They reached, however, far up the
Missouri and Yellowstone rivers, and thence into Oregon. The head-waters
of the Missouri became one of their great centres of population; but
their chief sites were upon the Mississippi and Ohio rivers. In
Wisconsin we find the northern central limit of their work; they seem to
have occupied the southern counties of the State, and the western shores
of Lake Michigan. Their circular mounds are found in Minnesota and Iowa,
and some very large ones in Dakota. Illinois and Indiana were densely
populated by them: it is believed that the vital centre of their
colonies was near the junction of the Ohio and Mississippi rivers.
The chief characteristic of the Mound Builders was that from which they
derived their name-the creation of great structures of earth or stone,
not unlike the pyramids of Mexico and Egypt. Between Alton and East St.
Louis is the great mound of Cahokia, which may be selected as a type of
their works: it rises ninety-seven feet high, while its square sides are
700 and 500 feet respectively. There was a terrace on the south side 160
by 300 feet, reached by a graded way; the summit of the pyramid is
flattened, affording a platform 200 by 450 feet.
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