Cores from a deep well at the crest of a dome about 2½ miles in diameter near Peoria, Illinois, show a normal sequence of Paleozoic strata down to the Upper Ordovician Maquoketa Shale, but the underlying 1,500 feet of rocks are intensely disordered. Middle and Lower Ordovician strata normally underlying the Maquoketa are not recognized in the disturbed zone. However, the core penetrated several jumbled blocks of Cambrian formations uplifted about 1,000 feet above their normal stratigraphic position.

The disturbed section consists chiefly of brecciated dolomite, with minor amounts of compact, mylonitic sandstone, and contorted beds of red and green shale. Dark argillaceous bands and shale partings show many small-scale faults. Dips are randomly oriented and attain up to 90° throughout much of the section, but in the lower 400 feet of the well the apparent bedding planes dip southeast and the angle decreases with depth from 70° to 40°. The chaotic condition of the pre-Maquoketa rocks suggests that this structure was caused by a violent explosion which probably took place in early Cincinnatian time. The writers believe the explosion was the result of meteorite impact.

Thinning and arching of strata overlying the breccia indicate that there was gradual and continuous structural development of the dome from Late Ordovician to at least Pennsylvanian time and perhaps to the present.