Mapping palimpsest karst features on the Illinois sinkhole plain using historical aerial photography

Abstract

Situated in southwestern Illinois, principally within Monroe, St. Clair, and Randolph Counties, the sinkhole plain contains the highest density of karst features within the state, and cover-collapse sinkholes are distinctive rural landscape features. However, after decades of using large-scale farming equipment, modification by farm operators using drainage stand pipes and fill material, coupled with the widespread adoption of conservation tillage methods beginning in the 1980s, numerous sinkholes are now partially or completely indistinguishable on current maps and aerial photography. The result is that the land surface in this region has developed a relict, or palimpsest karst appearance. To detect and catalog now relict karst features, county-wide historical aerial photography was digitized and orthorectified to create a geometrically accurate image base map of the entire region. Acquired during the summer of 1940 by the U.S. Department of Agriculture, Agricultural Adjustment Administration (AAA), the AAA aerial photography represents the earliest and most detailed visual record of the landscape prior to the general introduction of mechanized farming practices. Because horse-drawn farming equipment was still prevalent in the region, pesticide and fertilizer application mostly unknown, row crops were planted at much lower plant densities. Therefore, the near-surface geology was able to be discriminated through the mature summer crop canopy. Interpretation of the leaf-on AAA aerial photography, augmented with 2005 early spring, (leaf-off) U.S. Geological Survey (USGS), National Aerial Photography Program (NAPP) imagery determined that approximately 30 % more sinkholes are present on the land surface then are delineated on the most current USGS 7.5 minute topographic quadrangle maps of the study area. Conversely, in the densely wooded areas of the sinkhole plain, numerous sinkholes not visible on the summer 1940 AAA photography could be detected on the leaf-off NAPP photography.