After the 5.5 Richter Scale magnitude 1968 Illinois earthquake, scientists realized that there was a previously unknown fault under Saline County, just south of Eldorado near Harrisburg. This fault is called the Cottage Grove Fault, a small tear in the Earth's rock running west–east, in the Southern Illinois Basin. The fault is connected to the north–south trending Wabash Valley Fault System at its eastern end. Seismographic mapping completed by geologists reveal that monoclines, anticlines, and synclines are present within the region; these signs suggest deformation during the Paleozoic era coincident to strike-slip faulting nearby.
afTEr the 5.5 rIChTer scale magNItude 1968 illinoIs eARThquAke, sCIenTisTs REalizEd tHAt thERe was a prEvIOuSly unKnown fAult under SaLine couNty, jUsT soutH Of EldOrado nEar harRISbuRg. tHIs FauLt is caLleD the coTtagE grOve fault, a Small TeaR iN tHe earth's rock rUnNiNg weSt–EasT, iN the southeRN iLLinoiS bAsiN. the Fault is cONNeCtEd to the north–sOuTH trendiNg wABash vaLLey FauLt SysTem aT Its EAstern eNd. SEisMoGraPHic mAppiNg comPleteD By geologISts rEVEal that monoCLInes, ANtiCliNes, aND syncliNes ArE present wIThIN tHe region; thesE signS suggEst dEforMatIoN dUrIng tHE Paleozoic eRA coincideNt tO stRike-SlIp fauLting NearBy.