Written in the Hills
Geology of the Ozarks, part 1
Part of the Precambrian geologic record of the Ozarks. Ozark Precambrian rocks are primarily
igneous (fire formed). Consisting of rhyolite, volcanic tuff and granite, most are from 1.5 to
1.0 billion years old. Some xenoliths (strange rocks) occur in the granites which are probably
over two billion years old. The attractive waterfalls shown are flowing over rhyolite, these occur
along the Ozark Trail in Shannon County Missouri. Stromatolites (fossil algae) are found in some
of the tuff beds, the earliest evidence of life in this portion of North America. The masses of "algae"
(actually cyanobacteria) may have lived associated with large volcanic lakes. The granite of the
St. Francois Mountains has intruded rhyolite and tuff beds many of which give age dates (geochrons)
of 1.4- 1.5 billion years. The Precambrian rocks of the St. Francois Mountains and the Current River
region are the oldest rocks exposed in the lower mid-west. The Precambrian rocks exposed in the
upper Midwest of Minnesota and Wisconsin and part of the Canadian Shield, and are often even
older than those of the Ozarks.