The Platteville formation in the UMV represents the maximum transgression of the Blackriverian age seas into the mid-continent. The transgressing seas could not yet break through the Transcontinental arch to unite the seas occupying the western Williston Basin. The rock unit ranges from approximately 30 feet to in excess of 135 feet in the UMV. In Illinois the unit attains group status with thickness exceeding 780 feet in the southeast part of the state.

The age of the Mifflin is upper Blackriverian (approximately 453 million years old). For correlation purposes the unit is slightly younger than the Bromide formation in Oklahoma or the Benbolt formation of the Virginia area. The trilobite fauna has been correlated to the Watertown Formation (Chaumont) of New York.

Most of the Platteville consists of dolostone or dolomitic limestone. As such, much of the invertebrate fossil material has been lost through the dolomitization process. The middle member (portion of a member or formation depending on State) consists of a predominantly slightly dolomitic limestone unit referred to as the Mifflin. The Mifflin member or formation was named for an exposure near the southwest Wisconsin community of Mifflin in Iowa County. The Mifflin produces the preponderance of trilobites in a band of outcrops extending from the vicinity of Dixon, Illinois on the south through southwest Wisconsin and into northeast Iowa. The Mifflin loses its identity as one moves northwest out of Wisconsin and into the Iowa and Minnesota outcrop belt.

The Mifflin was deposited on a shelf environment, relatively close to the coast, where pulses of terrigenous material form the shale parting surfaces common in the formation. The formation was below normal wave base, possibly in the 50-75 foot water depth. Occasional large storms may have winnowed the sediments creating coarser grained carbonate beds that can be found on outcrops. More commonly, the storms may have created bottom currents that pushed shell material into sporadic areas of coquina beds.

The Mifflin can generally be described as pale yellowish brown to light gray micritic, thin wavy bedded, argillaceous limestone exhibiting greenish gray thin shale partings. The combination of thin wavy bedding and shale partings generally benefit the trilobite collector because farmers are able to utilize the formation, when it is near surface, as farm borrow for farm roads and yards. The Mifflin readily breaks up into 2 – 6 inch by 1 – 3 inch blocks with just the force of a ripping tooth attached to a tractor or bulldozer. These "farm borrow pits" can sometimes be great sources of trilobite specimens.

The trilobites tend to be found on the bedding planes but the thin shale partings tend to hide them on fresh surfaces. It is best if the exposed bedding surfaces are allowed to weather 3 – 6 months to optimize collection efforts.

Specimens of Gabriceraurus, Thaleops and Bumastoides dominate the Mifflin trilobite community though a total of 25 species representing 20 genera have been collected. This "community" suggests a "Midramp" shelf environment as suggested by other N. American Ordovician trilobite research.