It was the beginning of the Glacial Period or Ice
Age, when a large portion of the United States is supposed to have been
covered by a sheet of ice. The ice is believed to have entered South
Dakota from the northeast and its drift across the state limited by a
line so closely following the present course of the Missouri River that
many of us would be inclined to consider it the western bluff. Beyond
this line the ice failed to push its way, but the Hills were subject to
heavy rain storms that filled the streams and carried large quantities
of bowlders and other eroded material, both coarse and fine, down into
the valleys and over the lower hills, where much of the moderately
coarse can now be seen exposed on the surface, and fine specimens
collected without the use of a hammer. The brilliantly colored, striped
and mottled agates, and the bright, delicate tints of the quartz
crystal, are particularly attractive to the majority of visitors. The
beauty of these gaily colored rocks is quite extensively utilized by the
inhabitants of the southern and southeastern hills to supply the place
of growing plants which are generally denied by the inconvenience of the
water supply. The quartzite of the Hills is well crystallized and heavy.
I have one beautiful specimen of the dark Indian red variety through
which passes a narrow line of pale blue, and the yellow quartzite or
jasper sometimes shows dendrite markings.
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