DIVIDER ANALYSIS OF CENTRAL U.S. WISCONSINAN GLACIAL BOUNDARIES

We have performed a geometric analysis of the geographic maximum extent of the Wisconsinan glacial event (24 to 18 Ka) of the Laurentide Ice Sheet. These boundaries were digitized across the states of Indiana, Ohio and Illinois at scales of 1:250,000 and 1:500,000 and divided up into segments defined by areas of local geomorphology. We then analyzed these sections using the divider method to determine fractal characteristics of glacial boundary plan-view irregularity.

Six distinct sections can be established using geomorphologic criteria: Northern Illinois, Southern Illinois, Western Indiana, Eastern Indiana (which contains portions of western Ohio), Central Ohio, and Eastern Ohio. Richardson plots for the six sections include five having two linear segments at fine and coarse resolution ranges (approx. 1-6 km and 6-40 km, respectively), and one with a single linear segment spanning both ranges. Fractal dimensions at fine scales range from 1.04 to 1.13, while those at coarse scales range from 1.03 to 1.10. In most cases, individual boundary segments display greater degrees of wandering (higher fractal dimension) at fine scales. Maximum fractal dimension (D=1.13) values at fine scale occur in the Western Indiana and Central Ohio sections, both of which contact escarpments. The eastern portion of this study area is characterized by a more rugged landscape while the western portion is mostly comprised of lowlands; however, no general trend in fractal analysis results corresponds to this.

We have also analyzed two sections from a later Wisconsinan event (defined as 18 to 14 Ka) that directly correspond to the Northern Illinois and Eastern Ohio sections from the previous mentioned event. Both of these Richardson plots show larger D values at coarse scales of wandering than those at fine scales. The highest D value of this study (1.20) is noted at coarse scales for the later event of the Eastern Ohio section. It is also of interest that the later event in the Northern Illinois section has two distinct linear segments while the earlier event only has one segment that spans both scales. The earlier and later event boundaries in Eastern Ohio have similar geometric character at fine scales, but contrast strongly at coarse scales.