Abstract

The modal distribution of stone long-axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till. In this paper we offer a two-axis diagram that compares fabric modality to fabric isotropy (S 3 /S 1 ) and addresses the problem of eigenvectors falling between the modes of some well-developed till fabrics with low eigenvalues. Our simple five-fold scheme of modality categories includes: (1) unimodal clusters, (2) spread unimodal, (3) bimodal clusters, (4) spread bimodal, and (5) polymodal to girdle-like fabrics, and requires the analyst to study equal-area, lower-hemisphere (Schmidt) plots of the fabric data. After assigning the fabric to a modality category, isotropy is calculated and both results are plotted on the graph, which helps to separate two main fields of subglacial till: (1) lodgement and subglacial meltout tills, and (2) deformation till. On the basis of selected published fabrics from tills at modern glaciers, as well as our own Pleistocene till data, lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics. In contrast, deformation tills and tills that experienced multiple processes tend to have polymodal to girdle-like fabrics. Some overlap occurs between fields because of the complex nature of till formation (i.e., because pure end-member till facies are rare and most tills are hybrids). We strongly recommend that Schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till. However, fabric data alone is not enough. Multiple criteria including structural, lithologic, and stone morphologic data from the till must also be considered before drawing conclusions on till genesis. Furthermore, if eigenvectors fall between fabric modes, then they cannot be used to indicate former ice movement directions. Finally, our new modality-isotropy diagram may have wider applications.

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