SOIL OF THE MISSOURI ‘‘ BLUFFS.” 65 
are very largely developed, where the great limestone 
series of rocks, from which they derive their chief charac- 
ters, is found. 
Among these Western soils particular prominence should, 
however, be given to one of a peculiar character, consisting 
of an extensive bed of a fine, friable, silicious marl of a buff 
color, which is found to cover to a great depth those pic- 
turesque hills on the banks of the Missouri River, and of the 
Upper Mississippi, called ‘ the Bluffs,” from which the de- 
posit itself derives its name. It is most largely developed 
along the Valley of the Lower Missouri, and extends over 
. wide areas of the elevated country on either side, forming 
the surface of the richest of the upland prairies. The ex- 
traordinary and almost exhaustless fertility of these Bluff 
Lands, as well as other qualities which distinguish them, 
will be more readily understood when the nature of the 
agency to which they evidently owe their origin is con- 
sidered. There can be no doubt that during a compara- 
tively recent geological period, the era of the mammoth, 
the mastodon, and the huge primeval beaver, a large part 
of the Western country, comprising at least the basins of 
the great rivers, was deeply covered by a fresh water lake 
of vast dimensions. Upon the floor of the lake an exten- 
sive deposit of fine, rich, marly sediment was gradually 
formed, the remains of which, after the subsequent changes 
of level and drainage of the country through its present 
water-courses, are now to be found capping the highlands 
along the streams. This sedimentary bed has in some 
instances been found to be not less than 200 feet deep, and 
perfectly homogeneous throughout; but its average depth 
on the Missouri River, where it attains its best development, 
generally does not exceed 100 feet, and it is often much 
less. The soil formed from it contains a very large pro- 
portion of vegetable matter in its natural state, derived 
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