GRASSES OF IOWA. 371 



called the State Quarry beds. It is noticeable for the peculiar character 

 of the rish remains found in it. 1 



Lower Carboniferous. The Lower Carboniferous or Mississippian 

 is also very largely made up of non-dolomitized limestone, though its 

 lower member, the Kinderhook, includes an important shale bed, some 

 sandstone, and the magnesian limestone and oolite quarried at Marshall- 

 town. The middle and upper members, the Augusta, or Osage of some 

 authors, and St. Louis, include a smaller proportion of sandstone and are 

 almost wholly calcareous. 



Upper Carboniferous. The Upper Carboniferous (coal measures) 

 covers approximately the southwestern third of the state, and rests un- 

 conformable" upon the St. Louis, and occasionally on earlier rock. This 

 division includes two separate formations. The lower, Des Moines series, 

 is largely made up of sandstone and shale and carries most of the pro- 

 ductive coal seams. It occupies a strip of country extending approxi- 

 mately from Fort Dodge to Keokuk and from What Cheer to Winterset. 

 Southwest of a sinuous line drawn from southeastern Guthrie county 

 through Madison, Clarke and Wayne to the southeastern corner of De- 

 catur county, the upper division of the coal measures is exposed. This 

 division consists largely of shales and limestone, and carries but little 

 coal. It underlies the Cretaceous for some distance to the north, being 

 found along the Missouri river as far north as Monona County. 



Cretaceous. The Cretaceous as found in Iowa includes a basal 

 member, in part conglomeritic and largely arenaceous, running up into 

 shales, and known as the Dakota. Above it is a series of shales and chalk, 

 together belonging to the Colorado, and known separately as the Benton 

 and the Niobrara. In Sioux county there are a few patches of a still 

 later shale belonging to the Pierre. The latter is the youngest formation 

 in the state certainly earlier than the drift. There are some sands in 

 certain of the northwestern counties which may represent a portion of 

 the interval between the close of the Cretaceous and the invasion of the 

 ice, but the exposures are few and the relations are uncertain. 



THE DRIFT SERIES. 



The general distinctions between the older and younger drifts were 

 noted in describing the physiography. There is, however, a much greater 

 variety in the drift series than this single division indicates. There are 

 in fact evidences of at least four and probably five separate invasions of 

 the state by glaciers. These invasions were separated by interglacial in- 

 tervals in some cases much longer than the whole of the period since the 



♦Calvin : Iowa Geol. Surv. , 7 : 72-79. 



