OF THE UPPER MISSOURI. 
109 
its formation, but 1 prefer to delay until a larger number of facts can be gathered together 
from a more widely extended exploration. 
2(7. Yellow Marl or Bluffformation . 
The yellow marl formation is one of great interest, inasmuch as it exerts a favorable 
influence on the agricultural capacities of the country where it prevails. Its general 
character is a light, rather pulverulent silicious marl, in most cases unstratified, containing 
large quantities of yellowish white concretions of limestone, varying in size from half an 
inch to four inches in diameter. It varies much in color, however, depending upon the 
amount of ferruginous matter contained in it. In the southern and southeastern portions 
of Kansas and Nebraska this deposit is quite largely developed, often concealing the older 
rocks over large areas. According to Prof. Swallow it is seen throughout that portion of 
the State of Missouri bordering upon the river, but reaches its highest development from 
the Iowa line to Booneville. From thence to St. Louis it is very thinly represented. It 
is also largely developed in those portions of Kansas, Nebraska, and Iowa, which border 
upon the Missouri river, but attains its greatest thickness between Council bluffs and 
Sioux city, on the Iowa side. There it is two hundred and fifty to three hundred feet in 
thickness, forming a range of very precipitous hills, which are so steep that vegetation is 
with great difficulty supported on their sides. After passing the Big Sioux this deposit be¬ 
gins gradually to thin out upon the summits of the hills, and after passing the mouth of the 
Niobrara, it cannot be said to exert much influence on the surface of the country. It is 
observed, however, in outliers even to the mountains, both along the Missouri and Yellow¬ 
stone rivers, is quite conspicuous near the mouth of the Judith, with a thickness of one 
hundred feet, and containing its peculiar fossils. Wherever it occurs in the interior of the 
country it may be readily recognized by the more luxuriant and healthy vegetation which 
it supports. 
About three miles above Florence the marl bluffs are cut by the waters of the Missouri 
so as to present the following section: 
Feet. 
1. Yellow silicious marl, homogeneous in its character, containing numerous white calcareous concretions, 
varying in size from half an inch to several inches in diameter, ...... 100 to 150 
2. Yellowish sand and clay, very ferruginous, containing rounded pebbles sometimes forming a conglomerate, 3 
3. Dark ash-colored clay, with a slight mixture of sand filled with small waterworn pebbles, usually of small 
size, seldom more than one inch in diameter, ........... 2 
Near the mouth of the Big Sioux there is a vertical section of the yellow marl similar to 
the above. All the southeastern portion of Nebraska is covered with a greater or less 
