Clasts >1 cm were sampled from three drill sites (1101, 1095, and 1096) on the continental rise west of the Antarctic Peninsula during Leg 178 (Fig. F1; Table T1). In this study, each core was divided into three stratigraphic units based on the sediment characteristics at Site 1101 and using magnetostratigraphy for correlation between the three drill sites (Barker and Camerlenghi, 1999). Unit 1 (0-0.76 Ma), Unit 2 (0.76-2.2 Ma), and Unit 3 (2.2-3.1 Ma) have variable thicknesses at each site that depend on the linear sedimentation rate over the time period.
The lithology of each of the 331 pebbles was identified in hand specimen (Table T1). Thin sections were made for pebbles of fine-grained lithology. For simplicity, pebbles were also grouped as volcanic (felsic or mafic), intrusive, metamorphic, or sedimentary.
The long, intermediate, and short axes of each clast were measured using Vernier calipers. Sphericity was determined using Krumbein's (1941) intercept sphericity equation. Roundness was determined by comparing each dropstone to the Krumbein (1941) visual roundness chart. Shape was found using the Zingg (1935) shape diagram and assigning each pebble to a shape category of sphere, disc, blade, or rod.
Sphericity vs. roundness was plotted and contoured, then compared to similar plots by Boulton (1978) to determine in what zone within the glacier the pebbles were transported. Sixteen pebbles were damaged during drilling and were not included in analysis of roundness, sphericity, shape, and surface features.