A sandstone promontory weathering by spalling slabs of case-hardened surface layers.
This extreme southern Ohio outcrop exposes fluvial sandstone laid down during the Coal Age, Pottsville and Allegheny Groups (near transition), Pennsylvanian System, Lawrence County, Ohio. You can see this outcrop a kilometer off road in Wayne National Forest near N38 42' 59" W082 41' 11".
Case hardened rock surface exfoliating from base of promontory, note thin shadows cast on the belly of the outcrop by dark exfoliation slabs separating from lighter colored sandstone underneath. Removal of slabs may have been assisted by iron ore diggers*.
A remote promontory, Wayne National Forest, Lawrence County, Ohio. A large slump block separated along a pronounced, inclined rock joint is seen at upper right.
We found this cross-bedding sandstone promontory terminating a secondary ridge overlooking Wolcott Hollow. The promontory exhibits sedimentary structures, prominent jointing, and mass wasting by slump blocks and by spalling of exfoliation slabs of case-hardened surface sandstone.
Weathering and groundwater movement has dissolved iron oxide cement beneath the rock surface and redeposited the cement in the outer centimeters of the outcrop surface, resulting in case-hardened surface layers. Loss of cement underneath the case-hardened surface has reduced the bond between the surface layer and the rock mass underneath. Addition of cement in the surface layers increases their volume creating tension between the surface layers and the underlying sandstone. The outer layer is exfoliating, exposing less weathered sandstone.
Porous and permeable sandstone bedrock often preserves detailed structures, coarse and fine, long after the sediment is lithified. Differential erosion, harder structures outlasting softer structures, can emphasize rock structures. The chemical properties of bedrock cement, in this case, iron oxides, result in the variations of hardness as well as colorful appearance. Liesegang bands, alternating color bands and sometimes swirl and ring patterns, result from diffusion-reaction properties of mobile iron oxides and other cementing chemicals.
Cross-bedding interrupted by a cavity.
Liesegang banding, a colorful diffusion-reaction phenomenon common in some massive homogeneous sandstones, here, apparently migrated from origins along sedimentary laminae within a cross-bed set. Clear bands indicate top and bottom of a single bed. A mud dauber nest suggests scale.
*Lawrence County, Ohio is part of the Hanging Rock Iron District of southeast Ohio and Northern Kentucky. The clean removal of case-hardened slabs seen in the image may be the result of ore diggers using long-handled tools to break free iron rich slabs to top off a ox cart load of the Feruginous Ore gathered nearby.
We discover a weathered stylolite zone along outcrop bluffs of sub-Lockport Silurian limestone.
Stylolites in three dimensions, upper, lower, and intermediate dissolution seams bounding interlocking columns between.
39 50'46" 084 19'36"
(Currently closed to visitation. A new natural area restoration is underway here. Formal opening will be scheduled during June 2011.)
Weathered stylolites in this Silurian age limestone outcrop offer a 3-dimensional view of dissolution seams and interlocking prismatic columnar structures exposed along the vertical face of limestone bluffs overlooking the Stillwater River, West Milton, Miami County, Ohio.
Stylolites form after lithification of sediments, a diagenetic process. The original volume of rock is reduced along the seams of stylolites by at least the amplitude of the stylolites. The stylolite zone seen at this Stillwater River location is recessed into the face of bluffs in many places due to loss of chunks of material separated along dissolution seems.
L. Bruce Railsback offers a comprehensive internet atlas of stylolite features and discussion of formation:
"At the macroscopic scale, pressure dissolution between bodies of rock larger than individual grains leads to the development of dissolution seams and stylollites. Dissolution seams are subplanar non-serrate surfaces between two such rock bodies, whereas stylolites are serrate surfaces resulting from mutual interpenetration of two rock bodies, commonly as interdigitate columns. Because formation of dissolution seams and stylolites requires dissolution of bodies of rocks, rather than of individual grains, and requires coherent motion of those bodies toward the seam or stylolite to destroy any resulting pore space to and sustain pressure, lithification of the rock prior to development of seams or stylolites is required. Such lithification may occur through cementation or intergranular compaction."
We discover another interesting landscape boulder near roadside, Calumet Road, Miami County, Ohio. Location: N 39 59' 11" W084 19'23".
We found this shaped, grooved, and striated glacial erratic, a small example of a landform called a rochemoutonnee, plucked from its landscape of origin and rafted far south of the Canadian Shield where these teardrop-shaped glacier erosion features sculpt the surface of the ground. Evening's low sun shadows shallow linear groves and pitted chatter-marks cutting texture into the smooth surface of the granite boulder.
The grooves and striations follow the smoothly arching abrasion-shaped surface of the boulder. The large grooves and the deep striae are parallel. Lighter striations cross primary texture at obtuse angles. All roughly follow the length of this long glacial erratic, a chunk of granite rafted by the Wisconsin Ice Sheet. The upstream end narrows, the downstream end is wide. Both ends are shaped by glacier erosion. Roche moutonnees terminate in blocky ice-plucked ends like the one pictured.
This boulder tells a rock-story.
A rochemoutonnee (sheepback) is a landform found where the Wisconsin Ice Sheet eroded crystalline bedrock in situ (in place), shaping minor bedrock prominences into smooth whaleback forms with truncated downstream ends. Early descriptions named them for their resemblance to a sheep's back or to mutton-oiled wigs popular in the day.
This discovery of a rochemoutonnee made into an erratic is unique, geographically. The boulder is a landscaping feature at the entry to a private nature preserve in Miami County, Ohio, far south of the Canadian Shield. Today, the boulder rests in position with ice-sculpted side up as it formed in nature. Following formation in place by flowing glacier ice carrying abrasive rocks and sand, the shaped boulder was plucked by the south-flowing ice and rafted to its present position--well, nearby. The boulder has been repositioned from a field nearby to its feature position in landscaping bordering the entry to River Ridge Nature Preserve, The Lange Estate, West Milton, Ohio.
Ice sheets scoured the high spots and filled-in the low spots throughout the northwestern two-thirds of Ohio and much of the Midwest, North America's glaciated flat-lands. Glacial geology lessons abound throughout the landscapes of Ohio's glaciated counties, and beyond; but we rarely find well preserved examples of eroded bedrock like this striated boulder recently unearthed during back-fill operations for the new Nature Lodge under construction.
This rough boulder is a gem of a find, a 'diamond-in-the-rough' reflecting a sub-glacial erosional process that shaped Ohio and the Midwest.
A limestone boulder, plucked by glacier ice from the glacially scoured surface of the Middle Devonian age Columbus Limestone formation, shows angular fractures surrounding a striated facet. Much of the surface of Ohio's buried limestone bedrock is found flattened, striated, grooved, and chatter-marked when glacial deposits are removed, exposing the buried bedrock surface. This boulder offers a rare glimpse of the buried bedrock surface.
The flattish facet surfacing the boulder pictured above was chiseled by debris 'tools' once frozen in a basal ice-debris matrix, the 'dirty' ice at the base of the moving Wisconsin-age Laurentide Ice Sheet.
Resistant bedrock surfaces were scoured and gouged by debris held in glacier ice for tens of thousands of years underneath the Laurentide Ice Sheet that spread across northern North America. In central Ohio, the erosion process continued until at least twenty-thousand years ago.
The sharp fractures surrounding the boulder's facet suggest the boulder did not travel far after it was faceted, and then plucked from the bedrock surface. The striations would have been smoothed, and the sharp edges of the fractures would have been rounded by long dynamic transport within the ice or by transport in flowing melt water.
Detail of striations. Parallel striations were cut when the boulder was part of the bedrock surface the glacier slid over. Hard rocks, frozen in the basal ice-debris matrix, scraped the soft limestone bedrock like coarse sandpaper on wood. A second group of deep gouges, crossing the first, may have resulted when the bedrock block was rotated and plucked from its original position at the ice-rock interface (the bedrock surface) to begin its transport within the ice from its nearby origin to the site of the new nature lodge at Deer Haven.
The boulder was unearthed from clayey till, a ground moraine left in central Delaware County, Ohio by the most recent retreating ice sheet. Till is a mix of fine and coarse grained sediments kneaded together by pushing-grinding ice flow, and released from melting ice without the sorting action of flowing water.
During the 1.8 million years of the Pleistocene Epoch (beginning 1.806 million years ago*), much of the Midwest was buried by ice sheets for most of the time. We live in the Ice Age still, though we formally end the Pleistocene Epoch at 11,700 years ago.
Geology in a basement excavation in the Central Ohio Clayey Till Plain, Delaware County, Ohio.
Location: 40.24N 83.07W
(excavation now back-filled, construction site off-limits)
Geologist's love holes in the ground. Excavations for basements expose a brief glimpse beneath the surface to see soil profiles, sediments, and more. A muddy hole in the ground brings subsoil secrets to light.
I volunteered to gather photographs and specimens at the site of a new nature center under construction for eventual use in an, "Under Your Feet" educational program, following a morning walk discovering birds with the Ohio Young Birders Club, Delaware Chapter.
Basement excavation and poured walls of a new nature center under construction at Deer Haven Preserve, Preservation Parks of Delaware County, Ohio. This fourteen-feet deep excavation offered a rare glimpse under the surface of a typical muddy farm field in central Ohio slated for restoration to wetlands, forest and meadow.
Glacial till in central Ohio is composed of a lot of clay-size particles with some sand, gravel, and larger stones mixed-up in the clay and exhibiting little or no sorting normally seen when sediments are laid down by running water. The stones are matrix supported, floating in the mud.The till at this site is at least fourteen-feet deep. There is no sign of bedrock exposed in the bottom of the hole.
Stones in till often have many different lithologies (recipes) and origins (where the rock was made). Each grain, whether a tiny clay particle or a large granite boulder, was picked up somewhere to the northward by south-creeping glacier ice and transported to its resting place in the till by the motion and melting of glacier ice.
Views beneath the surface of a farm field in the Central Ohio Clayey Till Plain. Rounded cobbles of limestone and other rocks are locked in a matrix of heavy clay. Rounded rocks have spent time in running water (nature's rock-tumbler) before getting mixed into the clayey till.
A powdery-white residual silt and mineral precipitates coat a deep thin open till fracture side-wall in this excavated ground moraine. Clay has been flushed from the crevice by infiltrating water. The opposing side-wall fell away into the open pit during excavation.
Remains of old branching rootlets are seen in the lower image. These rootlets of long-gone trees penetrated along the open fracture between eight and ten feet below the surface, following the soil's natural plumbing.
One side-wall of the excavation separated and fell away along the uneven plane of the natural soil fracture pictured above, exposing the till's natural plumbing system. Water drainage through open interconnected fractures in till is an important natural process throughout glaciated areas of the Midwest, and worldwide. Physical and chemical changes in the till sediments due to surface rebound following unloading of glacier ice and weathering through time cause broad area surface sediment (till) stretching and local till shrinkage (drying out and thawing from freeze). Tension across the till layer opens natural soil fractures.
Interconnected deep fractures through the till layers function as a natural plumbing system aiding water drainage. Infiltration through secondary porosity (through the fractures) can operate several orders of magnitude faster than infiltration through primary porosity (between grains of till).
Fracture porosity, the existence of interconnected open fractures in glacial till, is an important modern focus of research for geotechnical scientists and engineers. Till used to be considered a good subsurface material for limiting downward movement of chemicals and leachates. Small amounts of clayey till tested in the laboratory are found to be very tight, greatly limiting seepage downward. Recent experiments with carefully collected large samples of soil test differently, they drain quickly through natural cracks in the clay--through natural fracture porosity. Field tests support the findings (Weatherington-Rice, J. et. al., 2000).
Resources:
Weatherington-Rice, J. et al 2000. Ohio's Fractured Environment: Introduction to The Ohio Journal of Science's Special Issue on Fractures in Ohio's Glacial Tills. Ohio Journal of Science,100 (3/4):36-38, 2000.
See large Ohio Shale carbonate concretions displayed along the roadside at a Galena area family stone company.
Location: 40.2630N 82.9375W
Twins are common among Ohio Shale concretions. Narrow zones in the lower Huron Member are often crowded with large and small concretions.
Large concretions line the frontage and decorate the exhibit grounds of Galena, Ohio area Hill Stone Company along 3B's & K Road just south of Route 36/37 (I71 Exit 131, just west of Cracker Barrel). The concretions will not be offered for sale anytime soon, they say.
The concretions seen here were excavated during highway construction in 2007 at the Route 315 interchange with I270 on the north side of Columbus, Ohio (40.11N 83.03W), then moved to the stone company premises in 2008.
Ohio shale concretions have captured public interest and sparked scientific debate about origins since they were first described during the First Geological Survey of Ohio, 1837-1838 by geologist, John Locke.
I was introduced to concretions as "fossil cannonballs" by my educator Grandfather, with a chuckle of course. He kept a nearly round ten-inch specimen with nice rusty-brown patina in his garden in Marion, Ohio all of the years I remember there. I regret the specimen is no longer with my family.
I've continued the family tradition with a nice oblate-spheroid concretion featured in my garden. It's from the same concretion zone in the Huron Member of the Ohio Shale north of Columbus from which the big ones pictured above were excavated recently. I collected the specimen in the photo below during the late 1970's from earlier highway ramp construction excavations into shale bedrock at the same interchange (R315/I270) .
Concretion as versatile garden feature seasonally serves as perch or pedestal for a birdbath.
A concretion as found at Camp Lazarus near Seymore Woods State Nature Preserve, Lewis Center, Delaware County, Ohio has eroded from nearby shale and rests a few feet away in a ravine bottom.
Occurrence of concretions...
My concretion formed under ancient Devonian sea water, as far as the eye could see, and deep. Our Devonian-age sea covered the whole of today's Ohio and much of North America's mid-continent. It was around 380 million years ago when our local Huron Member concretions were formed in deep organic sea-bottom ooze. Ohio was tropical then, located a little below the equator. The conditions for formation of concretions occurred rarely. Today, large concretions occur in limited narrow zones within the Huron Member of the Ohio Shale, not throughout formation.
Unique Devonian times...
The Upper Devonian between about 385 million years ago and 360 million years ago was a boundary time for life on earth. Terrestrial plants had developed throughout the Early and Middle Devonian Epochs from simple water-margin forms to complex tree-size forms able to vegetate the uplands. This singular event led to the global development of soils. Plants break down rocks and regolith mechanically and chemically, and add organic material, greatly increasing the breakdown of rocks.
Global vegetation and soil cover greatly increased weathering of rocks and nutrient loads in rivers and in seas. Some investigators are convinced the organic Ohio Shale and similar shales elsewhere are a result of huge algal blooms and general anoxia in oceans, especially shallow seas like ours, resulting from the success of plants on land. Global vegetation drew down CO2 levels in the atmosphere, cooled the planet, and brought about the second* Phanerozoic ice age and was precursor to the Ice House Climate** of the Coal Age!
Today, the Ohio Shale is thirty percent organic by volume due to accumulated tiny plankton organisms which were not fully decomposed in the anaerobic depths. Snap a piece of fresh black Ohio Shale today, and you will smell the organic echo of life formed under a young Sun long ago.
The short explanation for concretion formation...
A dead fish or chunk of fish flesh (occasionally a waterlogged chunk of wood) sank to the sea bottom, landing in the soft organic ooze. The slowly decomposing flesh gave off ammonia, urea, and other organic chemicals which formed a halo of high pH in the sediments surrounding the decomposing flesh. The pH gradient drew in calcium carbonate from within the ooze and through the ooze from nearby sea bottom waters which continually replenished precipitating minerals. Anaerobic bacterial no doubt formed an important link in the chemical chain by reducing sulfates to sulfides in the water. The mineral-rich water precipitated carbonates and iron oxides. Calcite dominated from the center of the growing concretion outward. Iron-rich dolomite and siderite replaced calcite from the outside inward, but rarely all the way to the core. The concretion grew until the accumulating ooze buried the sphere too deeply to allow continued mineral replenishment.
More detailed explanation (borrowing some from Hansen, 1994)...
The Devonian is often called, "The Age of Fishes" in elementary texts. Giant armored fish, the Placoderms "ruled" the oceanic food chain. They even preyed on ancient sharks then beginning to diversify late in the Upper Devonian. Placoderms were fleshy and cartilaginous like sharks, but with bony plates protecting their heads and arming their jaws. Dead sharks and placoderms not immediately consumed in the water column probably bloated and floated under hot tropical sunshine until their disintegrating carcass fell apart. Fleshy clumps of cartilaginous shark, and placoderm flesh attached to bony plates (or waterlogged wood), sank into the depths and came to rest in the water-saturated organic bottom ooze. A concretion began to form immediately.
There was little available oxygen in the depths of our Devonian sea. It was shallow by oceanic standards, and its bottom waters were stagnant. The continental basin holding the sea was closed off around much of its perimeter restricting formation of deep currents so waters didn't get mixed-up much. Decomposition was hindered more and more with increasing depth.
A series of chemical processes precipitated concretions within the anaerobic ooze of the Devonian sea floor. While the chunk of fish-flesh or wood became buried by ooze, organic chemicals; ammonia, urea, and so on, formed through decomposition, resulted in a high pH halo around the flesh or wood which precipitated calcium carbonate deposition. Later, Ca-carbonate was replaced outside the core by iron-rich dolomite and siderite. Again, anaerobic bacterial certainly played a role (wanted: descriptive chemical reference to bacterial role in Ohio Shale concretion formation).
An alternative (or conjunctive) process suggested by R.E. Criss, G.A. Cooke, and S.D. Day (1988) of the U. S. Geological Survey proposes the formation of a sphere of low-density soapy organic wax-like material dubbed, adipocere (grave wax), which formed a halo of organics surrounding the wood or fish flesh soon after it settled in the ooze. Adipocere formed and held the spherical concretion-shape during continued burial deep in ooze while mineral precipitates replaced the organics. Calcium carbonate precipitated from the center outward without fully displacing ooze sediments suspended in the halo of adipocere. Barth (1975) found intact pollen within concretions but flattened pollen in adjacent shale outside of the concretions. Slowly, calcium carbonate in the forming concretion matrix was replaced by Fe-rich dolomite and siderite.
Slowly, as the organic sphere was buried deeply by accumulating sediments, it was converted to a mineral concretion centered where the wood or flesh had decomposed, or on the remaining bony plate of a placoderm (bony plates are sometimes, but not always found at the center of Ohio Shale concretions). As burial continued, water was squeezed out of the ooze. Layers of sediments compressed into shale surrounding the concretion. The shale layers bent, wrapping around the solid concretion, top and bottom "like a marble pressed within the pages of a book." (Barth, 1975).
Many large concretions appear slightly compressed suggesting they were semi-solid as the shale compressed around them. Some investigators attribute the flattened shape of large concretions, and the conical indentation at tops and bottoms of large concretions, to recrystallization and shrinkage of the core while the outward sphere was still partly soft. Many concretions, especially concretion cores, exhibit fracture patterns suggesting shrinkage after mineralization. If deep burial below available mineral replenishment resulted before full replacement of organics by minerals, continued shrinkage would result in fracture. Many concretions are septarian. Secondary minerals have filled the cracks.
Large carbonate concretions are found in narrow zones in the Huron Member, usually in the lowermost fifty-feet or so of the Upper Devonian-age Ohio Shale formation. Smaller concretions are found higher in the rock column in the Cleveland Shale Member. Conditions had to be just right or concretions did not form.
Another streambed concretion (about nine feet across) typical of many found along shale ravines draining into the Olentangy River in northern Franklin and southern Delaware Counties, Ohio. Conical indentations, like the ones seen in two photos above, are found at the tops and bottoms of many very large concretions (localities pictured are are on private properties, not accessible).
Ohio shale concretions have captured the interest geologists and fueled nearly two centuries of debate about origins since they were first illustrated during the First Geological Survey of Ohio, 1837-1838 by geologist, John Locke.
The "Lundushelmontii" of Adams County, Ohio illustrated by John Locke. Illustration borrowed from Ohio Geology Newsletter (Hansen, 1994).
Septaria, the "Lundushelmontii" illustrated by John Locke (Locke, 1838) in the First Geological Survey of Ohio are fairly common among Huron Member concretions. Various septaria are known from varied locations worldwide, though usually very much smaller than the septaria illustrated by Locke. Smaller septaria were commonly called "fossil turtle shells" or "turtle rocks" long ago. A septarium results when well-pattered shrinkage cracks fracture a concretion into polyhedral blocks. Secondary minerals sometimes fill the cracks cementing the pieces together.
Joseph Vasichko's website offers a detailed catalog with nice images of many minerals associated with the Ohio Shale and its concretions.
"Huge concretion, known as "Huron River Boulder" fallen from the "Huron" shale. Huron River below Norwalk."
This image C.1910 is slide number 1006 in the Jesse Earl Hyde Collection. The "Huron" shale is the lowest member of the Devonian-age Ohio Shale geological formation.
Image and caption used with permission: The Jesse Earl Hyde Collection, Case Western Reserve University (CWRU) Department of Geological Sciences (http://geology.cwru.edu/~huwig/).
*Assumes the first Phanerozoic ice age was the terminal event of the Ordovician (60%+ extinctions of marine fauna).
**Ironically, current research suggests the Coal Age (the Carboniferous Period, our Mississippian and Pennsylvanian Epochs) with its characteristic cyclic sedimentation with deposition of coal seams due to huge accumulations of plants growing in steamy backwaters (cyclothems) occurred through a deep stretch of time during which sea level rose and fell along the broad shallow shores of continents gathered near the equator due to the repeated expansion and melting of global ice sheets.
Resources:
Michael Hansen summarized the knowledge of Ohio Shale concretion formation and context of occurrence in the Ohio Geology newsletter, Fall 1994.
Barth, V.D., 1975. Formation of concretions occurring in the Ohio shales of the Olentangy River: Ohio Journal of Science, v. 75, no. 3.
Carlson E.H., 1991, Minerals of Ohio: Ohio Division of Geological Survey Bulletin 69.
Case Western Reserve University (CWRU) Department of Geological Sciences, The Jesse Earl Hyde Collection. Locke, J., 1838, Geological Report, Southwestern district: Ohio Division of Geological Survey, Second Annual Report. Criss, R.E., Cooke, G.A., and Day, S.D., 1988, An organic origin for the carbonate concretions of the Ohio Shale: U.S. Geological Survey Bulletin 1836, 21p.
Hansen, M.C., 1994 Concretions: The "LudusHelmondii" of The Ohio Shale. Ohio Geology newsletter, Fall 94.
See lake shore shale cliffs and an erosion exposed preglacial ravine filled with glacial sediments. Glacial till and sand layers filled the ancient small ravine as the Laurentide Ice Sheet advanced through central Ohio during the Pleistocene Epoch.
Cliffs of Ohio Shale eroding under lapping waves along the banks of Alum Creek Reservoir.
These shale cliffs are visible from the Route 36 bridge over Alum Creek Reservoir, Alum Creek State Park, Delaware County, Ohio. The Devonian Ohio Shale bedrock formation forms the cliffs seen in several directions from the bridge. The inlet exposing a channel filled with glacial deposits is visible along the east shoreline in the distance from the parking area at the southwest end of the bridge. The deposits can be observed closely on foot from the parking area at the southeast end of bridge or by boat (my favorite approach is by kayak).
Views of the contact between Pleistocene glacial deposits and a buried Ohio Shale erosion surface. The contact between Devonian Ohio Shale (below keys) and Pleistocene glacial till (above keys) is found near eye-level along the bank of Alum Creek Reservoir. The Ohio Shale is 365+ million years old. The glacial deposits were left by the Wisconsin Ice Sheet between 20,000 and 100,000 years ago*.
Glacial till: Rocks locked in mud dampened for better contrast. The center rock is shale oriented flat-side toward the camera. The large rock is crystalline, probably carried by ice from upper Ontario. Texture seen in this till suggests it was squashed and pressed into the small ravine by heavy ice.
A white oak tree's roots cling to Ohio Shale while softer ground moraine erodes out from under them. Soon, this tree will fall into the lake. The geological contact seen in the photos above is located under the tree's root web.
Glacial till is composed of rocks and gravel in a mud matrix deposited by melt-release from ice absent flowing water. Mud (mostly clay with some silt and sand) supports the clasts (rocks) in near-random orientations. Water deposition would have sorted the clasts by grain-size and density. The small inlet pictured above exposes ground moraine interleaved with small deformed sand lenses filling a narrow, steep-walled ravine. A cross-section of the ravine-fill is exposed.
Picturesque Alum Creek Reservoir floods the principle valley and side ravines of Alum Creek in central Ohio. The north-south axis of the valley follows the sub-surface exposure of the Ohio Shale Formation.
Central Ohio bedrock formations are mapped on the ODNR Geological Survey's "Geologic Map and Cross Section of Ohio". Glacial deposits shroud most of central Ohio's bedrock under thin deposits of clayey till or gravel and sand. Highway-cuts, railway-cuts, quarries, shorelines, and natural gorges offer uncommon views of exposed bedrock.
*The age of the channel fill deposits is not dated using calibrated methods. The young relative date range estimate assumes the possibility that the immediate area was deglaciated during middle Wisconsin time. The older date applies if the area remained covered by the Wisconsin Ice Sheet throughout Early, Middle, and Late Wisconsin time.
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Banner photo: Rock shelter ceiling, Pennsylvanian Epoch fluvial sandstone with calamites (extinct tree-sized woody relatives of horsetail). Buckhorn Township, Lawrence County, Ohio
