ON THE INTERPRETATION OF SUBGLACIAL TILL FABRIC

The modal distribution of stone long-axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till. In t his paper we offer a two-axis diagram that compares fabric modality to fabric isotropy (S-3/S-1) and addresses the problem of eigenvectors f alling between the modes of some well-developed till fabrics with low eigenvalues. Our simple five-fold scheme of modality categories includ es: (1) unimodal clusters, (2) spread unimodal, (3) binodal clusters, (4) spread binodal, and (5) polymodal to girdle-like fabrics, and requ ires 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, wh ich helps to separate two main fields of subglacial till: (1) lodgemen t and subgiacial meltout tills, and (2) deformation tin, On the basis of selected published fabrics from tills at modern glaciers, as well a s our own Pleistocene till data, lodgement and subglacial meltout till s tend to have unimodal or bimodal fabrics, In contrast, deformation t ills and tills that experienced multiple processes tend to have polymo dal to girdle like fabrics, Some overlap occurs between fields because of the complex nature of till formation (i.e., because pure end membe r till facies are rare and most tills are hybrids). We strongly recomm end that Schmidt plots be visually analyzed and used in conjunction wi th eigenvalues when studying till, However, fabric data alone is not e nough. Multiple criteria including structural, lithologic, and stone m orphologic data from the till must also be considered before drawing c onclusions 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.