F.B. Meek and
F.V. Hayden originated the scientific names for the series of Cretaceous rocks in the central Great Plains of the
North American Continent. They gave the name "Benton" to the great shale deposits between the sandstone bluffs at
Dakota City, Nebraska, and the chalk bluffs at the junction of the
Niobrara and
Missouri rivers. At that time, the early 1860s, Meek and Hayden's "lower Cretaceous" series of the upper Missouri River,
Dakota-Benton-Niobrara, was already widely observed from Canada to New Mexico over the Great Plains up to the foothills of the
Rocky Mountains. In southcentral Colorado, southeast of
Pueblo, this series expresses topographical patterns that inspired subdivision of the Benton shales. Wind and rivers rapidly erode the shales, producing bluffs: massive blocks of Niobrara Chalk cap high slopes of
non-chalky Benton shale leading down a flatter plain that stretches miles to another bluff of
chalky shale with many thin limestones. Particularly, in places where rivers have cut deeply through this lower chalky shale, and furthermore have cut into the bases of the bluffs, banks of
non-chalky shale can be found. In 1896,
G.K. Gilbert named this lower argillaceous shale
Graneros (from R.C. Hills) for the exposures in Graneros Creek, a tributary of the
Arkansas River near Pueblo. and is therefore the complement of the similar Blue Hills Shale of the
Carlile Formation that records the regression of the same Greenhorn Sea. By 1938, the Graneros had been mapped into eastern Wyoming, southeastern Montana, South Dakota, Nebraska, Kansas, and northeastern New Mexico. The same Benton topography is also found in the
Smoky Hills of Northcentral Kansas, and the same Graneros Formation is found on the river banks there. In Iowa and Nebraska, the Graneros grades into the middle-
Cenomanian rock of the Woodbury Member of the
Dakota Formation. As discussed in the Description section, the Graneros exceptionally demonstrates
Walther's Law of Facies. As the Greenhorn Cycle of the Western Interior Seaway advanced eastward, formation of the Graneros also shifted eastward, but also upward in elevation and time, to such an extent that the top of the Graneros at Pueblo is older than the lowest Graneros in Iowa. North of Kansas, the application of the name has been somewhat different. In 1904, describing the geology of the
Black Hills of South Dakota,
N.H. Darton applied the name
Graneros Group to descending members; marine
Belle Fourche Shale and
Mowry Shale, terrestrial
Newcastle Sandstone (a tongue of Dakota Formation from the southeast of South Dakota), and marine
Skull Creek Shale. which, in sequence, traces to the Dakota type in the east. By the 1960s, Darton's definition was recognized as problematic, but many geologists continued to use this classification. It is understood that the listed formations are widely known individually, most with little relationship to the original Graneros Formation definition; and, the same or equivalent units are classified in Wyoming and Colorado as the
Dakota Group. As a result, newer reports include the Belle Fourche (Graneros equivalent), Mowry, Newcastle, and Skull Creek within the Dakota Group of this region. ==Description==