Ancient Reefs & Bioherms of Indiana, Part 20: Meditation on a Mississippian Mound | Stobo Bioherm, Bloomington, Monroe County
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 northward.
The best way to understand the difference between a biogenetic structure like this one, which is usually called a bioherm, and the reefs already discussed in this album, is to read the introduction (second italicized paragraph) of the Part 1 essay.
On top of that, there’s one other bit of geo-jargon, buildup, that some paleontologists have preferred in describing this site. It’s a very general term for any feature preserved in the rock record that was constructed by marine organisms. So, as I understand it, both bioherms and reefs are buildups.
As far as Stobo goes, it’s just a very local moniker of uncertain origin. There is no town here called Stobo. There is no well-defined neighborhood here named Stobo. Nevertheless, this place is Stobo. And that fact, and the way the word sounds, make it quintessentially Hoosier.
I took this photo from across the road and did not crop out the latter because I want to emphasize just how close the bioherm is to this surprisingly busy two-lane thoroughfare. If you take a class or tour group here, be prepared to spend all of your own energy keeping everyone else from wandering onto the pavement and getting scraped up onto the windshield of a passing one-ton pickup. You may need to bring along a bullwhip or an electric cattle prod.
(Because facetiousness is no longer recognized let alone appreciated, and because everything is taken literally these days unless you use puppets to illustrate, please note that I am kidding about the whipping and the zapping. But not about the dangerous traffic.)
And concerning the bioherm itself: it certainly has a well-defined convex or mound shape that was nicely revealed in cross section by the road’s builders. Its central portion is composed by layers of limestone with abundant crinoid (“sea lily") remains; bryozoan fossils are also found there. Between these strata lie others made of silty shale. And the whole structure is covered and surrounded by siltstone. All of this is stratigraphically assigned to the Edwardsville Formation of the Mississippian (Lower Carboniferous) Borden Group.
I have also featured the Borden Group in another context, because its shales and siltstones were mined for the manufacture of Crawfordsville Brick. That striking building material was often used by architect Louis Sullivan in his incomparable jewel-box banks.
The limestone beds, easily identified in this photo as the more resistant, projecting sections of the mound, are the remains of crinoid-bryozoan “meadows” that developed some 345 Ma ago in shallow marine conditions, on the topset beds of the Borden Delta. That great mass projecting into the Midwestern epeiric sea was made of sediments eroded from the Neoarcadian Mountains to the east. Like most modern deltas, it had a complex pattern of distributaries, branching streams that carried the water of major rivers into the sea. The bioherm was situated between distributaries.
The recessed and more easily weathered shale layers between the limestone strata probably represent pulses of mud, perhaps the result of tropical storms, that periodically entombed the organisms inhabiting the underwater meadows. Time and again the crinoid communities reestablished themselves after these burial events; but eventually the delta prograded (grew forward) and covered the bioherm once and for all.
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.
Ancient Reefs & Bioherms of Indiana, Part 20: Meditation on a Mississippian Mound | Stobo Bioherm, Bloomington, Monroe County
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 northward.
The best way to understand the difference between a biogenetic structure like this one, which is usually called a bioherm, and the reefs already discussed in this album, is to read the introduction (second italicized paragraph) of the Part 1 essay.
On top of that, there’s one other bit of geo-jargon, buildup, that some paleontologists have preferred in describing this site. It’s a very general term for any feature preserved in the rock record that was constructed by marine organisms. So, as I understand it, both bioherms and reefs are buildups.
As far as Stobo goes, it’s just a very local moniker of uncertain origin. There is no town here called Stobo. There is no well-defined neighborhood here named Stobo. Nevertheless, this place is Stobo. And that fact, and the way the word sounds, make it quintessentially Hoosier.
I took this photo from across the road and did not crop out the latter because I want to emphasize just how close the bioherm is to this surprisingly busy two-lane thoroughfare. If you take a class or tour group here, be prepared to spend all of your own energy keeping everyone else from wandering onto the pavement and getting scraped up onto the windshield of a passing one-ton pickup. You may need to bring along a bullwhip or an electric cattle prod.
(Because facetiousness is no longer recognized let alone appreciated, and because everything is taken literally these days unless you use puppets to illustrate, please note that I am kidding about the whipping and the zapping. But not about the dangerous traffic.)
And concerning the bioherm itself: it certainly has a well-defined convex or mound shape that was nicely revealed in cross section by the road’s builders. Its central portion is composed by layers of limestone with abundant crinoid (“sea lily") remains; bryozoan fossils are also found there. Between these strata lie others made of silty shale. And the whole structure is covered and surrounded by siltstone. All of this is stratigraphically assigned to the Edwardsville Formation of the Mississippian (Lower Carboniferous) Borden Group.
I have also featured the Borden Group in another context, because its shales and siltstones were mined for the manufacture of Crawfordsville Brick. That striking building material was often used by architect Louis Sullivan in his incomparable jewel-box banks.
The limestone beds, easily identified in this photo as the more resistant, projecting sections of the mound, are the remains of crinoid-bryozoan “meadows” that developed some 345 Ma ago in shallow marine conditions, on the topset beds of the Borden Delta. That great mass projecting into the Midwestern epeiric sea was made of sediments eroded from the Neoarcadian Mountains to the east. Like most modern deltas, it had a complex pattern of distributaries, branching streams that carried the water of major rivers into the sea. The bioherm was situated between distributaries.
The recessed and more easily weathered shale layers between the limestone strata probably represent pulses of mud, perhaps the result of tropical storms, that periodically entombed the organisms inhabiting the underwater meadows. Time and again the crinoid communities reestablished themselves after these burial events; but eventually the delta prograded (grew forward) and covered the bioherm once and for all.
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.