A River's Ancient Stone.

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-19^th 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.