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 i ...

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 is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mos ...

. The Decorah lies above the

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

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, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

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 northern subregion of the U.S. Census Bureau's Midwestern United States. Although the exact boundaries are not uniformly agreed upon, the region is usually defined to include the states of Iowa, Michigan, Minnesota and Wi ...

, 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 stat ...

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

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of 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 e ...

s, horn corals,

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

crinoid Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, 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 South Dakota Ordovician Wisconsin Ordovician System of North America {{US-geologic-formation-stub