ENHANCED LIDAR IMAGERY INDICATES THAT “GIANT CITY” KARSTIC FEATURES ARE NUMEROUS AND WIDESPREAD IN THE SHAWNEE HILLS OF SOUTHERN ILLINOIS, USA

Giant City is a National Natural Landmark within Giant City State Park in the Shawnee Hills of southernmost Illinois. These striking geomorphic features consist of large (3+ m wide and 6 to 13 m high) blocks of Lower Pennsylvanian sandstone (“city blocks”), separated by joint-aligned corridors up to 2.5 m wide (“streets or avenues”). In a talk that I gave at the last NC-GSA meeting in Champaign, I hypothesized that the origin of the “streets and avenues” of Giant City were formed by the very slow chemical erosion of the sandstone along joints, akin to dissolution of limestone in karstic environments. Similar features, although not as striking, are also present at the Garden of the Gods and Ferne Clyffe State Parks. Available aerial data in 1999 was not detailed enough to reliably identify other “Giant City” features in other areas of the Shawnee Hill. Since then, enhanced LiDAR elevation data has become available for most of Illinois, including the counties of southernmost Illinois. The above mentioned karstic features occur in Lower Pennsylvanian sandstones, which tend to be thicker, more quartzose, and very well-cemented, when compared to other sandstones of the area. This study thus was restricted to the areas where the mapped bedrock consists of these Pennsylvanian sandstones. Examination of the hillshade models of the areas was undertaken on a county-by-county basis, at an initial map scale display of 10:000. When potential sites were recognized, map scales down to 1:000 were used to confirm the occurrences of “streets and blocks.” Both features had to be present for the site to be included as part of the dataset. Features either lacking “streets” or consisting of “blocks” that had moved downslope were excluded. The preliminary results of this study indicate that sites with “Giant City” features are quite numerous--nearly 30 sites in the easternmost two counties of the Shawnee Hills. The recognition of these sites is important because they indicate that these features are the result of climatic changes that probably occurred during the Tertiary, prior to the Pleistocene Epoch.