Pandemic Discoveries #3
Bedrock Bend
Home Woods
Discovery made: 2021
Written in: 2024
Well, I'm not sure what happened there, how so much time slipped by between Pandemic Discovery #2 and Pandemic Discovery #3, but there you have it. I suppose it had to do with coming out of that period and having to jump back into what some call “normal”, but which I call “The craziness I lived before then.” Other aspects of life grabbed me by the collar, the shirt-tail, the belt, the ponytail, and dragged me away from writing projects.
So here I am, back again, after resurfacing from the craziness and trying to leave parts of it behind.
Let's get back to those discoveries. Pandemic Discovery #3 is different from #1 and #2 in a number of ways. The most obvious is that #1 and #2 were about plants. But #3 is about something foundational to all of the present-day plants in my woods, and to the soil that supports them. It's alive, too, but in a different way.
Under all of these plants and soil is … bedrock.
But why be so excited about bedrock? All over Indiana there are many areas of visible bedrock. Sometimes you drive through them where highways were cut through hills. You see buildings all over (and well beyond our state) made of Indiana limestone. You can enter caves through bedrock in Indiana.
But that's because not all of Indiana was created equally. So many changes took place – continental drift, major uplifts, erosion, sedimentation, glaciation, outwash, more erosion – creating vastly different kinds of topography throughout the state (no, Indiana is not all flat cornfields and racetracks …).
Why get excited about bedrock?
Well, for one thing, we rarely see it in Knox County, here in the southwest portion of the state. At Ouabache Trails Park, about 6 or 7 miles north of the Home Woods, the “big creek” eventually cuts through a valley between high sides, relatively low “cliffs”. Once every several years or so, the creek runs so high that it reaches the tops of the cliffs. At those times you can't even see the bridge going across the creek. At those times, the rushing water, which eventually reaches the Wabash River, carves out the valley and gnaws away at the cliff sides even more.
When it does so, it exposes even more of the … bedrock … that lines the valley. This bedrock, so unusual to see exposed in Knox County, is ancient sandstone, created from sediment deposited during the Pennsylvanian Period. This sandstone is of the McLeansboro Group of this period.
I tell people that if they want to see bedrock in Knox County, go down to the “big creek” at Ouabache Trails Park. You can touch rock formed 30,000 years ago that is always under our feet, but otherwise covered by so much glacial outwash that we don't see it.
But here it is, bedrock, in the Home Woods.
Yes! That's Pandemic Discovery #3!
I was on one of my little exploratory rambles, following our little creek after it passes Cottonwood Pond (see my blog “Cottonwood Pond”.) That little creek (with some tributaries), through the bottom of our little woods, does some twisting and turning, getting deeper and wider. After cutting deeply through our property, it ambles on through other countryside, twisting and turning even more, until it empties into the “River Deshee”, or “Riviere du Chien.”
But, back to my ambles along our little creek, where it winds through mainly mud and cut-aways of soil that reveal tree and other plant roots. Any debris in the creek bed (pebbles, gravel, bits of broken crockery, broken bottles, an occasional beer can, a soccer ball, etc.) tend to originate somewhere way upstream, somewhere up from the other side of the road, through the old homeplace of the neighbors' property and further up from there, or they are items lost or tossed from the road that work their way down. Much of the gravel probably came down from road work and construction projects, trucked in from elsewhere.
Again, I was ambling and exploring down the creek, getting to a spot where it makes a relatively sharp turn. This spot is also deeper, and is steeper on one side. Over the years, the creek has carved away at that bank, cutting into it. The bank is part of the hill that starts below our barn area. As the water has carved this side, it has exposed many roots and weakened the root system of a tree that fell over the creek, its limbs and branches splayed across. Below this jumble is a pool that, during wet times, is deep enough to hold some life. In fact, on this day there were the ker-plunks of Green Frogs, and I saw a couple of them. Their tadpoles need deep enough water that is present long enough for these somewhat large frogs to develop, so this pool had maintained its cool, shady depth all spring and into summer. Sometimes it does dry up, but not often.
April 15, 2024: Looking toward the location of Bedrock Bend. Across the vegetation in the foreground can be seen patches of pale lavender Woodland Phlox and deep purple Dwarf Larkspur.
I lingered there a bit, looking for frogs, examining moss and fungi on the fallen limbs, looking into the gravel that gathers here, when … what to my wondering (wandering) eyes did appear? Did I really see what I thought I was seeing?
Rock!!
Actual rock! In our woods! Bedrock! Layers of rock at the bottom of the cut, just above the water.
And I mean … layers. The rock was obviously layered – layers and layers of grayish-brown rock, broken here and there. Definite fissures. Various sedimentary rock words came forth from my head: shale, siltstone, mudstone. My Earth Science background from way back was starting to come forward, and I was happy to meet it again. It was gradually uncovered, layer-by-layer, like water revealing layers of bedrock.
Below the exposure, on top of the gravel, some under flowing water, were flat, gray rocks. Some were shaped like various states, including Indiana, though that one needed a piece broken of the top left corner to accommodate the southern tip of Lake Michigan, from where I hail.
I picked up one rock that was larger than my palm and I was acutely aware of having in my hand a piece of rock formed, how many years ago?, in a spot where it has never before been seen by humans. When I ran my thumb over the wet rock, some “mud” appeared. No grains were visible, which left out sandstone, though I already figured it wasn't that. They were extremely fine, essentially undefinable grains.
I put pressure on the rock and it broke easily, revealing layers along the broken edges. “Friable” was the word that came from my past brain. This rock is very friable, meaning it breaks apart easily.
But, what is it? Shale? Siltstone? Claystone? Mudstone?
I looked up descriptions on geologyscience.com and ruled out mudstone, which is, of course, hardened mud. It is a mixture of silt- and clay-sized particles (too small to see), which would fit with this rock. But, mudstone breaks off in blocky pieces, not flat ones. Claystone has clay-sized particles and is “lithified, non-fissile mudrock”. “Lithified” just means sediments that were transformed into stone. But that “non-fissile” part ruled out Claystone. This rock is definitely “fissile”, meaning it splits easily (not just simply breaking easily). Fissile. Another word in my past brain.
Now I was down to Siltstone and Shale. Which is it? In Siltstone, “more than half of the composition is silt-sized particles.” Well, that sounds kind of general. I found more information in Geology.com: “It forms where water, wind, or ice deposits silt, and the silt is then compacted and cemented into a rock.” “It represents a level of current, wave, or wind energy between where sand and mud accumulated.” Well, that sounds possible, given what I've learned about formations in our area. “Sedimentary structures such as layering, cross-bedding, ripple marks, erosional contacts, and fossils provide evidence of these environments.”
Whoa! Look at the ripple marks on this rock!
We are seeing evidence of the action of water on sediments – how many years ago??
But it also says “Siltstone is much less common than sandstone and shale. The rock units are usually thinner and less extensive. Only rarely is one notable enough to merit a stratigraphic name.”
Huh. Well, I have no idea how thick this bedrock layer is, since this top part of it has been only recently revealed. Is it a “notable” layer, with a name? Or too thin to merit one? Or is it even Siltstone?
What about Shale?
From geologyscience.com I find that Shale will “break into thin chips with roughly parallel tops and bottoms.” That's true of my rocks. But, how thin is “thin”? I have so many questions. So much is relative, and the items that are very specific (like measurements of particle size) are things I don't have the resources to determine.
They also say, “Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.”
This certainly sounds like my rock, too.
So, is my bedrock the “much less common” Siltstone? Or is it the “most common” Shale?
It goes without saying that I'd like for my bedrock to be less common and more special. But, I'm stumped at this point (my past brain doesn't go quite far enough), so I'll be consulting geology friends and will add an update to this blog post when I find out the truth.
Meanwhile you, dear readers, shall remain on the edge of your seats, waiting to hear how common is my bedrock.
The Pennsylvanian bedrock was lain atop of a Precambrian metamorphic and igneous “basement complex” - in other words the first, original rock formed by fire, upheaval, etc.. The Precambrian is 9,000 to 10,000 feet thick in our part of the country, the oldest rock in Indiana, and was formed billions of years ago.
After the Precambrian Era came the Paleozoic Era, consisting of seven Periods (from oldest to youngest: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian.) During those first Periods, starting with the Cambrian about 570 million years ago, through the Mississippian, ending about 320 million years ago, what is now Indiana, plus much of inland North America, was under the sea! Though levels fluctuated over the years, this sea was usually a shallow one, with sediments (and remains of sea creatures, which became fossils) collecting along the way.
Getting gradually closer to Bedrock Bend …
During the second-to-last Period of the Paleozoic Era, the Pennsylvanian Period, starting about 320 million years ago, what is now Indiana was not only located near the Equator (think about that … ), it also was then above sea level. Our area consisted of large rivers and swamp forests then (and, yes, there are some places here that still seem that way, and in the summertime still feel rather tropical, so you can imagine yourself back in the Pennsylvanian Period … ). During this period, lasting to about 299 million years ago, sediments were lain that became coal, shale, sandstone, and clay. Coal eventually formed from deposits of carbon-rich sediments (by the way, my husband tells me of another creek-gouged place on our property, downstream from Bedrock Bend, where some coal is exposed! Coal has been mined beneath our place and in the general vicinity.)
Well, since the fissile rock I found at Bedrock Bend is not coal, sandstone, or clay, it must be … shale.
The Pennsylvanian Period is divided into three Groups (based on rock features) – from oldest to youngest they are; Raccoon Creek, Carbondale, McLeansboro. In my limited knowledge of this, I'm not sure what Group my place would be in, but I'm inclined to say McLeansboro. This Group is divided into four Formations: Shelburn, Patoka, Bond, and Mattoon. Again, I do not know which one contains Bedrock Bend and my place.
I would love for anyone knowledgeable in local Geology to further educate me and correct any errors, clarify any suppositions.
So, I, and we, have learned some things about the bedrock underlying my place near Verne, Indiana, of which rock is exposed at what I've dubbed Bedrock Bend.
But, getting back to a point made long ago in this blog post … why is so little bedrock exposed in Knox County?
Well, that's because … along came the glaciers, otherwise knows as the Pleistocene Epoch, or Ice Age, which started about 2.6 million years ago re-sculpting the landscape and dumping sediment and other debris during glacial movement and retreat. There is no certainty about how many glacial advances and retreats actually existed in Indiana, but there is clear evidence of two major ice sheets – the Illinoian and Wisconsinan. Before these, valleys were filled and streams diverted by ice sheets starting about 700,000 years ago, as they extended into central Indiana.
The Illinoian ice sheet existed about 300,000 to 132,000 years ago, reaching the furthest into Indiana, but going around the uplands of south central Indiana (which became known as the “Driftless Area” and is well-known for magnificent hills, valleys and rivers.) This means, of course, that the Illinoian ice sheet covered what is now Knox County, scraping it flat and depositing debris. After the retreat of this glacier, the land was exposed for about 125,000 years.
Starting about 50,000 years ago, the Wisconsinan glacial sheet advanced, managing to get south of present-day Indianapolis, so it did not reach our area.
But even the Wisconsinan had a strong effect on our area. As it (and previous ice sheets) melted and retreated, glacial meltwater dashed down valleys, reforming them, and also leaving great deposits of sand and gravel, called outwash. So, Knox County is thickly covered by glacially-deposited outwash, which led to a lucrative sand and gravel industry, and which had to be penetrated to get to coal in the bedrock (later, some wind-borne, or aeolian, silt created rich deposits of loess – some of these deposits formed hills, such as Dicksburg Hills, and the mounds that were later used by Late Woodland people.)
Oh, here is some more vocabulary:
Lacustrine – sediments deposited in a lake or lakes
Fluvial process – erosion or deposition by a stream or river.
Fluvial landform – produced by the action of a stream
Alluvial – deposits left by running water
Over historic time (some pre-historic human time, but mostly historic), humans have affected the shape and condition of the land that was formed by all of these volcanic and uplifting and erosive and depositional and glacial processes. We have pushed and prodded, dug (sometimes very deeply) and lifted, and generally rearranged our landscape. But without our “help”, natural processes have also done their work, mainly through erosion by water and wind, creating varying conditions for different ecosystems and plants, which then make their own changes.
Humans have done some “work” on what is now our place, but it all sits on top of deep layers of sediment that collected and compacted thousands of years ago. And that sits on top of bedrock formed millions of years ago from oceanic and swampy sediments. At our place, the only way to see the bedrock is to dig way down there (as in coal mining) or for the power of water to keep working down and down through the soil and glacial layers, all the way down to those ancient rocks.
And that's what happened to create Bedrock Bend, in the bottom of my little woodland, where the creek kept working away at the side of a hill. Now I can go down there and look at something that was formed about 300 million years ago, and watch frogs in the pool of water at the bend, going through their life cycle in the current year. I can pick up a flat, friable rock, possibly one shaped like Indiana, and hold deep history in my hand, find its fine sediment on my thumb, then bring it up to our house that was built less than 40 years ago, to be on display. I can point to it and say to a guest -
“This is our bedrock.”
May 12, 2024
Some Sources:
Indiana Rocks!: A Guide to Geologic Sites in the Hoosier State; Indiana Geological and Water Survey; Mountain Press Publishing Company, Missoula, Montana; 2018.
Roadside Geology of Indiana; Mark J. Camp and Graham T. Richardson; Mountain Press Publishing Company, Missoula, Montana; 1999.
Bedrock Geologic Map of Southern Half of Knox 30-X-60-Minute Quadrangle Indiana.
IU DataCORE
datacore.iu.edu/concern/data_sets/js956f80d?
Geology.com
Geologyscience.com
October 25, 2023
