Drought-Induced Crop Lines Characterize Karst Terrain in the Driftless Area of Illinois
The persistent 2012 summer drought in the U.S. Midwest adversely affected the health and vigor of agricultural crops. An unforeseen outcome of the severity of the drought was that it provided a rare opportunity to glimpse the structure of the buried bedrock surface within the Driftless Area of northwestern Illinois. Complex vegetated networks, termed “crop lines,” began to manifest across the dry summer landscape of Jo Daviess County, Illinois (see inset map). An August 2012 aerial photograph shows vegetated crop lines within an alfalfa field situated between a cornfield (left) and a harvested hayfield (right). The most prominent crop lines visible in the photograph measured approximately 4 feet in width.
Primarily confined to alfalfa fields, the vegetated crop lines resulted from the extremely dry conditions, coupled with relatively thin (3- to 5-foot) unconsolidated deposits, overlying fractured and creviced Ordovician age Galena Dolomite. Alfalfa is a drought-tolerant, deep-rooted perennial hay crop possessing the longest taproots (up to 20 feet deep) of any agricultural crop. Groundwater and associated soil moisture within the vadose zone of the buried bedrock fractures and crevices provided the necessary moisture to sustain the overlying healthy alfalfa plants, whereas the remaining field area exhibited stunted and sparse plant growth.
Newly acquired multitemporal aerial photography was used to identify 17,885 individual fractures and digitize them into a GIS database representing more than 100 separate locations across the county. Azimuth calculation revealed two dominant fracture orientations, 93°–99° and 181°–184°. Photointerpretation and field investigations revealed that the vegetated crop lines closely mimic the creviced pattern of the underlying karst bedrock. Ongoing research into their character and distribution is providing important new insights into the structural geology of northwestern Illinois.