The Flat Rock River is less than 800 feet above sea level in
Shelby County, Indiana. The river travels along and through outcroppings of
limestone much of the way from Decatur to Shelby County. I’m a geology nerd and
there are incredibly beautiful, very non-Indiana looking spots on the Flat Rock
where rock meets river and runs along beneath the water.
In 1862, a geologist wrote that the origin of the name “Flat
Rock” for this river couldn’t be explained. Apparently that geologist didn’t wade
along the limestone bed or travel down through the terraced cliffs or
outcropped banks that attracted me here. Those rocky outcroppings and the flat
stone bed of the river are a big part of what drew me to this place.
Still that mid-19th century geologist knew a lot
that I don’t about the rock of this area. In his Report of a Geological Reconnaissance of Indiana Made during the Year
1859 he had this to say about the stone of the Flat Rock. “The Upper
Silurian provides good building material on the Flat Rock in Shelby County.
Junction of Upper Silurian and Devonian is found in the quarries in the valley
of the Flat Rock. The silicious limestone in the beds of the Flat Rock did not
afford many fossils." A “southerly dip was quite perceptible in the bed of the
Flat Rock; at an old dam, the rocks were much rippled, marked and grooved by
the wearing effects of the water running here with the strike. Immense
quantities of chert and confervae covered some portions."
The 1880 Atlas of
Shelby County, Indiana noted that “in the extreme southeast corner of Shelby
County… the face of the country is rolling and undulating, and the land is well
watered by numerous streams. Flat Rock, Conn’s Creek, Lewis Creek, Tough Creek
and Duck Creek are the principal. Part of St. Paul is in Shelby County … on the
west bank of the Flat Rock River. At this point are located the famous
limestone quarries, from which stone is shipped all over the country. The stone
for the new State House of Indiana and for the United States Custom House in
Cincinnati is from this quarry.”
According to the Indiana Geological Survey, the name “Geneva
Limestone” was used in 1882 to describe the exposures of buff dolomitic
limestone along the Flat Rock River near Geneva, Shelby County, Ind. The same
rocks were later called the Shelby Bed. More recently the term Geneva Dolomite
Member, part of the Jeffersonville Limestone has been used for this rock. “The
Geneva Dolomite Member is massive to thick-bedded in its lower part and more
commonly thin-bedded in its upper part. The distinctive colors are due to a
high organic content, and near-surface beds are commonly oxidized to pale tan,
cream, or even white. White crystalline, coarsely cleavable calcite masses, ranging
from 1 inch to more than 1 foot in cross section, resulting from calcification
of fossils, are scattered through the fine-grained dolomite matrix. Chert is
present in some sections, and quartz sand is especially common in basal rocks.”
By 1876, the limestone of this county was already
appreciated as a building material. That year a History of Shelby County, Indiana reported that “The river beds
furnish us with a most excellent substance for the construction of turnpike
roads; and to what extent our people have availed themselves of it may be seen
in such facts as these, that fifteen different travel roads center in
Shelbyville, and that there are almost two hundred miles of turnpike in the
county!”
It looks like the stone has been harvested along the banks
of the river near my house. There are literally blocks of stone on the banks.
Maybe that happens naturally, or maybe it’s leftover from when early
homebuilders here took advantage of a rocky river bank to chunk off foundation
stone. Road builders hauled stone from
my river to build their turnpikes. Pioneers nearby managed to harvest slabs big
enough to make a porch (out of one huge rock) for their fine stone house in the
1850s.
That stone has been useful to and used by humans for as long
as humans have been here, I’m sure. But it’s ancient stone. It was formed from
the deposition of critters who lived in our shallow inland sea in the Upper
Silurian to the Devonian periods---that’s more than 400 million years ago.
And after that shallow sea receded, the glaciers came and went and their melt-off carved out this river valley, leaving behind, when they receded, this beautiful peaceful river. And the 400 million year-old limestone that runs beneath it.
