Soils of Nebraska — Hicls. 41 
Cretacic system, extending through the east central portions of 
the state. Its eastern boundary is the Dakota group as ah'eady 
indicated. Its western boundary runs from the north-west cor- 
ner of Brown county in a south-easterly direction to Boone 
county, thence almost due south to the Kansas line, near the 
point in Nuckolls ci nty where the Republican river leaves 
Nebraska. Besides this great central belt of the Colorado group 
there is another area in Chf-yenne, Keith, Lincoln, and Perkins 
counties along Lodg3 Pole creek and the South and North 
Platte. The rocks of the Colorado group are limestones, shales, 
and clays, which exert the same influences upon the soil as 
already ascribed to such rocks occurring in the Carbonic system. 
A characteristic difference, however, between the Carbonic lime- 
stones and those of the Colorado group is that the latter are 
extremely soft and friable, thus the more readily breaking up 
and yielding their nutritive elements to the soil. So well 
marked and general is this characteristic that these strata of 
the Colorado group are generally known as "rotten limestones.'" 
They are full o£ shells of the genera Inoceramus and Ostrea. 
Some of the outcrops look like dumping grounds of an oyster 
market, as if the shells had been thrown down by the wagon 
load and partially cemented together by the solution and redep- 
osition of the calcareous matter. This process of solution and 
redeposition has in some places produced large masses of crystals 
of calcite. In northern Nekraska the limestone of the Colo- 
rado group is quite chalky, so that it is known as "chalk rock." 
Fine sections of it may be seen in the hills bordering the 
Missouri river in Cedar and Knox counties. In western 
Nebraska the limestone of the Colorado group is firmer in tex- 
ture than in the central belt, but still readily yielding to the 
action of the elements, and thus enriching the soil. 
The Cretacic rocks both of the Colorado and Dakota groups 
were deposited in seawater. Marine fossils occur in them 
from top to bottom. After the close of the Cretacic period 
much of Nebraska was dry land, but a fresh water lake covered 
all the western and central counties, [n this lake, which varied 
in its extent from time to time, were deposited the rocks of the 
Tertiary era the most widely distributed of all the geological 
formations of Nebraska. It is indicated on the map by oblique 
strokes from the right above to the left below. The rocks of 
