This up and down motion continued at intervals until the Fox Hills epoch
of the Cretaceous Age, at the close of which the sea retired forever
from that portion of the country. In the next epoch fresh water work
began and extensive marshes were formed, with an abundant growth of
vegetation and reptiles. There was also much volcanic violence which
resulted in the fine scenery in the north end of the Black Hills, and
probably opened the fissures to form Wind Cave, the Onyx Caves in the
southern hills and Crystal Cave near the eastern edge toward the north.
This was near the close of the Cretaceous Age. But here is a point on
which the best authorities who have studied the porphyry peaks, have
failed to agree; Prof. N.H. Winchell believing that the intrusion
occurred, probably, during the Jura Trias, but as Cretaceous beds, of
more recent date, are found to have been distorted by the outflow, it
seems that Professors Todd, Newton and Carpenter hold the stronger
position and that the later time is correct.
No record of the next geological stage, which was the Eocene, or earlier
part of the Tertiary Age, has been found in the Hills, because they were
at that time dry land with gently flowing, shallow streams, and
consequently no strata were laid down; but they are supposed, through
later evidences, to have had a tropical climate and vegetation, enjoyed
by large animals of strange new forms.
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