714 ON THE GEOLOGY AND NATURAL HISTORY 
and characteristic fossil, Ostrea congesta. From there it continues to be the predominant 
formation until we reach the foot of the Great Bend, when it passes by a gentle dip beneath 
the water-level of the Missouri. In many places, as opposite the mouth of Niobrara river, 
it is shown in the form of a long series of precipitous bluffs, giving a pleasing variety to 
the general monotony of the scenery. This is one of the principal characteristic external 
features of this formation. (See fig. 10.) 
Fig. 10. 
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The upper portion of this rock is a yellowish and gray calcareous marl, very soft and 
yielding, so that it is easily cut up into numerous ravines by the temporary streams, and 
thus the bluffs along this part of the Missouri often present the appearance of a series of 
cones. At the mouth of the Niobrara the upper portion, about twenty feet in thickness, is 
much softer than that below, and is of a deep yellow color from the presence of ferruginous 
matter, and the lower portion is in the form of large square masses of gray limestone, set 
in the yellow material, which acts as a kind of mortar or cement. The vertical walls of 
the bluffs resemble very much the labors of some gigantic mason, so regular are they in 
their structure. i 
