PEBBLE AND SAND LITHOLOGY OF THE MAJOR WISCONSIN GLACIAL LOBES OF THE CENTRAL LOWLAND

Abstract

Pebble and sand-grain counts were taken from the axial portions of moraines of six Wisconsin glacial lobes of the Central Lowland (Erie, Saginaw, Lake Michigan, Green Bay, Des Moines, and Iowan), and the observed distribution of lithologic types was interpreted in terms of provenance, lithologic properties, and glacial processes. It appears unlikely that the glacial lobes of the Central Lowland can be differentiated on the basis of a single “indicator” lithology. Although several lithologies were found to be distinctive of some lobes and not of others, only the Saginaw lobe contained lithologies not found elsewhere (jasper conglomerate and a Precambrian conglomerate resembling tillite). Investigation of the relative proportion of lithologic types appears to be the most fruitful method for the differentiation of glacial lobes. Several pebble counts taken from Kansan till in eastern Iowa and western Illinois substantiate this general method by showing strong lithologic affinities toward the Iowan drift and thus indicating a Keewatin source. This conclusion has been reached by others using various methods of approach.