Ancient Reefs of Indiana, Part 17: Focusing on the Nearside | Wabash Reef, Wabash, Wabash County

At the railroad cut just to the northeast of the eastern terminus of Market Street. Looking north-northeastward.

My primary sources for this and my other Wabash Reef posts are cited in the Part 16 essay

Before proceeding to the core of this small patch reef I’m taking a moment to record the steep dip of the dolostone flank beds on the southwestern side of the railroad cut.

These strata serve to remind us that the universe we inhabit loves to mock the human need for simple understandings. One such understanding is a hallowed dictum of geology, Nicolaus Steno’s Principle of Original Horizontality. This rule states with uncomplicated elegance that layers of sedimentary rock form flat. If one ever finds them tilted, overturned, or otherwise deformed, it signifies that some sort of subsequent disturbance has taken place.

But here, in the stable core of the continent, these beds have remained as they’ve always been, serenely unruffled by tectonic activity. They’re a classic exception to Steno’s otherwise still extremely useful observation. Sediments deposited on a slope faithfully reflect the glaring lack of an originally horizontal surface. They tilt the way their local environment did.

I have recently made this same point in a post about a much larger biogenetic structure, the great Capitan Reef so gloriously exposed at Guadalupe Mountains National Park in Trans-Pecos Texas. Here’s one shot of the toe of its foreslope that is quite analogous to the image above.

Being Middle Permian in age, the Capitan is 150 Ma younger than the structures seen in this series. Curiously, it was made not by the same sorts of marine creatures—the ancient types of stromatoporoids and tabulate and rugose corals that constructed this and other Midwestern Silurian reefs. Those species had gone extinct by the waning of the Paleozoic era. Instead, the Capitan was built by calcareous sponges, algae, and bryozoans.

Of course, a new lineage of corals, the scleractinians, ultimately came into being. They are the architects of our current reefs, which are now so sadly threatened by ocean acidification, overexploitation, and global heating. All of these are caused by a separate growth-obsessed, planet-wrecking animal species. The less said about that heedless critter, the better.

You'll find the other photos and descriptions of this series in my Ancient Reefs of Indiana album.

Taken on: June 26, 2007

Tags:

geology

nature

rocks

stone

bedrock

Indiana

Wabash, Indiana

Wabash Reef

Silurian reefs

ancient reefs

dolostone

strata

flank beds « less

Sets appears in: • Ancient Reefs of Indiana • Grab Bag of All Things Silurian more » • Natural History: Indiana • Rock Forms, Rock Formations « less

Camera: NIKON COOLPIX S4, f/4.0, 1/182 sec, 6.3mm, ISO 50 (more info) (hide info)

Camera:	NIKON COOLPIX S4
Aperture:	f/4.0
Exposure Time:	0.005 sec (1/182)
Focal Length:	6.3mm
ISO:	50
Exposure Bias:	0 EV
Flash:	Off, Did not fire
Exposure Program:	Program AE
Metering Mode:	Multi-segment
White Balance:	Auto
Color Space:	Uncalibrated
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