution (b). The third (c) has a steep gradient and consists of bi-modal tills with a dominant proportion of monomineralic grains. Each of these distributions is explained by using the grain-size characteristics of physically weathered rock debris as a starting point for discussion. By a process dominated by the continual addition of weathered-grain populations (population addition), tills with (a) characteristics are formed. By a process dominated by comminution and the selective removal of grains by ablation (population subtraction), tills with (b) characteristics are formed. Any of these tills, when subjected to flowing water, rapidly lose their glacial characteristics and assume QDa—Md characteristics of fluvial deposits (although they are fluvioglacial). |
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