MEDIEVAL OR MESOZOIC TIMES. 187 



The Fort Benton Group. 



The preceding period was closed by the changed conditions 

 brought on by a further subsidence of" the region where its deposit 

 are found. Where shallow seas and extended sea beaches and flats 

 full of low islands had obtained, now rolled deeper waters and 

 quieter seas. The deposits formed during these times have been 

 called by Havden the Fort Benton Group. They are dark gray 

 laminated clavs, sometimes alternating near the upper part with 

 seams and layers of soft gray and light colored limestone, filled in 

 many places with marine shells. Occasionally in Nebraska this 

 group contains seams of impure lignite and other carbonaceous 

 matter. It lies conformably on the Dakota Group below. It is so 

 friable and easily eroded and disintegrated, that wherever it is left 

 exposed, so far as I have observed, it has disappeared. In many 

 places, however, where deep sections have been made by canyons 

 and railroad cuts through the Niobrara. Group, which lies above, its 

 deposits are almost invariable present, and often in notable thick- 

 ness. One of the finest of these exposures is seen below the 

 mouth of Iowa Creek, in Dixon County, along the Missouri bluffs. 

 Here for a long distance the line of demarkation between the Da- 

 kota, Fort Benton and Niobrara groups are distinctly seen and clear- 

 ly outlined. Below Milford, on the banks of the Blue, and at other 

 points in Seward County, in deep sections, it is also observed. 



That this period was a long one is evident from the fact, as ob- 

 served by Havden, that its deposits are in some places Soo feet 

 thick. The materials, too, are of a kind that are slowly deposited. 

 It is probable that the numerous low islands that had existed in 

 Nebraska during the previous epoch, had now mostly disappeared 

 beneath the constantly deepening seas. Some land surfaces still 

 existed in southeastern Nebraska, but no such memorials of its con- 

 dition have come down to us as marked the preceding epoch. 

 Marine life, however, was abundant. Meek alone has described 

 from this group five species of Inoceramus, a mollusk distantly re- 

 lated to the oyster, and nine species of chambered shells, some of 

 which were of great size and beauty. He has also given eleven 

 additional marine molluscan forms*. The seas swarmed with fishes. 

 Reptilian life was abundant, but this feature will be presented in 

 the discussions of the next epoch. 



♦SeeMeek's "Cretaceous Invertebrate Fossils 



