Near Spring Grove in Houston County
The upper part of this formation is
Decorah Shale. The limestone is in the Platteville Formation, both part
of the Ordovician Age. Both of these rock formations have numerous
fossils. We came here with the express purpose of collecting
fossiliferous limestone. Many of the fossils are small "shell" type but
there are also larger fossils, especially the cephalopods that can
reach six feet in length. A full cephalopod fossil is very rare.
Notice in the photos the
layering effect of the rock. These sedimentary rocks were deposited in
layers from the remains of sea life. Notice that the layers vary in
thickness. The varying thiskness of the layer depends on the depth of
the water at the time of the formation of the layer. Each layer is an
indication of a local climate or geologic
change. In addition to the thickness change from the water is the rock
type change. In shallow water larger particles like sand are deposited
and in deep water very small particles of soil are deposited to form
shale (that may be metamorphosed into slate). The intermediate depths
are where the carbonate rocks are formed. Within the layers of
carbonate rock the thickness variations continue to reflect the change
in the depth of the water where the thickness of the layer is
controlled by the depth of the water where the required sealife can
form the rock. The cyclic variation in the thickness of the rock layers
as well as the variation of sandstone, limestone and shale are
indications of the changing water depth over time.
Builders often make use of the different thicknesses to build
the interlocking patters typical of limestone walls as those in the
building in the photos below.
The rock in this
location is mined for aggregate or what in most of the rest of the
state would be termed gravel. In this part of the state the "gravel"
roads and the "gravel" in the concrete is very often crushed limestone.
|In the photo at left there is a wonderful "real life" (as
drawn diagrams) example of a joint. This vertical crack in the rock is
a water pathway for surface water into the bedrock. With additional
weathering from water seepage this joint could become a sinkhole or
a cave entrance.