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The Garden of the Gods Area | |
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Credit: The following field trip is adapted from: Frankie, Wayne T. and Jacobsen, Russell J. Guide to the Geology of Garden of the Gods Area, Gallatin, Hardin, Pope, and Saline Counties, Illinois. ISGS Field trip guidebook 2001–A/B. Illinois State Geological Survey: Champaign, IL. 2001. 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: Garden of the Gods Recreation Area Gather your students near south (left) entrance to the Observation Trail located near the middle of the upper parking lot. You will be able to examine the Pennsylvanian age Pounds Sandstone Member of the Caseyville Formation and discuss the structural history of the region and development of the Eagle Valley Syncline. The Garden of the Gods Recreation Area is located in the Shawnee National Forest in southern Illinois. This area is bounded on three sides by the Garden of the Gods Wilderness Area, which was established by an act of Congress in 1990. No motorized vehicles or mechanized equipment is permitted in the 3,300-acre Garden of the Gods Wilderness Area, a relatively undisturbed wilderness area.
The Garden of the Gods Observation Trail is a one-fourth mile long interpretive trail. It is made of natural flagstone and leads to areas near the bluffs where there are outstanding views of the Shawnee Hills and the Garden of the Gods Wilderness Area. Starting at the south (left) entrance to the Observation Trail you will pass by many interesting, rock formations, given names such as Table Rock, Camel Rock, Devil’s Smokestack, and Honeycomb Rock. The Observation Trail has some short, steep grades and a few steps, but as a whole, is not tiring. Caution should be used because there are high cliffs in the area. GEOLOGY TIP: Glaciation stopped about 15 miles north of the Shawnee Hills. As a result, you can notice a distinct change in topography between the area near Harrisburg to the north and this area. The landscapes in the southern tip of the state are very hilly; hence, their name. Geological History The Shawnee Hills took millions of years to form. The rock formations and cliffs at Garden of the Gods are made of Pennsylvanian age Pounds Sandstone and are about 320 million years old. Long ago, most of Illinois, western Indiana, and western Kentucky were covered by an inland sea. For millions of years, great rivers carried sand and mud to the sea where it settled along the shoreline. Over time, the weight of the accumulating sediments, and chemical reactions between the sediments and fluids in them, turned them into layers of rock, thousands of feet thick.
At Garden of the Gods, the sediment layers now exposed were originally
buried about one mile deep. Beneath the Garden of the Gods there is still
some 20,000 feet of sediments piled on top of the crystalline basement.
Eventually an uplift occurred, raising the land well above sea level.
The uplift also fractured the bedrock, exposing it to nature’s erosive
forces. Since that time, windblown sand, rain, and freezing and thawing
actions have worn down the layers of sediment, Garden of the Gods, located on the south limb of the Eagle Valley Syncline, is one of the most scenic areas of Illinois. The rock layers exposed at the Garden of the Gods are part of the Pounds Sandstone Member of the Caseyville Formation. Long-continued erosion of the uplifted southern limb of the syncline has resulted in deeply disected northward facing dip slopes and high knobs and ridges that consist of strongly weather resistant sandstone.
The Pounds Sandstone is a fairly pure, slightly micaceous, quartz sandstone with numerous white rounded quartz pebbles. About 100 feet of sandstone is present in this member throughout much of the area. The sandstones of the Caseyville are very resistant to erosion, and where exposed, they are cliff-formers. The sandstones are river channel sands laid down by an ancient Pennsylvanian river system that crossed this part of Illinois from northeast to southwest. A number of sedimentary structures, typically formed by river currents within a delta system building out into and along a shallow continental sea, are well developed in the Pounds Sandstone. These sedimentary structures include wedge-shaped crossbedding and ripple marks. The purity and coarseness of the sandstone indicate that the currents along the shallow seashore (both river and nearshore currents) were swift and that much of the fine material and softer non-quartz materials were sorted out before deposition. Other noteworthy sedimentary features of the sandstone include graded bedding, bimodal sorting of the medium to coarse-grained sandstone containing white quartz pebbles, and Liesegang banding.
The unusual concentric and parallel Liesegang banding of iron oxide-rich layers in sandstone, which is so common in outcrops of the Caseyville, is well-displayed along the observation trail. Geologists generally attribute this banding to the so-called "Liesegang Phenomenon." For this phenomenon to occur, a fluid containing a salt must be introduced into a colloidal suspension within a porous medium (such as this coarse sandstone). During mixing of the fluid and the colloid, when the dissolved salt reaches a supersaturated level, precipitation occurs at regular intervals, resulting in the banding just described. Stop #2: The Lee Mine You will need to hike along the forest road, approximately 3,000 feet, to Lee Mine, an abandoned fluorspar mine. You will visit the abandoned Lee Mine located along the northern edge of the Illinois fluorspar district. Small samples of fluorite can be found in the old mine refuse piles.
Lee Mine was operated by Hillside Fluor Spar Mines. The company extracted ore from a vein along the Lee Fault. Geological mapping in this area has traced the fault three miles to the southwest and five miles to the northeast of the mine. The fault can be traced on the surface for about 1,000 feet at the mine. Although mapped as a simple fault, Lee Fault consists of several more or less parallel, closely spaced faults. At the mine this fault has a displacement of 450 feet, downthrown on the southeast side. The Cypress Sandstone forms the northwest wall, and the Menard Limestone forms the southeast wall. Fluorspar mineralization occurred for more than 1.5 miles along the course of the fault or fault zone. The mine was closed in 1938 and has not been worked since then. The U.S. Bureau of Mines did exploratory drilling in the mid-1940s, but that was the extent of activities until the early 1990s when the Illinois Department of Mines and Minerals sealed the three shafts for this mine. The depths of the three shafts, depending upon their distance from the fault, ranged from 35 to 150 feet. The ore along the fault zone occurred generally at depths of around 100 feet in the Menard Limestone, which apparently is much higher stratigraphically than is common in the fluorspar district. Most of the fluorspar ore zones in the district are associated with the thick limestones of the St. Louis and St. Genevieve Formations. According to mine notes at the ISGS, the Lee vein was up to 6 feet wide but averaged under 4 feet, with about one-third of the vein made up of fluorite. The fluorite was of high quality, and no other minerals were present in abundance, except perhaps calcite, and only minor amounts of galena and sphalerite.
Lee Mine is located in the northern edge of the Illinois-Kentucky fluorspar district. This region is a complexly faulted area lying between the Illinois Basin on the north and the Mississippi Embayment to the south. The Illinois portion of the district, with a history of fluorspar mining that dates from 1842, still has important deposits of minable fluorspar and related minerals. Fluorspar from Illinois was in demand because of its high purity and the absence of the toxic trace elements often found in imported ore. Fluorite (calcium fluoride, or CaF2), was designated as the Illinois state mineral by the 74th General Assembly in July 1965 (see Geobit 4: Fluorite—Illinois' State Mineral). Illinois fluorspar occurs almost exclusively in Pope and Hardin Counties. The main production has come from the Rosiclare vein system in the Rosiclare District and from bedded replacement deposits north of the Cave-in-Rock area. Less significant amounts of fluorspar have been mined from several areas outside these main areas. Ore Deposits Ore bodies in the Illinois-Kentucky fluorspar mining district are of three general types: (1) bedded deposits formed by selective replacement of limestone strata, (2) fissure-filling or vein deposits along faults and fractures, and (3) residual deposits derived from one of the other types. Vein Deposits The mineralization at the Lee Mine is a vein-type deposit. The primary controlling factor determining the location and extent of mineralization of vein deposits is faulting. Vein deposits occur in steeply inclined, sheet-like deposits as fissure fillings along faults. The width and continuity of the vein deposits depend on the size of openings between the fault surfaces in which they were formed. Fault planes (surfaces) are rarely perfectly parallel. The rock surfaces on either side of a fault are generally wavy and irregular. Stop #3: Horseshoe Upheaval You will need to hike along the gravel access road, approximately 500 feet, to the north side of the Horseshoe Upheaval. The Horseshoe Geological Land and Water Reserve is a part of the Saline County State Fish and Wildlife Area. A sign at this stop provides a great introduction to this geologic exposure: “The Powerful Earth.” The rocks in front of your are about 350 million years old. The tremendous forces of the earth forced this rock up from 3,500 feet below. These upturned rocks, known as the Horseshoe Upheaval, are silica-rich limestone and chert of the Fort Payne Formation. These rocks represent marine sediments deposited during the Mississippian period 350 million years ago. Several feet of the Upper New Albany Shale is also exposed at the northwest end of the upheaval. These rocks were brought to the surface along the Shawneetown Fault Zone, a great fracture zone that extends more than 100 miles across southern Illinois and western Kentucky. Cave Hill to the south is composed of sandstone, shale, conglomerate, and coal of early and middle Pennsylvanian age, about 310 to 330 million years old. Coal-bearing middle Pennsylvanian rocks also underlie the low hills to the north. The Fort Payne rocks here are confined in a narrow wedge of older rocks, sandwiched by younger rocks. This is unusual—most faults simply have older rocks on one side and younger rocks on the other. The Horseshoe Upheaval suggests that the Shawneetown Fault Zone underwent two episodes of movement in opposite directions. First, the rocks south of the fault zone were uplifted, bringing Fort Payne rocks to the surface, and then the southern block dropped back down. The wedge of Fort Payne rocks was sheared off and jammed in place within the fault zone. The tremendous forces involved are evident in the shattered and contorted rock layers before you. In an abandoned quarry, located on the northeast corner of the Horseshoe Upheaval, the Fort Payne was extracted for use as roadstone. The area covered by the abandoned quarry is about 250 feet wide and 850 feet long, a total area of about five acres. More than 200 feet of the Fort Payne Formation of the Valmeyeran Series (middle Mississippian) is exposed at the quarry site. The Fort Payne consists of highly shattered, partly siliceous and calcareous shale and siltstones and limestone. At depth, the Fort Payne consists of calcareous siltstone and limestone, but weathering has resulted in silicification (replacement by silica) of these rocks at the surface. The New Albany Shale, which consists of thin-bedded, black to dark gray carbonaceous shale that contains numerous partly phosphatic siltstone and claystone nodules, has also been intensely fractured and silicified. Calcite-filled fractures occur in the calcareous shale and siltstone of the Fort Payne Formation. The Fort Payne Formation was deposited as an irregular tongue-shaped body in southern Illinois that partially filled a deep-water basin. Uppermost units of the New Albany Group (Mississippian, Devonian) are exposed in the extreme northwestern part of the quarry site. The New Albany Shale occurs at a depth of about 3,500 feet below the surface on the north side of the fault. Intense fracturing is present in all units exposed at the quarry site. The most intensely deformed units are present near the eastern end of the quarry and about 100 feet south of the access road located on the north side of the upheaval. These crumpled and complexly deformed units consist of soft, somewhat brittle shale, which failed under shearing stresses. These soft shale units lie between siliceous siltstone and shale of the upper part of the Fort Payne Formation and thick units of the New Albany Group, which deformed competently. Suggested Activities:
Resources:
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 ©2002-2003 Illinois State Museum Society. Last updated 10/30/2003. Contact. |
