ICE-PRESSED RIDGES OF THE PENULTIMATE GLACIATION IN THE KASKASKIA SUBLOBE, SOUTH-CENTRAL ILLINOIS
Recent 1:24,000-scale surficial geologic mapping in the Stolletown Quadrangle (Clinton and Bond counties) has identified several hills that are circular in map view (~ 250 to 500 m diameter, ~ 5 to 15 m high) and protrude from a flat Illinois Episode outwash plain. Initially thought to be moulin kames, at least a few of these hills are composed of dominantly fine-grained material, mainly till, based on borings and two electrical resistivity transects. The till contains many inclusions of shale residuum or older paleosol, likely evidence of glacial shearing or a deformable substrate. We hypothesize that till-bearing hills in the Stolletown Quadrangle may be ice-pressed ridges (Boone and Eyles, 2001; Stalker, 1960). In this model, the hills formed subglacially from differential loading as stagnant ice sank into fine-grained, deformable, water-saturated till. The presence of irregular basal ice topography, crevasses, or supraglacial ponds would have provided areas with highly variable ice thickness that are susceptible to failure. For instance, an ice thickness of ~150 m with supraglacial ponds inset > 50 m (>1/3 less ice), results in differential loading that can cause internal deformation of the ice and till substrate, perhaps leading to the formation of the observed 10 m-high till hills. Ice thickness in the area may even have been up to 700 m based on recent glacial modeling and high pore water pressures may have been facilitated by sub-till proglacial sand deposits. Although ice-pressed hills are probably not the dominant hill-type in the region, this process should be considered for other hills with fine-grained material in the middle Kaskaskia Basin.