The Flintrock Nature Trail of Blue Mound State Park is a 1-mile
trail that focuses on the geology of the area. At the location of
this EarthCache and along the trail, you will find chert boulders,
but did you know that these boulders did not start out as chert?
About 400 million years ago, the area where we are now was covered
by warm and shallow seas full of sea life (coral and brachiopod
fossils can be seen in some rocks here). These seas deposited a very
thick layer of sediment which slowly hardened into limestone (made
of calcium). Over millions of years, some of the calcium was
replaced by magnesium to become dolomite. Here at Blue Mound, chert
slowly started to form as silica-rich water flowed through dolomite.
Chert is a sedimentary silica rock mostly made of silicon dioxide,
commonly known as quartz, or silica. Silica often moves around
during lithification, the rock forming process, producing
layers of chert or nodules within sedimentary rocks. Temperature
changes, including water flow, can also move or re-deposit chert in
the rock during a process called diagenesis.
Chert also often contains tiny cavities that may be coated with
small quartz crystals called druse. On sunny days, these
quartz crystals sparkle on nearby chert boulders and in the rocks
that make up the walls of the Nature Center. They formed in small
cavities as silica-rich water moved through the rock.
The earthy colors of chert come from the inclusion of iron oxides,
organic matters, and other materials. Bright bands of color usually
indicate the presence of certain chemicals. We know that red,
orange, and yellow bands indicate the presence of iron oxide. Brown
or black bands likely contain manganese oxide.
Finally, chert is very hard and resistant to erosion, Blue Mounds
still stand today because they are capped by resistant dolomite or
chert.