Intense deformation occurs in an area of about 25 square miles, centered at Des Plaines in northeastern Illinois. Previously called a “cryptovolcanic structure,” the Des Plaines disturbance is here called a “cryptoexplosion structure,” a term that implies no specific explosive mechanism. Silurian rocks, essentially flat lying and gently warped, surround the roughly circular disturbed area in which beds ranging in age from Mississippian to Early Ordovician underlie the glacial drift. Structural derangement is complex, with many high-angle normal faults but without any over-turning or pronounced tilting of blocks. A graben in which Mississippian and Pennsylvanian rocks are preserved partly surrounds a central uplifted core. The main structural movement has been upward; maximum displacement is at least 900 feet.
Only a post-Pennsylvanian age can be established for the faulting, although the possibility exists for multiple ages. A negative gravity anomaly of 8 mgals roughly coincides with the structure. Because of this, it is proposed that the Des Plaines disturbance may be the result of meteorite impact and elastic rebound of rocks in post-Pennsylvanian time. Erosion has removed about 1000 feet of rocks so that only the “root” portion of the impact structure remains at Des Plaines. Because of the pronounced gravity anomaly and the possibility that faulting occurred at more than one time, the concept of faulting produced by the focusing of regional forces at a point cannot be entirely rejected.