The Decorah Shale is a

fossiliferous A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

shale that makes up the lowermost formation in the

Galena Group The Galena Group or Galena Limestone refers to a sedimentary sequence of Ordovician limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the ...

. The Decorah lies above the

Platteville Limestone The Platteville Limestone is the Ordovician limestone formation in the sedimentary sequence characteristic of the upper Midwestern United States. It is characterized by its gray color, rough texture, and numerous fossils. Its type locality is P ...

and below the Cummingsville Formation in the sedimentary sequence that formed from the shallow sea that covered central North America during

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

Time. The Decorah consists of three members (from bottom to top): Spechts Ferry, Guttenberg, and Ion.Thompson, Thomas L., 2001, ''Lexicon of Stratigraphic Nomenclature in Missouri,'' Missouri Department of Natural Resources, Division of Geology and Land Survey, Report of Investigation Number 73, p. 80
/ref> The Spechts 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 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 the exact boundaries are not uniformly agreed-upon, the region is defined as referring ...

, especially in the

Twin Cities Twin cities are a special case of two neighboring cities or urban centres that grow into a single conurbation – or narrowly separated urban areas – over time. There are no formal criteria, but twin cities are generally comparable in statu ...

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

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

s, horn corals,

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

crinoid Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

s, and large numbers of bryozoans.Mossler, J. and Benson, S., 1995, 1999, 2006
Fossil Collecting in the Twin Cities Area
Minnesota at a Glance: Minnesota Geological Survey: University of Minnesota. Cephalopods may also be found in the lower layers of the Decorah Shale.

References

Shale formations of the United States Ordovician Missouri Ordovician Iowa Ordovician Illinois Ordovician Minnesota Ordovician Nebraska Ordovician geology of South Dakota Ordovician geology of Wisconsin Ordovician System of North America {{US-geologic-formation-stub