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The Buffalo Rock Area | |
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Credit: The following field trip is adapted from: Nelson, R.S., Malone, D.H., Jacobson, R.J., and Frankie, W.T. Guide to the Geology of Buffalo Rock and Matthiessen State Parks Area, LaSalle County, Illinois. ISGS Field trip guidebook 1996C-1997B. Illinois State Geological Survey: Champaign, Illinois. Objectives: As a result of this field experience and the preparation prior to the trip, students should be able to:
Materials:
Preparation:
Field Trip Basics:
Itinerary: Stop #1: Buffalo Rock State Park Buffalo Rock is a large, steep-sided monolith, rising 100 to 120 feet above the floodplain of the Illinois River. The monolith is elongate (1.2 miles long by 0.3 miles wide) and is oriented parallel to the Illinois River Valley. The top of Buffalo Rock is about the same elevation as the bluffs on either side of the river. Development of the Illinois River and the Origin of Buffalo Rock
Before glaciers advanced into the midwest, the topography and drainage system was much different from today's. Then, there were three main drainage basins in the region. Northeastern Illinois, eastern Wisconsin, and western lower Michigan drained north through the Lake Michigan Lowland. Western Wisconsin, eastern Minnesota, northeastern Iowa, and northwestern Illinois drained south through the Ancestral Mississippi River. Portions of Ohio, Indiana, and central Illinois drained westward through the Teays (Mahomet) River. In Illinois, the Ancestral Mississippi River flowed southeastward from near Cordova to near Hennepin and then southward until it joined the Teays River System in southeastern Tazewell County. The combined river then drained southwestward along the present course of the Lower Illinois River. In La Salle County, the crest of the La Salle Anticlinorium formed a low discontinuous ridge that acted as a local drainage divide. West of this ridge, short, steep streams drained to the west, into the Ancestral Mississippi River. East of the ridge, long streams with gentle gradients flowed south-southeast and drained into the Teays. Although the region was subjected to several pre-Wisconsinan glaciations, these glaciations did little to modify the general drainage patterns. The Late Wisconsinan (Woodfordian) glacial episode caused significant changes in the drainage pattern of the midwest. Major ice lobes advanced westward out of the Lake Erie Lowland and south out of the Lake Michigan Lowland into Illinois. Where these lobes of ice converged, lobate moraines were formed. An early consequence of this glacial advance was the damming of the southward-flowing Ancestral Mississippi River, forming a series of lakes filled by glacial meltwater and streamflow. As each lake basin filled, it overflowed into the next basin downstream. Ultimately, this process established the present course of the Mississippi River between Cordova, Illinois, and Muscatine, Iowa. The back-melting of ice lobes also formed a series of short-lived lakes filled with meltwater trapped between the morainal ridges and the retreating ice front. As each moraine was overtopped and breached by rising meltwater, vast amounts of water were released in a torrent. These torrents rapidly cut new channels and ripped sediments out of some pre-existing channels. The course of the Upper Illinois River (upstream from Hennepin) was formed by a series of these torrents. In La Salle County, the first of these torrents coursing down the Upper Illinois River rapidly cut a valley about one mile wide and a few tens of feet deep. The top of Buffalo Rock (Buffalo Rock Terrace at an elevation of about 540 feet) marks the floor of the initial channel. Later, even more spectacular torrents incised more than 100 feet deeper into bedrock. It is possible that the deeper incision may have been produced by the rapid retreat of a spectacular falls or steep cascade that formed where the torrent was cutting through relatively resistant strata. Waterfalls in Starved Rock State Park and at other locations along the Illinois River may still mark the retreat (headward erosion) of the falls or cascades up tributary streams. Buffalo Rock may have been left as an island during the fall's retreat, much like Goat Island will be left as a monolith in the Niagara River as the American Falls (at Niagara, New York) retreat along the east side of the island and the Horseshoe Falls (Canadian Falls) retreat along the west side of the island. Geology of Buffalo Rock At Buffalo Rock, the Colchester Coal Member of the Carbondale Formation and its underclay rest directly on St. Peter Sandstone. This unconformity represents a time gap in the stratigraphic succession of about 170 million years and erosion (or nondeposition) of more than 1, 100 feet of strata. This thickness estimate is based on the known thickness of strata between the Colchester Coal and the top of the St. Peter Sandstone in a deep well at Lostant, about 14 miles to the southwest, and the 1,250 to 1,300 feet of strata between the Colchester Coal and the St. Peter Sandstone found elsewhere in western Illinois. The Colchester was the first coal to extensively blanket northern and western Illinois. Prior to its deposition, the distribution and thickness of older Pennsylvanian sediments was strongly influenced by the hills and valleys that had been eroded in the pre-Pennsylvanian bedrock surface. St. Peter Sandstone The St. Peter Sandstone is of special interest geologically because of its widespread occurrence and remarkably high purity. The St. Peter occurs throughout an area of the upper Midwest that stretches from northern Michigan to Kentucky and from Kansas to Ohio. The principal areas of its exposure in Illinois are in the Dixon-Oregon area and the Ottawa-La Salle area, where the rocks of Middle Ordovician age are folded and brought to the bedrock surface along the flanks of the Oregon Anticlinorium, the Ashton Arch, and the La Salle Anticlinorium. The St. Peter Sandstone forms high bluffs along the Illinois River (such as at Starved Rock State Park), and Starved Rock itself presents a north facing cliff approximately 125 feet high. Elsewhere in Illinois, the unit has been penetrated in the subsurface by many wells. Except locally where it has been removed by erosion, the formation underlies almost the entire state. The St. Peter is a remarkably pure, fine grained sandstone consisting of well rounded grains of quartz. Under a microscope, many of the grains exhibit a peculiar frosted or dull appearance. At many exposures the rock is light gray to pure white, but commonly it is slightly brownish in color because of staining by iron oxide. The sandstone also exhibits well developed, inclined laminations called crossbedding. The rock exhibits a sugary texture and is typically friable and soft, and is easily disaggregated in the hand. In the subsurface, the St. Peter is sometimes tightly cemented by calcium carbonate, suggesting that its loose texture at the surface has been produced by the removal of cement by weathering and by leaching by percolating groundwater. The origin of the St. Peter Sandstone has interested geologists for decades. An early theory suggested that the sandstone was deposited on land in a vast interior desert of drifting dune sand. Similar crossbedding, roundness, and frosting are found in sands of present-day deserts, such as the Great Sahara Desert of North Africa. The rounding and frosting is caused by the grains' striking and abrading each other as they are moved along by the wind. Today most geologists accept evidence indicating that the St. Peter Sandstone formed in a marine environment. All of the properties of the sandstone can also be explained as the products of wave and current action in a shallow sea. The sand grains that make up the St. Peter were derived principally from the weathering and erosion (reworking) of pre-existing sandstones. The ultimate origin of the quartz grains was probably the Precambrian igneous and metamorphic rocks exposed in south central Canada. The reworked sand grains were transported southward by streams into the Middle Ordovician sea about 480 million years ago. Conditions in the sea remained very stable for a long time, and the sand deposits were extensively reworked by waves and currents that wore away the less resistant mineral grains and winnowed out the finer muddy particles, leaving behind the well sorted, highly resistant quartz sand. As the quartz grains were moved along the sea bottom by currents and washed back and forth by waves, they gradually became rounded.
The contact between the St. Peter Sandstone and the older sedimentary rocks upon which it rests is another major unconformity or erosion surface like the one that underlies the Pennsylvanian rocks. After deposition of the Lower Ordovician strata, crustal movements raised the midcontinent region above sea level, and along the interval of erosion (about 15 million years), accompanied by widespread development of solution features (karst topography), cut deeply into the rocks exposed at the surface. There is evidence that a river system drained across northern Illinois from the northeast and cut deep channels into the bedrock surface. The St. Peter Sandstone has been mined for a long time for silica sand. Silica sand has many uses ranging from sand blasting to making glass. Several active St. Peter Sandstone quarries are in the area, and abandoned St. Peter Sandstone quarries are present on the north side of Buffalo Rock and between Buffalo Rock and the Illinois and Michigan Canal. Old underground St. Peter Sandstone mines are present along the base of the bluffs north of Buffalo Rock. Stop #2: Matthiessen State Park, Dells Area Matthiessen State Park was named for the late Frederick William Matthiessen, a prominent industrialist and philanthropist from La Salle. The land was originally purchased by him near the end of the 19th century and used as a private park for a number of years. Matthiessen employed nearly 50 people to construct trails, bridges, and stairways and to check dams. At that time, the park was called Deer Park, referring to the large deer population in the area.
The original 176-acre park consisted of a long narrow canyon with a small stream flowing through it. The sandstone formations in the canyon (St. Peter Sandstone) were originally called the Dells, and this name has stayed through the history of the park. After Matthiessen passed away, the park was given to the Department of Conservation (now Department of Natural Resources). It was opened as a public park in 1943 and renamed in honor of Mr. Matthiessen. Since then, acquisitions of more areas along the main Dell, some former prairie land, and some forest land to the south have increased the park's size to a total of 1,938 acres. Geology of Matthiessen State Park The many unique and beautiful rock formations exposed in the Upper and Lower Dell areas of the canyon are composed primarily of St. Peter Sandstone. The Upper Dell begins at Deer Park Lake and continues to Cascade Falls, where the canyon descends 45 feet to the Lower Dell. In total, the canyon is about 1 mile long from Deer Park Lake to the Vermilion River. Closer to the Vermilion River, where the Platteville Limestone overlying the St. Peter Sandstone is visible, the rocks are folded, and the strata dip steeply toward the river along the west flank of the La Salle Anticlinorium. At the park and nearby areas, the Platteville Dolomite (Ordovician) rests directly on the St. Peter Sandstone (Ordovician). The interval marked by the erosion surface on the top of the St. Peter Sandstone represents one of several unconformities present in the Paleozoic succession. Pennsylvanian strata are also well exposed along the Vermilion River in the park. That these rock layers dip (tilt) less steeply than the Platteville and St. Peter Formations indicates that the older Ordovician rocks were tilted both during one of the episodes of uplift along the La Salle Anticlinorium before the Pennsylvanian strata were deposited and again when the Pennsylvanian strata were folded. In this area, the unconformity between the Pennsylvanian and Ordovician rocks, marked by the significant difference in their dips, is known as an angular unconformity. Platteville Dolomite The Platteville Dolomite is classified as a group by geologists and consists of five formations, many of which are recognized by only slight differences in subsurface samples. Because the units of the group are difficult to distinguish in surface exposures, we will discuss the Platteville as one rock unit. The Platteville Dolomite underlies all of the region except for a small area in the north-central part. It also occurs in small outliers west and southwest of Ottawa that represent remnants left behind when the surrounding rocks were eroded away. The Platteville Dolomite consists of brown, buff, and gray, finely crystalline, compact dolomite and fossiliferous brown and gray, finely granular (or lithographic) dolomitic limestone that has a distinctive mottled appearance. At several horizons, lenses and nodules of chert are common. The Platteville is 125 to 140 feet thick in the vicinity of Marseilles, but in the immediate area, east of the La Salle Anticlinorium, thicknesses of more than 100 feet are attained only locally. Development of the Dells The Dells area, developed in St. Peter Sandstone, is characterized by box canyons. The ends of the box canyons mark the present positions of waterfalls and rapids that have retreated by headward erosion up a small, unnamed tributary of the Vermilion River. The development of these prominent waterfalls is related to the much larger-scale events that created Buffalo Rock. As the Illinois River was deepening and cutting cascades through the St. Peter Sandstone, the Vermilion River was deepening and cutting through the overlying Pennsylvanian strata and the Platteville Group. The rate of deepening and headward erosion along this unnamed tributary was primarily controlled by the level of the Vermilion River until the tributary had cut down to the St. Peter Sandstone. Falls developed because the sandstone was more resistant to headward erosion than the Platteville Group or the overlying Pennsylvanian rocks.
As rain water percolates downward through the surficial glacial sediments, Pennsylvanian strata, the Platteville Group, and finally into the St. Peter Sandstone, it dissolves a variety of chemicals from the sediments and rocks. By the time the groundwater reaches the St. Peter Sandstone, it is highly charged with iron as well as other chemicals. Bright yellow, brown, or orange strains along the canyon walls mark the locations of seeps and springs, many of them controlled by joints, where the groundwater evaporates and the dissolved iron precipitates. Some dissolved minerals precipitate at these seeps as the soft, light colored powders called efflorescence. Strawberry Rock and Devils Paint Box are two places to see these effects of chemicals in water. Erosion of the St. Peter Sandstone The Dells are an excellent place to examine the various effects of erosion on the St. Peter Sandstone. Potholes are formed where strong stream currents swirl cobbles and pebbles in eddies. As the swirling cobbles and pebbles grind holes in the sandstone, they are ground down themselves. The strong current flushes the sand from the hole. As long as fresh cobbles and pebbles are available and the current can flush fines, the potholes enlarge. The Giant Bath Tub is a good place to see a pothole. Stop #3: Cyclothems in the Carbondale Formation This site is located in the Margery C. Carlson Nature Preserve owned by the Illinois Department of Natural Resources. Groups of more than 25 people must check with the site superintendent at Starved Rock State Park before entering the preserve. Park on the shoulder of East 675th Road and walk down the gravel road (not passable by vehicles) to a concrete block building. Go around the left (west) side of the building and walk to your right (east). Follow the path along the top of the bluff to the site. As you will find, three bedrock outcrops along the cutbank of the Vermilion River are periodically eroded and exposed by the river. Significance of this Site This site is one of the best exposures for the observation of cyclothems
in the northern Illinois Basin. Four complete cyclothems and part of a
fifth are present. As elsewhere in the region, the Pennsylvanian strata
unconformably overlie Ordovician rocks. Dolomites of the Ordovician Galena
and Platteville Groups can be seen in the riverbed, and the unconformity
itself may be exposed locally. Udden (1912) recognized cyclic repetition of strata near Peoria southwest of this locality; he grouped these units into cycles of deposition beginning with a coal at the base of each cycle. Savage (1927) observed the wide distribution of unconformities at the base of some sandstones and proposed separating the Carbondale Formation from underlying rocks along such an unconformity. Weller (1930) adopted the base of the sandstone as the horizon separating adjacent sedimentary cycles. They have been referred to variously as cycles (Udden 1912, Weller 1930), suites (Wanless 1929), and cyclical formations (Wanless 1931, Weller 1931). Finally, Wanless and Weller (1932) proposed the term cyclothem "to designate a series of beds deposited during a single sedimentary cycle of the type that prevailed during the Pennsylvanian Period." Suggested Activities:
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