Long Island Clast Orientations

Long Island glacigenic surface deposits, where exposed, are ideal for geologic field studies of Pleistocene glacial settings. Numerous outcrop exposures, found scattered throughout the island, delineate many of the deposit types associated with a full spectrum of glacial facies currently observable at active continental ice sheet margins. By directly field sampling axial orientations of a number of clasts in a particle distribution (e.g. a section of a stratigraphic layer) and with vector analysis of this orientation data we can classify the degree of preferred orientation for the total population of clasts in a specified volume. A clast fabric distinguishes the preferred axial orientation of particles for a particular section in a stratigraphic unit. The overall preferred axial orientation for a large outcrop unit may be inferred only after many isolated clast fabrics within the unit are amassed. Clast fabric analysis, despite limitations, is a useful tool in Long Island glacigenic sediment studies because, in its mathematically robust form (eigenanalysis) it allows quantification of otherwise descriptive fabrics. Glacial deposits (e.g., tills) described by clast fabrics convey the degree of relative strain preserved within a volume of an outcrop unit. Analysis in the field, combined with theory, has enabled us to document glacially induced deformation at Ditch Plains, Long Island.