The Dakota units are generally composed of sandstones, mudstones, clays, and shales deposited in the Mid-Cretaceous opening of the Western Interior Seaway.

[7] The usage of the name Dakota for this particular Albian-Cenomanian strata is exceptionally widespread; from British Columbia and Alberta to Montana and Wisconsin to Colorado and Kansas to Utah and Arizona.

It is famous for producing massive colorful rock formations in the Rocky Mountains and the Great Plains of the United States, and for preserving both dinosaur footprints and early deciduous tree leaves.

Owing to extensive weathering of older rocks during the Jurassic and Triassic, the Dakota strata lie unconformably atop many different formations ranging in age from Precambrian to Early Cretaceous.

With a few local exceptions, it is the oldest Cretaceous unit exposed in the northern Great Plains, including Kansas, Nebraska, Iowa, Minnesota, and Wisconsin, as well as the Desert Southwest.

This group ranking recognizes sequences and members that exhibit local formational characteristics, especially marine shales that are less developed or absent further from the center of the seaway.

[11] The Skull Creek Shale and Plainview Sandstone of Colorado are also included in the Dakota Group, as is the Cedar Mountain Formation of Utah and its Buckhorn Conglomerate member.

"Drillers", who navigate deep strata by monitoring material brought to the surface in the drilling mud, alphabetically designated the hydrocarbon "producing sands" of the Dakota in the Denver Basin as (highest) "D", "J", "M", and "O" (lowest).

Each sequence represents a cycle of major progression of the seaway into the western interior of North America followed by retreat (see Walther's Law of Facies).

The Greenhorn Cycle is the final relevant sequence as it overlays all Dakota classifications, with the exception of certain sandstones of Graneros age, such as classified in Wisconsin and Iowa.

[18][5] Over the range of the usage of the Dakota name, the unit is primarily known for its massive beds of sandstone, which commonly shows shades of red, but also gray, yellow, or white.

The sand was carried and deposited by rivers or accumulated in dunes or shoreline strands, and later cemented by red iron oxide or white calcite, depending on the local groundwater conditions that followed the sedimentation.

Nevertheless, near the western limits, where the Dakota "pinches out" between the Morrison and the Mowry, the unit returns to the totally terrestrial sand-mud-sand pattern and fossil pollens of the Nebraska type location.

Witzke and Ludvigson have argued that use of the name "Dakota" must reflect actual, not presumed correlation based on stratigraphy and composition of the sedimentary rock.

[17] To the west of the Rocky Mountains, such as on the Colorado Plateau, this sequence of Upper Cretaceous, predominately sandstone, sedimentary rocks was recommended to be known as the Dakota Group,[19] to dispel any suggestion of direct facies correlation.

[21] In eastern Utah and western Colorado, Young introduced the term Naturita Formation for a series of facies in the larger "Dakota Group".

[19] However, despite Witzke and Ludvigson logic, geologist have continued to refer to the Lower Cretaceous sequence of formations on the Colorado Plateau and the Rocky Mountains as the Dakota Group.

The Dakota sandstones form crucial supplies of water on the Plains, especially on uplands between river valleys wherever it is found outside the boundaries of the Ogallala Aquifer.

[25] The widespread bog environments of the Dakota period resulted in concretions of iron, forming hematite, limonite, and beds of "ironstone", which are common in the Janssen clay member of Kansas.

[27] The sands and muds of the Dakota represent wet lowlands, rivers, flood plains, and beaches with a shoreline deeply undulating between deltas and brackish marine embayments.

Ground water flowing from inland reacted with changes in pH, oxygenation, and salinity as it encountered seawater, depositing iron oxide and calcite in underground layers near shorelines.

[5] Sandstone with dinosaur tracks in the upper Dakota above the marine Skull Creek Shale near Denver demonstrates that the seaway occasionally retreated from the area.