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Looking at a roadcut on the northern side of Indiana 46, located 3.3 mi / 5.3 km east of that road’s T-intersection with Indiana 446. This is on the eastern and rural fringe of the city of Bloomington.
Facing northeastward.
This slightly more oblique view of the bioherm complements the Part 20 synoptic photo. Here the emphasis is on the stratification in the mound’s core.
And that stratification consists of alternating beds of resistant, ledge-forming limestone and much softer and more crumbly silty shale. All of it is part of the Edwardsville Formation of the Mississippian (Lower Carboniferous) Borden Group.
As noted previously, the limestone, rich in crinoid remains, represents once-flourishing marine “meadows” composed primarily of the graceful sessile echinoderms know to fossil collectors as sea lilies. These crinoid-dominated communities had established themselves about 345 Ma ago on the topset beds of a delta that projected into an epeiric (continent-covering) sea. On the other hand, the shale layers were deposited as surges of mud that periodically buried the meadows and their inhabitants.
What an amazing stroke of luck: this relict of an ancient and repeatedly resurrected ecosystem now stands like an opened history book by the side of the road. It’s there to be read by anyone who passes by.
Still, the opportunity is a fleeting one. Soon the layered stone will be weathered and eroded away, and so will we. But in the act of learning what outcrops like this can tell us, we make our lives into something more than the puny quanta of time and effort they otherwise would be.
Main Sources Consulted for This Essay
- Carozzi, Albert V. and J. G. William Soderman. “Petrography of Mississippian (Borden) Crinoidal Limestones at Stobo, Indiana.” Journal of Sedimentary Research 32 (3): 397–414 (1962).
- Becker, Michael J. and J. Robert Dodd. “Depositional History of a Mississippian Crinoidal Mound on the East Flank of the Illinois Basin.” Carbonates and Evaporites 9: 76-88 (1994).
- Hynes, Maureen P. and Cuffey, Roger. “Bryozoan Species and Roles in the Stobo Crinoid-Bank Bioherm (Lower Mississippian, South-Central Indiana).” Abstract of a presentation at the 39th Annual Meeting of the Geological Society of America North-Central Section (May 19–20, 2005). Accessed November 9, 2025. gsa.confex.com/gsa/2005NC/webprogram/Paper81957.html,
- Ettensohn, Frank R., R. Thomas Lierman, Devi B.P. Udgata, and Charles E. Mason. “The Early-Middle Mississippian Borden–Grainger–Fort Payne Delta/Basin Complex: Field Evidence for Delta Sedimentation, Basin Starvation, Mud-mound Genesis, and Tectonism during the Neoacadian Orogeny.” In From the Blue Ridge to the Coastal Plain: Field Excursions in the Southeastern United States. Field Guide 29, Martha Cary Eppes and Mervin J. Bartholomew, eds. Boulder, CO: Geological Society of America, 2012.
You'll find the other photos and descriptions of this series in my Ancient Reefs & Bioherms of Indiana album.