ABSTRACT

Carbonate contents of tills deposited by ice of the Scioto Lobe decrease in the direction of ice flow from the carbonate rocks of the Central Lowlands across the clastic rocks of the Allegheny Escarpment and then increase over clastic bedrock on the Allegheny Plateau. Thus, the distribution of carbonate in till units does not fit glacial dispersal models in recent literature. Regression analysis of fine carbonate (< 0.074 mm) content with distance down-ice along traverses from the carbonate-clastic contact to the Allegheny Plateau are not statistically significant because of the high carbonate contents of till units on the plateau. However, data from the contact to the escarpment do yield significant regression coefficients. One hypothesis to account for the increased carbonate con ent on the plateau is that entrainment of local shale quickly decreased carbonate concentrations. Because of its low durability relative to carbonate, shale was rapidly comminuted and caused a relative increase in fine carbonate on the plateau. An alternative hypothesis suggests that as soft shales were eroded, the debris content of the basal ice increased, causing the basal ice to stagnate along the escarpment. More active ice from the englacial zone, containing carbonate debris upsheared from the basal zone, may have overridden the stagnant basal ice and may have grounded as basal ice on the plateau. Subsequently, dilution of englacially transported carbonate began in the newly formed basal zone as the glacier moved over the plateau and glacial dispersal continued.