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6. There is no better place than Illinois to illustrate the interaction between flowing water and landscape. If in fact Illinois is no longer a prairie state (approximately .03% of its original grassland survives), it nevertheless remains a river state. And this is a typical Illinois river scene: midsummer along the Rock River, at Castle Rock State Park, in Ogle County. Note the outcrop of St. Peter Sandstone in the right middle ground. This Ordovician-Period formation constitutes an immense blanket of pure-quartz sandstone over the middle section of North America. The frosted, well-rounded sand grains in this rock have had a very long journey: it's believed they have been recycled between loose sand and sandstone at least several times, over what may be billions of years.
31. Not the Mississippi this time, but a tributary of a tributary of one of its tributaries. Here Rock Creek has cut a small canyon through tough Silurian dolostone to reach the level of its master stream. A large slump block, fallen of the canyon's vertical cliffs, is visible on the left. When is rock like this, which certainly looks like limestone, dolostone instead? -- When it contains, in addition to the calcium carbonate characteristic of limestone, at least some magnesium. The term dolomite, still often used as a synonym of dolostone, is best restricted to the carbonate mineral that contains both calcium and magnesium. In other words, dolomite is the mineral; dolostone is the rock type that is made of dolomite.
60. This pretty waterfall, named with simple elegance Falling Spring, is situated on the east bluff of the Mississippi River, in the small town of North Dupo, in St. Clair County. The rock layers here date from the geologic period (360-320 million years ago) that takes its name from the river. The Mississippian was a time of very extensive limestone deposition in North America. One practical way to tell limestone apart from dolostone is to put a drop of diluted hydrochloric (muriatic) acid on the rock sample in question. If it's limestone, it will fizz dramatically. If it's dolostone instead, it will react much more sluggishly.
51. Tough stuff. The Bailey Limestone at Inspiration Point. This rock is full of highly resistant chert, which makes a good material for staging a long delaying action against the forces of erosion. Formed in the Devonian Period (408-360 million years ago), this formation crops out along the eastern bluff of the Mississippi Valley south of Carbondale and forms one of the most scenic stretches along the state's Great River Road. .
53. It may not be common knowlege, but Illinois is a land of many scenic canyons of great geological and biological significance. This is one of the best -- the National Forest's Bell Smith Springs, in Pope County. Here a pyramidal slump block of Pennsylvanian-Period Pounds Sandstone has come to rest in the waters of Bay Creek. This site also features the state's longest natural bridge, superb ripple-mark exposures, rare flora and fauna, and excellent spiritual dividends for the hiker.
24. Sunrise over the Ohio River at Shetlerville. This mighty waterway served as the primary transportation system for a nation moving relentlessly west in the eighteenth and nineteenth centuries. Individuals and families who came to live in Illinois usually went down the Ohio to Cairo, and then up the Mississippi. In contrast, the extensive wet-lands of northwestern Indiana tended to discourage migration into northeastern Illinois. Consequently, the western part of the Prairie State was the first to be extensively settled.
2. This scene shows one locale in the rocky and hilly Driftless Section of northwestern Illinois. In the past two to three million years, continental glaciers have profoundly changed preexisting drainage patterns, even in places the glaciers haven't reached. Approximately 200,000 years ago, the far-reaching Illinois ice sheet halted about two miles short of this spot, which is now the heart of this peaceful park. However, the glacier blocked the flow of the Apple River farther down the valley, and consequently the stream backed up until it spilled over a ridge and carved itself a new canyon through solid rock. In the process, the Apple, originally a tributary of the Rock River to the southeast, became a tributary of the Mississippi, to the southwest.
41. One of the best places in northern Illinois to see interesting structural geology is in this park, just a short drive south from Starved Rock. The site straddles the crest of the La Salle Anticlinorium, a major upward folding in the Earth's crust. When the Vermilion River is low, the outcropping Ordovician dolostone on the west side of the anticlinorium is clearly visible outcropping in the stream bed. Here it dips downward steeply to the west (see Photo 11, below). The younger and higher Pennsylvanian strata, seen in the distance, dip westward, too, but at a much less pronounced angle (see photo 12, below). The orientation of these two rock units suggests that there were at least two episodes when the crust in this area was compressed -- probably in response to the collision of eastern North America and northwestern Africa some 300 million years ago.
44. Looking south down America's greatest river. Here the Mississippi must squeeze its way through a relatively narrow channel defined by the highly resistant bluff of Silurian dolostone. Just a few miles downstream, its valley widens dramatically, where the uppermost bedrock are the easily eroded Maquoketa Group shales instead. This photo was taken one grim spring morning, when the river was at flood stage. Note that the closest midchannel island is under water--and is visible only because of its trees.
30. A little farther upstream on the Mississippi. While the Illinois, Jo Daviess County side of the valley can be seen in the right background, this photo was in fact taken facing north from the scenic blufftop overlook at Bellevue State Park, Iowa. The handsome and aptly named river town of Bellevue is visible at left; Lock and Dam Number 12, one example of extensive human meddling with the river, is in the center. The river valley may seem wide here, but it isn't. The floodplain is a paltry mile and a half wide; in the St. Louis area, it is up to five times that size.
12. Because of the thick mantle of glacial drift, there are not many places in northeasten Illinois to see bedrock exposed at the surface. Consequently, commercial quar-ries are much appreciated by geologists and fossil-hunters alike. Here, excavation at the Elmer Larson Quarry in the town of Sycamore reveals Silurian dolostone and the Ordovician Maquoketa Group that lies under it.
13. A closeup of the Silurian-Ordovician contact at the Larson Quarry. The blue-tinted upper portion of the Maquoketa Shale owes its coloration to volcanic ash from eruptions of the Taconian mountain-building event. That episode took place about half a billion years ago, several hundreds of miles to the east, in what is now westernmost New England and eastern New York State. The buff-colored stone above is the Silurian-Period Kankakee Formation dolostone. Detached bits of the Kankakee dolostone have also fallen down in front of the Maquoketa, to partially obscure it.
9. The justly famous Thornton Quarry, just south of Chicago, is one of the largest open-air facilities of its kind in the world. For decades, the excavation at this site has revealed the inner structure of a coral reef that flourished in the warm Silurian seas about 420 million years ago. At that time, this part of North America was situated about 20 degrees south of the equator. The brown staining on the wall is a colony of algae that thrives where subsurface water emerges and trickles down the quarry face.
10. Students from one of my geology courses hunt for fossils in the lowest section of the Thornton Quarry, under the reef structure. Among the fossils found at this level are graptolites -- colonial marine animals that may have some distant affinity with vertebrates. At upper center, just above the opening in the quarry wall, is the overpass that carries the heavy traffic of Interstate 80 through the quarry complex.
56. Another popular spot: the Garden of the Gods, in Saline County. These naturally sculpted shapes are outcrops of the Pounds Sandstone. They stand at the crest of the Caseyville Cuesta, a steep-faced ridge that runs east and west across much of southernmost Illinois.
57. A second Garden of the Gods view. The Pounds Sandstone is a well-cemented sedimentary rock that was originally laid down by a river system flowing from north-east to southwest. Notice the joints in the exposed rock. These are fractures where no significant displacement has occurred on either side.
1. Not all preserved sediments eventually turn to stone. The white, iron-stained sand that makes up most of this borrow-pit face near Fayville in far-southern Illinois belongs to the Cretaceous-Period McNairy Formation. Laid down as part of a river delta in the last part of the age of the dinosaurs, this deposit has remained loose, uncemented sand for over 65 million years. The uppermost layer of buff-colored sediment is not glacial drift -- no Pleistocene glacier made it this far south -- but Tertiary-Period Mounds Gravel. It is unconsolidated, too. An Alfisol forest soil has developed on top of it.
23. The Mississippian Period is best known for its widespread limestone deposits, but there are places in southern Illinois that other types of sedimentary rocks of the Mississippian can be found. This is the massive Bethel Sandstone, exposed on the precipitous Ohio River bluff at Shetlerville, on the Hardin County-Pope County line. This spot is a favorite of a large flock of turkey vultures that circle about in the early morning, waiting to catch updrafts along the cliff face.
15. Also at the Larson Quarry. Here silt and clay derived from the Maquoketa Shale resumes its role as loose sediment after half a billion years. This spot was recently a small pond. As the water evaporated and the sediment dried, polygonal mudcracks formed. Patterns such as these are sometimes buried and turned to stone.
8. The entrance to Cave in Rock, in the state park of the same name. This karst cavern in the Mississippian St. Louis Limestone once extented farther to the south, but the swirling waters of the Ohio River, just a few feet to the right of entrance, have truncated it.
16. In the past two million years, approximately 85% of Illinois has been covered by glaciers at one time or another. The state's southern tip was one of the places that wasn't. Such flamboyant forms as this turret of Penn-sylvanian Caseyville Sandstone in the park have not been buried in glacial drift or swept away by the ice.
18. The glistening face of the Herrin Coal, 600 feet under-ground at West Frankfort's National Coal Museum. This dank and utterly realistic facility, formerly the Old Ben No. 25 Mine, is the place to see real rather than simulated coal-mining conditions. The seam shown here is bitum-inous, and its high sulfur content eventually led to mine's demise as a profit-making operation. Unfortunately, the Museum version of the mine is currently closed, too.
47. More for your money. At Pere Marquette, Illinois' largest state park, the blufftop view offers not one world-class river, but two. In the middle, just visible through the treetops, is the Illinois River. Just beyond it, and separated only by a narrow strip of land, is the Mississippi. The Illinois finally merges into the bigger stream a few miles to the left (south).
37. Mass wasting can sometimes be a slow and subtle process. This steep slope, also at McCormick Ravine, is experiencing soil creep. The surface material is inching down the slope at so leisurely a pace that trees can compensate by curving their trunks to remain upright.
36. Upstream at McCormick's Ravine. Geologists use the term mass wasting to refer to movement of materials down a slope. As these exposed tree roots in the ravine show, erosion on steeply inclined surfaces can be severe.
54. An exposure of Pounds Sandstone on the wall behind the Natural Bridge, Bell Smith Springs. The inclined bed-ding planes, called cross bedding, indicate the flow direction (here left to right) of the stream that deposited these sediments 310 million years ago.
52. The long-range view from Inspiration Point. In the foreground lies LaRue Swamp, in a former meander of the Big Muddy River. In the distance, beyond the temporarliy flooded farm fields, lies the modern channel of the Mississippi and the Missouri bluff.
55. Rock shelters at Bell Smith Springs. These shallow caves in the otherwise vertical canyon walls have been slowly scooped out by natural processes. Water from the top of the cliff has seeped or poured downward through recessed cracks in the Pounds Sandstone, only to hit the impermeable Drury Shale (the gray, lower layer of rock in the right foreground) . At that point, the water has been forced to flow out horizontally, thus undermining the base of the sandstone.
58. The shadows of tree limbs play against the corrugated surface of the Pounds Sandstone, at the Garden of the Gods. The weird, distended bull's-eye patterns are Liesegang rings, formed by varying concentrations of resistant iron minerals precipitated in this porous stone.
50. A handsome gateway to the National Forest. The Pine Hills Escarpment --the eastern, Illinois bluff of the Mississippi River -- by Union County's La Rue Swamp. The top of the bluff, directly ahead, ia aptly called Inspiration Point. On this cliff the older surfaces of the Bailey Limestone have weathered to a battleship gray. Weathering products often obscure the true color and texture of a rock formation. That's why geologists have the compulsive need to bang on outcrops with rock hammers: they break open rocks to reveal a fresh, unweathered face.
59. French Canyon, the most visited of the eighteen canyons of Starved Rock State Park. The bedrock here is the same formation found at Castle Rock State Park (see above) -- the "St. Pete," or St. Peter Formation. The waterfall in this scene is slowly carving through the sandstone to reveal the rock's zones of varying hardness. The park's canyons and cascades are an expression of tributary streams adjusting to the base level established by the nearby Illinois River.
21. This steep-sided ridge in Glacial Park has been interpreted as a delta kame. Know as the Camelback, this intriguing landform came into being approximately fifteen thousand years ago, when the margin of the Wisconsin ice sheet stood at this point. Meltwater streams pouring off the glacier's edge deposited coarse outwash in a heap. When the edge of the ice receded, the Camelback was a long, narrow island in a proglacial lake.
25. Up a lazy river. This is the "mouth" of the Dead River, the largest Illinois stream flowing into Lake Michigan. Its terminus is situated along what is currently an aggra-dational coastline of Lake Michigan. In other words, it's a place where the lake is actively building up the real estate of Illinois, instead of eating it away. For part of the year, the river's mouth is plugged by sand and gravel delivered by the lake's surf and longshore current. As a result, the river jusr sits there, unable to make it the last few yards to the open water. The Dead River is a good model of what the Chicago River, farther down the coast, used to look like, and how it used to behave.
7. The park in a different season. Anyone who thinks Illinois is scenically depauperate should visit this spot. This is the upstream view of the Rock River. Originally, the Rock flowed far to the east of this locale. Try to figure out what powerful force, moving from east to west, completely buried the ancient Rock's valley and shoved it into its present location. The correct answer has nothing to do with aliens, killer asteroids, or dinosaurs.
27. In recent decades, geologists have paid more attention to the crucial role that living organisms play as a geological agents. On this Illinois Beach State Park dune grows marram grass (Ammophila breviligulata) -- a re-markable organism, which, thanks to its symbiotic rela-tionship with microorganisms, can grow in pure sand. Vegetation such as this often holds dunes and even whole shorelines in place. The worldwide rate of erosion has slowed considerably since the late Devonian Period (ca. 385 million years ago) , when plants first succeeded in covering large portions of the dry land.
3. A mixed message, if ever there was one. At the confluence of Coffee Creek (foreground) and the lower Wabash River. When the State of Illinois purchased this land to preserve the priceless old-growth forest it contains, the subsurface rights could not be obtained as well. As a sad consequence, there are shafts dug under the park from a neighboring coal mine, and there are still oil pumps in the midst of the swamp habitat. (The reason this pump is on stilts is pretty obvious if you consider what the Wabash likes to do each spring.) On the larger version of this photo, you should be able to spot a vein of coal sticking out of the creek bank, about two-thirds of the way down to the water.
4. You can' t escape geology; it's everywhere and you'd better get used to it. This boulder in an old gravel pit in Kane County's Blackhawk Forest Preserve may not seem worthy of a photo. But in fact it is, because it shows the effect of weathering -- one of the most profound geologic processes in the entire solar system. After years of exposure to the elements, the surface of the boulder has begun to disintegrate and peel off like the outermost layer of an onion. This type of weathering, exfoliation, is characteristic of granite -- which is what this rock is.
29. A nineteenth-century, one-man lead mine shaft in the Illinois bluff of the Mississippi River. This outcrop of Ordovician-Period Galena-Platteville Dolostone is in the Driftless Section, too, just a few miles south of Galena. That city, at one time a good deal more populous than Chicago, took its name for the lead sulfide ore, once so abundant in this area, that spurred one of the country's first big mining "booms." After the galena was largely played out, miners turned their attention to the sphalerite -- a zinc-sulfide ore -- also present in great quantitites. Mining in this region has now ceased.
45. The same vista in summer. The level of the big river is a little lower, but it is still quite apparent that there is very little room for the railroad tracks and two-lane road below to squeeze through this tight defile.
42. On the bed of the Vermilion. The clinometer of this Brunton compass indicates that the Odovician Platteville Group dolostone dips to the west at about 20 degrees.
43. Here we're actually a little north of the park, up on the bluff above the Vermilion. Here the Pennsylvanian-Period La Salle Limestone dips westward ever so slightly -- at about 2 degrees.
40. A rock in a rock. In Illinois, many erratics (detached rocks that have transported from their place of origin by a glacier) are pressed into service as lawn ornaments. This erratic, on the park's grounds, has been bedecked with a commemorative plaque. Most of the boulder is a light-colored, granitelike rock, but it also contains a large, angular fragment of another, much darker intrusive igneous rock. The latter is a xenolith -- a chunk of preexisting stone that fell into a matrix of invading molten magma without being destroyed by it.
14. A closer look at the strata of the Ordovician Fort Atkinson Formation, a part of the Maquoketa Group exposed at the Larson Quarry. Relatively thick layers of dolostone alternate with thinner layers of crumbly shale. This suggests a cyclical pattern in which the shoreline of an ancient saltwater sea repeatedly advanced and retreated not far from this locale.
11. Along a southern face of Thornton Quarry, not far from the reef core. In general, rock layers (strata) are formed in a horizontal orientation, and they stay that way unless there is a disturbance in the Earth's crust that causes them to tilt, bend, or break. But there seems to be an exception for every rule. These tilting strata were formed that way; they represent sediments that came to rest on the sloping flank of the reef.
32. Coarse glacial outwash at the Fox Ridge Stone Company pit in Oswego. Some of the gravel shown in this picture has already become cemented to larger cobbles -- the first step in unconsolidated sediment turning into the rock type called conglomerate. The cement is calcium carbonate -- lime -- that was caried by water percolating through the outwash from above.
33. Another of Oswego's claims to geological distinction is this outcrop along Waubonsie Creek, close to its con-fluence with the Fox River. This is an excellent exposure of the Ordovician Brainard Shale, at the top of the Maquoketa Group.
34. When the Waubonsie is at low water, the Brainard Shale is most worthy of patient scrutiny. It is highly fossiliferous, and one of the most interesting finds is Tentaculites oswegoensis, the preserved hardpart of a marine animal of uncertain affinity. This fossil looks like a slighty curved wood screw. Once again, note the blue cast to the Brainard, due to its volcanic ash content.
35. The degradational coastline of Lake Michigan, at the foot of Lake Forest's McCormick Ravine. The face of the bluff, actually the exposed interior of the Highland Park Moraine, is being chewed away by the surf, despite the various erosion-control structures that have been placed along the coast in this area. The small streams that cut down throught the glacial drift from the moraine crest to the lake have very steep gradients (slopes) -- often their beds drop 100 feet per mile.
38. Beginning just a mile to the west of the steep ravine streams of the Lake Michigan shore are some of the Midwest's flattest stream valleys. This creek, named rather grandiosely the Skokie River, has a gradient of only about two feet per mile. Such stream valleys to the north and west of Chicago drop so little over such a long stretch that they often trouble moving flood water downstream after a thunderstorm or heavy snowmelt.
39. The Valparaiso Morainal Complex in northern Lake County. End moraines -- long, curving ridges of unsorted glacial rock debris called till -- are the defining element of the rolling terrain that surrounds downtown Chicago' s lake plain. Driving from the Lake Michigan shore westward, one crosses over one moraine after another. End moraines form when there is a delicate standoff between the glacier ice advancing from the rear and the ice melting at the glacier's margin. When conditions are right, the ice margin melts just enough to stay in the same place, even though new ice and rock debris is still arriving. This creates a sort of conveyor belt that keeps dumping till along one line, where it piles up into a broad ridge. The upper surface of that ridge is an uneven, "swell and swale" terrain.
48. The kame is one of the typical glacial landforms of northeastern Illinois. Either a moundlike hill or a short, steep-sided ridge or terrace, it is composed of sand, gravel, or cobbles. This picture shows Prairie Kame. It's an example of the type produced when a deposit of sediments, originally sitting atop or within a stagnant glacier, is exposed when the ice finally melts. (For a look at one important plant species found here, and for a detailed view of the material of which this landform is made, see my Plants of Illinois Gallery, No. 25).
17. On the crest of another kame, in the upscale suburb of Glen Ellyn. This old cemetery is one indicator of its true identity. In earlier times, before the introduction of back-hoes and other earh-moving machinery, gravediggers preferred to ply their trade in kames, dunes, and other places where the subsurface is sandy and easily ex-cavated, rather than in the stony and clayey glacial till of moraines. Anyone who has ever dug a tree pit in the latter material will understand why.
5. This is superb subject matter for the student of what I call pseudogeomorphology (the study of phony landforms). The hill shown certainly looks like a kame and is located in bona fide kame territory, but it's actually a high-rise ex-landfill known as Mount Hoy. It is part of what is now a popular forest preserve. Conscientious geologists must constantly be on the lookout for the devious works of nongeologists who want to confuse us and make us look less intelligent than we obviously are.
22. Actually, kettles are a dime a dozen in northeastern Illinois. Many suppose that Lake County was named after Lake Michigan, but in fact it commemorates the in-numerable kettle ponds that dot the landscape there. In this park there are three kettles, each with a different plant community and ecosystem. This one is a full-fledged bog; its peat mat is covered with the dwarf, acidity-loving shrub called leatherleaf (Chamaedaphne calyculata).
61. Another shot for the annals of pseudogeomorphology. In this case, what looks like a kettle is really a large, abandoned gravel pit, which now goes by the name of Lake Renwick. It's near the town of Plainfeld. Quarries and gravel pits, if not pumped constantly, fill up with water when the excavations go below the water table -- the upper surface of the zone of groundwater saturation.
19. This shot may not seem a masterpiece of the photo-graphic arts, but it does accurately portray the sense of low-ceilinged gloom that confronted miners at the beginning of every work shift. Except that this scene is, believe it or not, too bright. The light bulbs, as well as the guide ropes and overhead netting, were not present when the place was a working longwall mine. For illumination, miners had their helmet lights and, occasionally, the head-lights of vehicles operating underground. When all the lights go out in this place, there is a darkness no sighted surface-dweller can ever experience.
20. This sign, from the final days of the Old Ben No. 25, is sufficiently self-explanatory. But now, at least for the time being, the Fat Lady has sung for the Museum as well.
49. Whether they know it or not, the main responsibility of civil engineers and road construction crews is to per-iodically provide fresh roadcuts for geologists. This cut, in the Little Egypt country of southernmost Illinois, reveals a wide swath of the unconsolidated Tertiary- and Quaternary-Period Mounds Gravel. Note how rills have begun to form on the nonvegetated slope.
26. Looking eastward from what now really is the mouth of the Dead River. For the time being, the sand barrier is breached and the sediment-laden stream pours into the bluer water of Lake Michigan. In unusually wet springs, much of the park preserve's wet prairie is flooded if the river remains blocked. On at least one occasion, park workers and a backhoe have helped by excavating a drainage channel across this narrow band of beach.
28 To walk along a Lake Michigan beach is to stroll through an open-air museum. And this museum has works of art dating back over two billion years. These surf-rounded pebbles and cobbles were transported from tens to hundreds of miles to the north by the latest glacier, then sorted and smoothed by the agitated water of the lake. Among the rock types show here are Archean and Proterozoic igneous rocks from northern Wisconsin and the Canadian Shield (basalt, gabbro, and granite) and Paleozoic sedimentary rocks from eastern Wisconsin and Michigan's Upper Peninsula (sandstone and dolostone).
46. Some glacial landforms have "negative topographic expression" -- a highfalutin way of saying, essentially, that they're holes in the ground. A good example are kettles, depressions caused by the slow melting of blocks of detached glacial ice buried, or partially so, in glacial drift. This hole, Nelson Lake Marsh, is a big one. There is some discussion whether it is a kettle in the classical sense, but for all intents and purposes it can be considered one.
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ALEXANDER COUNTY
BLACKHAWK FOREST PRESERVE (KANE COUNTY)
BEALL WOODS STATE PARK (WABASH COUNTY)
APPLE RIVER CANYON STATE PARK (JO DAVIESS COUNTY)
CASTLE ROCK STATE PARK (OGLE COUNTY)
CAVE IN ROCK STATE PARK (HARDIN COUNTY)
COOK COUNTY
DEKALB COUNTY
DIXON SPRINGS STATE PARK (POPE COUNTY)
DUPAGE COUNTY
FRANKLIN COUNTY
GLACIAL PARK (MCHENRY COUNTY)
HARDIN COUNTY (NOT INCLUDING ITS PARKS)
ILLINOIS BEACH STATE PARK (LAKE COUNTY)
JO DAVIESS COUNTY (NOT INCLUDING ITS PARKS)
KANKAKEE RIVER STATE PARK (KANKAKEE COUNTY)
LAKE COUNTY (NOT INCLUDING ITS PARKS)
LOWDEN STATE PARK (OGLE COUNTY)
MATTHIESSEN STATE PARK (LA SALLE COUNTY)
MISSISSIPPI PALISADES STATE PARK (CARROLL COUNTY)
NELSON LAKE MARSH PRESERVE (KANE COUNTY)
PERE MARQUETTE STATE PARK (JERSEY COUNTY)
KENDALL COUNTY
PULASKI COUNTY
PRAIRIE KAME PRESERVE (KANE COUNTY)
SHAWNEE NATIONAL FOREST (VARIOUS SOUTHERN COUNTIES)
ST. CLAIR COUNTY
WILL COUNTY
STARVED ROCK STATE PARK (LA SALLE COUNTY)
BLACKWELL FOREST PRESERVE (DUPAGE COUNTY)
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