Ancient Reefs of Indiana, Part 1: Welcome to Hanging Rock National Natural Landmark | Lagro, Wabash County
(Last updated on January 18, 2025)
On the southern bank of the Wabash River, and facing westward. We're a few meters/yards from the upstream side of this prominent waterfront feature.
Please note that in this series I will not distinguish between the technical terms "reef"' and "bioherm." If the scientific literature is any guide, different specialists have somewhat different definitions of each, and I'm not going to get embroiled in that semantic debate. My apologies in advance if I seem to be imprecise in describing everything I'll cover as a reef.
I first heard of Hanging Rock long before the American Endarkenment had begun, i.e., in the early 1970s, when my invertebrate-paleontology professor at Purdue University mentioned it to me. The fact that there was an exhumed Silurian-period reef about 75 mi (121 km) up the Wabash River from where my school stood was to me a thrilling discovery demanding immediate exploratory action. But that locale was definitely out of the range of my only vehicular transportation system at that time, a ten-speed bicycle. It had taken me to some quarries about 20 mi (32 km) away, but that was about the limit. Fortunately, I was eventually able to cadge a ride to Hanging Rock from one of my car-owning friends.
For some reason, though, I didn't pay a return visit till the winter day I took this photo, two decades later. At that point I was working as a National Park Service ranger at Indiana Dunes National Lakeshore. I tried to convince some of my colleagues to go with me and learn some more Hoosier State geology. Alas, while they were only too happy to have me herd them around the Chicago Loop, they had no interest whatsoever in a trek to see a big hunk of rock sticking up in the middle of nowhere.
In fact, any ancient reef you find anywhere on Earth is the direct opposite of being "nowhere," but there was no convincing my fellow rangers of that. So I made the drive alone, which, I've learned since, is the best state to be in if you want to develop a good Ortsinn, the Clausewitzian concept of accurately assessing and appreciating the landscape you're traveling through.
Hanging Rock itself is actually only that portion of a reef that the Wabash River and the late-Pleistocene Maumee Megaflood have not removed. Stratigraphically speaking, it's composed of the predictably named Upper Silurian Wabash Formation, which dates to approximately 425 Ma ago.
This unit includes two members, the Liston Creek Limestone above and Mississinewa Shale below, as well as reef facies here and elsewhere in this region. All three of these have a tendency to interfinger in reefal zones, and it can be difficult to differentiate one from the other.
Supposedly the Mississinewa can be distinguished at the base of this reef. Or so E. R. Cumings and R. R. Shrock averred in a 1926 report ("The Silurian Coral Reefs of Northern Indiana and their Associated Strata,” Papers on Geology, Geography, and Archaeology 36).
Designated a National Natural Landmark back in 1986, this site offers the visitor willing to scramble up to its summit a wonderful view of the great, megaflood-scoured Wabash Valley. And underfoot there's the rather thin-bedded, slabby, and flaggy carbonate bedrock, which varies from limestone to dolostone. The latter of these, being somewhat less reactive, probably has contributed, along with the harder reefal texture, to the survival of the southern side of this hill.
Photos to follow, taken in 1993, 2001, and 2007, will show different aspects of this locale, including a shot or two of the good ol' Wabash at flood stage.
Addendum: A few days after writing this post I got my paws on the Geological Society of America Field Guide No. 56, Ancient Oceans, Orogenic Uplifts, and Glacial Ice. It contains a field-trip writeup, "The Maumee Megaflood and the Geomorphology, Environmental Geology, and Silurian–Holocene History of the Upper Wabash Valley and Vicinity, North-central Indiana," (Anthony H. Fleming et al., 2018).
The Fleming paper provides a good modern synthesis of research on both the Maumee Megaflood and the significance and stratigraphy of Hanging Rock. It divides the latter into three rock units, from bottom to top:
- Dolomitic siltstone resembling the Mississinewa Member; this was laid down before the reef began to form.
- Layers of dolomitic and calcitic siltstone that alternate with thin, fossiliferous, and southwestward-dipping strata of limestone resembling the Liston Creek Member.
- Hard, massive caprock of dolomitic limestone that also dips southwestward. According to this source, it's this section that most resembles true reef core, but it too was probably part of the reef's southern flank.
This will be stratigraphy I'll use in future Hanging Rock posts.
You'll find the other photos and descriptions of this series in my Ancient Reefs of Indiana album.
Ancient Reefs of Indiana, Part 1: Welcome to Hanging Rock National Natural Landmark | Lagro, Wabash County
(Last updated on January 18, 2025)
On the southern bank of the Wabash River, and facing westward. We're a few meters/yards from the upstream side of this prominent waterfront feature.
Please note that in this series I will not distinguish between the technical terms "reef"' and "bioherm." If the scientific literature is any guide, different specialists have somewhat different definitions of each, and I'm not going to get embroiled in that semantic debate. My apologies in advance if I seem to be imprecise in describing everything I'll cover as a reef.
I first heard of Hanging Rock long before the American Endarkenment had begun, i.e., in the early 1970s, when my invertebrate-paleontology professor at Purdue University mentioned it to me. The fact that there was an exhumed Silurian-period reef about 75 mi (121 km) up the Wabash River from where my school stood was to me a thrilling discovery demanding immediate exploratory action. But that locale was definitely out of the range of my only vehicular transportation system at that time, a ten-speed bicycle. It had taken me to some quarries about 20 mi (32 km) away, but that was about the limit. Fortunately, I was eventually able to cadge a ride to Hanging Rock from one of my car-owning friends.
For some reason, though, I didn't pay a return visit till the winter day I took this photo, two decades later. At that point I was working as a National Park Service ranger at Indiana Dunes National Lakeshore. I tried to convince some of my colleagues to go with me and learn some more Hoosier State geology. Alas, while they were only too happy to have me herd them around the Chicago Loop, they had no interest whatsoever in a trek to see a big hunk of rock sticking up in the middle of nowhere.
In fact, any ancient reef you find anywhere on Earth is the direct opposite of being "nowhere," but there was no convincing my fellow rangers of that. So I made the drive alone, which, I've learned since, is the best state to be in if you want to develop a good Ortsinn, the Clausewitzian concept of accurately assessing and appreciating the landscape you're traveling through.
Hanging Rock itself is actually only that portion of a reef that the Wabash River and the late-Pleistocene Maumee Megaflood have not removed. Stratigraphically speaking, it's composed of the predictably named Upper Silurian Wabash Formation, which dates to approximately 425 Ma ago.
This unit includes two members, the Liston Creek Limestone above and Mississinewa Shale below, as well as reef facies here and elsewhere in this region. All three of these have a tendency to interfinger in reefal zones, and it can be difficult to differentiate one from the other.
Supposedly the Mississinewa can be distinguished at the base of this reef. Or so E. R. Cumings and R. R. Shrock averred in a 1926 report ("The Silurian Coral Reefs of Northern Indiana and their Associated Strata,” Papers on Geology, Geography, and Archaeology 36).
Designated a National Natural Landmark back in 1986, this site offers the visitor willing to scramble up to its summit a wonderful view of the great, megaflood-scoured Wabash Valley. And underfoot there's the rather thin-bedded, slabby, and flaggy carbonate bedrock, which varies from limestone to dolostone. The latter of these, being somewhat less reactive, probably has contributed, along with the harder reefal texture, to the survival of the southern side of this hill.
Photos to follow, taken in 1993, 2001, and 2007, will show different aspects of this locale, including a shot or two of the good ol' Wabash at flood stage.
Addendum: A few days after writing this post I got my paws on the Geological Society of America Field Guide No. 56, Ancient Oceans, Orogenic Uplifts, and Glacial Ice. It contains a field-trip writeup, "The Maumee Megaflood and the Geomorphology, Environmental Geology, and Silurian–Holocene History of the Upper Wabash Valley and Vicinity, North-central Indiana," (Anthony H. Fleming et al., 2018).
The Fleming paper provides a good modern synthesis of research on both the Maumee Megaflood and the significance and stratigraphy of Hanging Rock. It divides the latter into three rock units, from bottom to top:
- Dolomitic siltstone resembling the Mississinewa Member; this was laid down before the reef began to form.
- Layers of dolomitic and calcitic siltstone that alternate with thin, fossiliferous, and southwestward-dipping strata of limestone resembling the Liston Creek Member.
- Hard, massive caprock of dolomitic limestone that also dips southwestward. According to this source, it's this section that most resembles true reef core, but it too was probably part of the reef's southern flank.
This will be stratigraphy I'll use in future Hanging Rock posts.
You'll find the other photos and descriptions of this series in my Ancient Reefs of Indiana album.