If you have driven around the Black Hills area you more than likely have noticed the red colored dirt. Drive along Interstate 90 through Rapid City, Sturgis, and Spearfish you should have noticed the exposed red dirt from the road construction work. I have often wondered about this dirt and how it got its color. I found a report by the Journal of Geology about The Upper Red Beds of the Black Hills. 

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The Upper Red Beds of the Black Hills
Author(s): George B. Richardson
Source: The Journal of Geology, Vol. 11, No. 4 (May – Jun., 1903), pp. 365-393
Published by: The University of Chicago Press
Stable URL: https://www.jstor.org/stable/30056747
Accessed: 18-03-2019 12:31 UTC

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