The Decorah Shale is a fossiliferous shale that makes up the lower most formation in the Galena Group. The Decorah lies above the Platteville Limestone

Platteville Limestone

The Platteville Limestone is the Ordivician limestone formation in the sedimentary sequence characteristic of the upper Midwestern United States. It overlies the thin Glenwood Shale, which overlies the thick Saint Peter Sandstone...

 and below the Galena Limestone in the sedimentary sequence that formed from the shallow sea that covered central North America during Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 Time. The Decorah consists of three members (from bottom to top): Spects Ferry, Guttenberg, and Ion. http://www.iowageology.org/gb70/stop-09.htm The Spects Ferry member is organic-rich and suggests a large influx of terrigenous sediment during deposition. The Guttenberg is characterized by nodular calcareous beds and contains several K-bentonite deposits. The Ion Member, present in the southern Decorah in Iowa, is characterized by alternating beds of shale and limestone.

Where it crops out in the Upper Midwest

Upper Midwest

The Upper Midwest is a region in the northern portion of the U.S. Census Bureau's Midwestern United States. It is largely a sub-region of the midwest. Although there are no uniformly agreed-upon boundaries, the region is most commonly used to refer to the states of Minnesota, Wisconsin, and...

, especially in the Twin Cities

Twin cities

Twin cities are a special case of two cities or urban centres which are founded in close geographic proximity and then grow into each other over time...

, the Decorah is a popular place for amateur fossil collecting. It contains trilobite

Trilobite

Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

s, brachiopod

Brachiopod

Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s, horn corals, gastropods, crinoid

Crinoid

Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...

s, and large numbers of bryozoans. Cephalopods may also be found in the lower layers of the Decorah Shale.