The rocks so wonderfully displayed in Backbone State Park were originally deposited as lime sediments in a shallow tropical sea that covered the Iowa area about 430 million years ago, a time geologists term the Silurian Period. These sediments were chemically altered to form rock composed of dolomite, a magnesium and calcium carbonate mineral, with scattered nodules of chert. The Silurian rocks at Backbone belong to the Hopkinton Formation, an interval of dolomite strata that forms a productive part of the Silurian aquifer across much of eastern Iowa. Solutional openings, fractures, caves, and active springs provide evidence of water movement through these strata at Backbone.
Fossils are preserved in the rocks at Backbone as natural molds or as silica (quartz) replacements. The lower strata display an abundance of corals and sponge-like stromatoporoids, all fossils of long-extinct forms. The upper strata are crowded with molds of clam-like brachiopod shells, many oriented as they would have appeared in life. Geologists have termed these shell-rich layers the “Pentamerus Beds,” named after the characteristic brachiopod fossil. The fossils seen at Backbone provide a glimpse of ancient life that once inhabited the tropical sea bottom.
The rocks now seen at Backbone lay buried beneath younger rocks and sediments for untold millions of years. It was the inexorable onslaught of erosion that ultimately exposed these rocks at the surface. The bedrock surface in Iowa evolved in response to recurring episodes of erosion, interrupted by periods of renewed deposition and burial. In particular, the waxing and waning of continental ice sheets across Iowa over the last 2 million years or so was accompanied by a complex record of erosion and sedimentation on the Iowa landscape.