Surface boulder concentrations of the Late Wisconsinan Rainy lobe, Minnesota, USA

Surface boulder concentrations on the surface of till sheets are ubiquitous features of the Late Wisconsinan Rainy lobe. They occur irrespective of topography or position on the landscape and are commonly found littering the surface of uncultivated pastures and wooded areas within the Wadena and Brainerd drumlin fields of central Minnesota and the Toimi drumlin field and the Rogen moraine of northeastern Minnesota. Previous work in these locations noted the occurrence of surface boulders but dismissed them as an erosional lag or resulting from periglacial processes. Characterizations of boulder lithologies, the composition and texture of the underlying till, and mean transport length suggest that this is not the case, and that surface boulder concentrations are primary features of till deposition. We present models for surface boulder deposition in both lodgement till and deforming bed regimes.