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Exploration sponsored by the Central Illinois Light Company has delineated a structural high, 12 miles southwest of Peoria, near the village of Glasford, Illinois. The structure is nearly circular and consists of a normal sequence of Paleozoic strata down to the Ordovician Maquoketa Shale. The Maquoketa is abnormally thick over the dome, and the underlying 1,500 feet of rocks are faulted and severely brecciated. The structure appears
to be the result of a violent explosion caused by meteorite impact in very early Cincinnatian time.
Each recognizable stratigraphic interval thins over the brecciated core of the structure. Part of this thinning may be the result of differential compaction, comparable with that found over Silurian reefs. The possibility also exists that the brecciated core has been gradually and continuously uplifting, either by rebound after impact or by internal pressures locally exerting an upward movement through the area of disturbed rocks.
Isopach and structure maps indicate that the structural deformation continued into the Pennsylvanian Period and perhaps to the present time. Relative uplift of the dome has taken place at a gradually reducing rate, particularly after Devonian time.
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