40 Soils of Nebraska — Hicks. 
which no rock-forming sediments were deposited in the region 
now known as Nebraska, During the Carbonic period the sea 
prevailed over this region and in it were deposited the mud^ 
sand, and lime which have hardened into the shales, sandstones, 
and limestones of the Coal Measures and Permian. Then the 
sea retired, and during the Triassic and Jurassic periods while 
thick strata were formed in other parts of the continent, 
Nebraska was dry land. The streams of water upon its surface 
cut deep valleys during this interval. Then as the land slowly 
sank again the sea once more invaded this region. In the val- 
leys formed by running water, which were approximately in 
the same lines with the valleys now in existence^ the incoming 
sea formed deep bays along its eastern shore. In these bays 
and all over the region covered by the incoming sea, was de- 
posited a series of sands, clays, shaly sandstones, hard sand- 
stones, quartzite, lignite, conglomerate, and shale, which now 
forms the Dakota group of the Cretacic system. This is found 
in Dakota, Burt, Cuming, Dodge, Washington, Douglas, Sarpy, 
Saunders, Lancaster, Seward, Saline, Jefferson, and Gagecounties, 
as the reader will see by reference to the map. Some of these 
counties are almost wholly occupied by the Dakota group, 
others are just touched by this formation. 
Of the varions kinds of rock above mentioned as occurring in 
the Dakota group, the sands, sandstones, and clays are most 
abundant, and exert the greatest influence upon the soil. The 
clays are valuable for brick and pottery. Where they form 
continuous strata of considerable extent, with a level surface, 
the water is retained, causing boggy or swamp land. By them- 
selves these clays impart too great heaviness and tenacity to 
the soil, but with a suitable proportion of sand intermixed a 
good loam is formed. An abundance of sand and sandstone is 
everywhere present in the Dakota group to temper the clays. 
In some places the sandstone pred ominates so much as to form 
sandy knolls with a thin and poor soil or none at all. But these 
bald knobs are not numerous, and are never of great extent. 
The glacial drift and loess cover the country occupied by the 
Dakota group so generally that it is only on the high points 
projecting into the valleys that the sandstone foundation is in 
sight, making thin land. 
West of the Dakota group comes the Colorado group of the 
