ENHANCED LIDAR DATA REVEALS MORE WIDESPREAD EXTENT OF LATE QUATERNARY “PAHA” AEOLIAN LANDFORMS IN NORTHWESTERN ILLINOIS

“Pahas” (low, elongated, rounded ridges or hills of glacial origin) were recognized in Carroll, Whiteside, and Rock Island Counties in northwestern Illinois in the 19th century by Leverett and in the 20th century by Flemal et al. This area, mostly between the Mississippi River to the west and the Rock River to the southeast, encompasses about 1150 sq. km. Study of recently acquired LiDAR elevation data has revealed that “pahas” and associated dune deposits occur over a larger area and are recognizable 50 to 80 km in almost all directions (within IL) from the original Illinois “paha” study area. Most “pahas” occur on loess-covered uplands bordering the Mississippi River where strong northwest-west prevailing winds eroded into the thick loess deposits to form these distinctly linear and parallel features. Sediment from these “pahas” were deposited downwind as dome and possible barchan dunes in the proximal areas of the uplands, as prominent parabolic dunes on the Green River lowlands, and both longitudinal (seif) and parabolic dunes in more distal areas eastward and southeastward. In some areas, dune deposits also occur on the windward side of the uplands. The “paha” uplands likely were a northwestern source for aeolian deposits in the Green River lowlands. Previous studies indicated that the primary period of deposition (and erosion) occurred during the Late Quaternary (17,000 to 18,000 years ago) when Wisconsin Episode glaciers still covered much of northeastern Illinois.

A preliminary comparison with the features in northeastern Iowa where pahas were originally defined suggests that the Illinois “pahas” may be more accurately referred to as yardangs. Yardangs are prominent ridges bounded by rounded troughs formed by aeolian erosion oriented parallel to the prevailing wind direction. Pahas therefore would be restricted to hills, plateaus, or uplands that are adorned by yardangs.