If I told you the gods were rock gardening in southern Illinois, would you believe me? No, I know, I thought the same: American Midwest. Flat. Full o' cornfields. Bo-ring. I'm originally from neighboring Indiana. I would've sworn there wasn't a surface rock more exciting than a pebble anywhere within 500 miles.
Get beyond those areas where vast continental ice sheets planed the land and left everything buried under thick deposits of nothing visually stunning. You won't have to go far, just fifteen miles (24 km) north of the Shawnee Hills. Things begin to get rather more interesting. There are rocks. Keep going south, into the hills. Then: stunning.
Suddenly, it looks nothing like the Midwest. No wonder folks named this wilderness full of fantastical formations The Garden of the Gods. It looks remarkably like some supernatural entities have been growing stone mushrooms. And judging from some of the psychedelic patterns in the rocks, those aren't your basic dinner shrooms, either.
How does this even happen?
That's the question I set out to answer when my FreethoughtBlogs colleague Brianne posted that remarkable photo. In my quest, I've traveled back in time to an Illinois far more exotic than the one we know today, and discovered simple natural processes that create extraordinary automatic art - no gods required. Which is for the best, really. The last thing Illinois needs is a bunch of gods growing megalithic magic mushrooms all over the place.
Life was often a beach in ancient Illinois; sometimes, even a sea bottom. From the late Cambrian to at least the end of the Carboniferous, the middle of the North American continent was repeatedly host to a fine, large inland sea. Titanic forces warped the crust as plates collided and mountains rose elsewhere, causing the ocean to mosey north, then recede south. Ocean sediments piled up over the hundreds of millions of years this went on: time, pressure, and chemical reactions turned them into a layer cake of sandstones, limestones, and shales. Erosion nibbled some of the layers away from time to time, but tens of thousands of feet (thousands of meters) remain.
320 million years ago, during the Pennsylvanian epoch, the region that would become the Garden of the Gods was a realm of rivers traveling from the northeast to the southwest. Sands were deposited by the fast-flowing waters in river channels and a delta fanning out to sea. We can see the remains of those old systems in the cross-bedding and ripple marks preserved in the rocks. The river and nearshore currents were vigorous, carrying away the soft, fine sediments, leaving quartz grains and pebbles behind. These would have been lovely beaches to laze the warm Paleozoic days away upon. Shame the piña colada ingredients hadn't evolved yet.
A Rocky Start
The Pennsylvanian paradise couldn't last forever, of course. After much depositing of lovely sands, la mer and la rivière said au revoir. Our particular sands, by now buried under around a mile of sediments, strongly lithified by the experience, were uplifted high above sea level, fractured, and stripped of their protective covering. Beaches had become the Pounds Sandstone Member of the Caseyville formation, and were ready for the gods to garden.
How to Grow Stone Mushrooms
So you've got your firmly-cemented sandstone, and you want to wear some bits away to form funny shapes. What are you, natural processes, going to do in order to grow a garden of stone mushrooms?
Well, you'll need to break up your sandstone a bit. Joints and cracks and faults do nicely. And, just like for ordinary grocery store mushrooms, you'll need a nice, moist bed of soil.
This is how those fantastic shapes happen: under a deep blanket of soil, kept damp by the vagaries of local drainage or the water table, chemistry and biology happens. Chemical reactions and microbial activity dissolve the cement binding those ancient grains of beach sands together. Carbonates and silicas get flushed away, leaving loose sand grains behind. Blocky shapes are rounded as the grains loosen. Bits buried deeper, and thus subject to more vigorous chemical and biological attack, are whittled away to stems. The caps, nearer the surface, aren't subject to quite the same intensity of etching, and thus remain more firmly cemented. Although it's the same rock as the sorely-vexed stuff beneath, its relatively cushy conditions have allowed it to become - wait for it - cap rock.
(You know, mushroom cap... stone mushrooms... cap rock.... Okay, yes, sorry. Please put down the rotten veggies. I'll show myself out.)
Now, before the stems are dissolved completely away, we need to harvest our mushroom garden with some nice erosion. Perhaps it's those lovely stresses in the crust causing bits to rise. The soil is weathered away in the usual manner - wind, rain, streams, etc. The mushrooms emerge from their dark, silent soil tomb. The caps shed water to a nicety; the weaker etched bits are weathered off, and the shapes of today are assumed, ready to awe the squidgy nekkid apes who eventually found their way to this seeming Garden of the Gods. It in, in fact, a magnificent rock garden that geology grew. Amazing.
And we haven't even gotten to the most psychedelic display of nature's automatic art.
Rings Around the Mushrooms
Have a meander along the Observation Trail, and you may see a remarkable sight:
We still don't know precisely how these Lisegang bands form, but we have the basics down. At some point in the past, these bits of sandstone were bathing in iron-rich ground water. Fluids find it relatively easy to travel through porous sandstones - the stuff soaks it up like a rocky sponge. Think of the water soaking through these bits as a sort of runny iron gelatin - it's called a colloidal suspension, and that basically is a lot of extremely fine particles of something, like iron, suspended in water. Now, if your colloid encounters a salt, which is quite easy for it to do in the near-surface conditions here, something rather amazing happens. The iron particles begin clumping into a solid. But they don't do it in one big lump. They begin forming bands and rings. Some researchers think it's because the added salt isn't introduced uniformly, causing gradients to form.
The bands swirl through the sandstone, leaving incredibly colorful patterns behind. This can happen more than once, creating lovely abstract art. And since iron is one of those things that can cement sandstone, the rings stand up to erosion more strongly than the surrounding stone does. You can see where Liesegang banding was after the colors fade by the bizarre, contorted layers left in the stone.
All this, all done by nature. Lovely!
Adamovič, Jiří (2005): Sandstone cementation and its geomorphic and hydraulic implications. Sandstone Landscapes in Europe - Past, Present and Future. Proceedings of the 2nd International Conference on Sandstone Landscapes. Vianden (Luxembourg).
Frankie, Wayne T. (2009): Guide to the Geology of the Garden of the Gods Recreation Area, Shawnee National Forest, Saline, Gallatin, Pope, and Hardin Counties, Illinois. Geological Science Field Trip Guidebook 2009B.
Grab, S. et al (2011): Sandstone geomorphology of the Golden Gate Highlands National Park, South Africa, in a global context. Koedoe - African Protected Area Conservation and Science.
Mueller, J.E. and Twidale, C.R. (2002): Geomorphic development of the Giants of the Mimbres, Grant County, New Mexico. Bureau of Geology and Mineral Resources.
Palmer, J. and Dutcher, R.R. (1979): Depositional and structural history of the Pennsylvanian System of the Illinois Basin. Illinois State Geological Survey Guidebook Series 15, Part 1: Road log and descriptions of stops.
Thiry, Medard (2005): Weathering morphologies of the Fontainebleau Sandstone and related silica mobility. Sandstone Landscapes in Europe - Past, Present and Future. Proceedings of the 2nd International Conference on Sandstone Landscapes. Vianden (Luxembourg).