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Megabreccia Bed

     Look closely at the layers of rock pictured in this outcrop. The layer formed with large, irregularly shaped fragments of rock is a megabreccia bed. This layer of large, broken rock chunks is of great interest to geologists because its origins remain a mystery. Generally, strata containing such large fragments are found near ancient mountain or volcano environments. The Matthiesen Park area has had neither of these in its history. The most plausible explanation is that this strata formed during a severe storm, by intense wave action. It began as a skeletal grainstone that had not fully lithified. As this grainstone was buffeted by the strong waves, all uncemented materials near the top of the formation were carried away, only to be incorporated back again into the bed later. Another explanation is that the bed was formed by a large debris flow that lithified.

     Beneath the megabreccia layer is the Platteville Dolomite. This rock unit was deposited in a high energy, marine environment during the Ordovician Period. It is known to contain a large quantity of greatly variegating fauna. Overlying the megabreccia is Pennsylvania strata. Each layer of the Pennsylvania strata represents a cyclothem, or cycle of sedimentation. Most of the sedimentation of this strata probably occurred in large deltas on the subsiding Illinois Basin, as the environment changed from marine to non-marine. The dip of this strata does not correlate with that of the Platteville of St. Peter. This indicates that the Pennsylvania strata was deposited after the Platteville and St. Peter units were tilted by uplift along the LaSalle Anticlinorium.


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