Drought-Induced "Crop Lines" in the Driftless Area of Northwestern Illinois
The persistent drought impacting the health and vigor of Illinois' crops has provided a rare opportunity to glimpse the bedrock surface and underlying karst aquifer within the driftless area of northwestern Illinois. First reported to the ISGS by Mr. Matthew Alschuler of Warren, Illinois, complex networks of vegetated crop lines crisscross the summer landscape of Jo Daviess County (Figure 1), prompting an investigation by scientists from the Illinois State Geological Survey and the Illinois State Water Survey.
In addition to a ground investigation of the crop lines, the phenomenon was also photographed from a low flying aircraft at an elevation of approximately 1,000 feet. The orientations, widths and horizontal separations of the crop lines were measured at several locations and found to be consistent with solution-enlarged crevices exposed in road cuts and quarries in the area, as well as lineaments mapped using LiDAR elevation data of Jo Daviess County.
Figure 2. Aerial photography of the alfalfa fields showing how the vegetated crop lines delineate fractured and creviced bedrock, which is approximately three feet below the land surface in this location. North is to the left in the photograph. Photo by S.V. Panno.
Primarily confined to alfalfa and soybean fields, the crop lines are due to a combination of extremely dry conditions, coupled with a thin soil zone overlying fractured and creviced dolomite bedrock. The plants forming the crop lines are denser and taller (approximately 1.5 ft) as compared to the plants in the remaining field area, which are only about 0.5 ft. in height. The near-surface bedrock fractures and crevices host a karst aquifer, providing the necessary moisture to sustain the overlying healthier plants, while much of the field exhibits stunted and sparsely separated plant growth (Figure 1). Aerial photography reveals a complex pattern of intersecting crop lines, with the most prominent ones exhibiting generally north-south and east-west orientations (Figure 2). Note also the distinctive rectangular drainage pattern in the stream adjacent to the crop lines — the right-angle angle bends in the stream course are the result of control by crevice and fracture patterns at the top of the near-surface bedrock. Streams tend to follow the paths of least resistance. The orientation of the stream channel is consistent with the orientation of the crop lines, and the straight reaches of the stream channel follow trends in crevices which have been observed in outcrops in the region (Figure 3).