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Mississippi Palisades Park and the Savanna Area | |
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Credit: The following field trip is adapted from: Frankie, Wayne T. Guide to the Geology of the Mississippi Palisades State Park and the Savanna Area, Carroll and Jo Daviess counties, Illinois. ISGS Field trip guidebook 2001-C/D. 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: Sentinel Trail, Mississippi Palisades State Park
A hike of the upper loop of the trail leads to the Sentinel Rock Overlook and Bat Cave. Geologic features present here are the shear dolomite bluffs known as the palisades and the dolomite column known as the Sentinel. This unique geologic feature is a free-standing dolomite column, rising nearly 200 feet above the talus slopes. The Sentinel appears to be a fractured pillar from adjacent bluff lines. Small limestone outcrops also occur scattered above the sides of the slopes and provide important cliff habitat for many plant and animal species.
The bluff face provides habitat for the state-threatened cliff goldenrod. The towering bluffs also provide sanctuary for several ferns, including the bulblet fern, and provide some of the northernmost limits for the smooth cliffbrake and baby lip ferns. Along the bluff are a number of small sinkholes and at least one cave structure. Bat Cave, located on the north loop of the trail, provides an important habitat for bats.
Karst topography occurs throughout the park. Typical features include several deep solution pits or sinkholes. In addition, some small caves have formed from solution along joints and cracks in the dolomite. The sinkholes act as large funnels, collecting rainwater and channeling it into underlying joints and cracks.
Four conditions contribute to the development of karst topography. First, soluble rock, flat or nearly so, lies at or near the surface. Second—and most important—the limestone or dolomite is dense, highly jointed, and generally thin-bedded. If the stone were porous, rainwater would be absorbed and move through the whole body of the rock rather than concentrate along joints and bedding planes. Third, major valleys are entrenched below the uplands and act as outlets toward which the groundwater moves in the subsurface. Fourth, rainfall is ample. The name "karst topography" comes from the karst region of the Dinaric Alps in Yugoslavia, where such features are common. Stop #2: Lookout Point, Mississippi Palisades State Park.
Lookout point provides a great overview of the Mississippi River valley. The most impressive topographic feature in the field trip area is the Mississippi River valley, especially where it has incised the resistant Silurian dolomite to form the Mississippi Palisades. High on the bluffs east of the Mississippi valley near East Dubuque, Survey geologists discovered deposits of early pre-Illinois glacial outwash—evidence that the river was probably not entrenched in its present valley in the Savanna area until after that early glaciation. These pre-Illinois outwash deposits consist of coarse gravel overlying lake sediments in small, shallow channels eroded into the surface of the dolomite bedrock 200 feet above the present Mississippi River floodplain. The position of the Mississippi River valley from the vicinity of St. Paul, Minnesota, southward along the West side of the Driftless Area closely follows the margin of the early pre-Illinois glacial deposits. Apparently, this early glaciation established the position of the valley, which developed from an ice-marginal stream during the maximum advance of the ice sheet. Just south of Galena in Jo Daviess County, the Mississippi Valley cuts through a prominent northfacing escarpment of Silurian dolomite. The front of the escarpment stands about 200 feet above the level of the Lancaster Peneplain. This escarpment, or cuesta, is the erosional edge of the Silurian dolomite formations that dip gently southwestward off the Wisconsin Arch. From this spot, at Lookout Point, the Mississippi River valley is slightly more than 2 miles wide, whereas a short distance to the north near Hanover Bluff, the Mississippi River valley is a little more than 4 miles wide, and, to the south near Sloam Marsh, the valley is 4.5 miles wide. The widening of the Mississippi River valley to the north and south is because the Mississippi River has eroded its valley into the relatively soft Ordovician age Maquoketa shale below the resistant Silurian dolomite. The valley is as much as 6 miles wide at the Thompson Causeway where the river has eroded into the softer bedrock strata.
At the beginning of the "Ice Age," the topography of Upper Mississippi Valley region of northwestern Illinois, northeastern Iowa, and southwestern Wisconsin was a plain of low relief at the level of the present uplands (Lancaster Peneplain). The Silurian escarpment was a continuous divide extending across northwestern Jo Daviess County into Iowa. Streams flowed northward and southward from the divide in broad, shallow valleys, high above the present drainage. No evidence shows that a major south-flowing stream existed in the Savanna area, but several of the major tributaries to the present Mississippi River may follow courses established at that time. Farther south, an ancestral river called the Ancient Iowa River had its headwaters in east-central Iowa and followed the present course of the Mississippi Valley below Muscatine, Iowa. With the advance of the early pre-Illinois glacial episode, the first of the Pleistocene glaciers to invade the Upper Mississippi Valley region, northward drainage was blocked and a meltwater lake formed in front of the Silurian Escarpment north of Galena. The meltwater eventually spilled over the divide at Galena and eroded a channel through the escarpment. The water then flowed southward and eroded a valley along the margin of the early pre-Illinois glacier. By the end of this early glaciation, the Mississippi River had established its valley along the west side of the Driftless Area southward as far as Fulton in Whiteside County. From there, it followed a southeastward course, along the Princeton Valley, to the present big bend of the Illinois River valley near Hennepin in Putnam County. The river then followed the present course of the Illinois Valley southward to its confluence with the Ancient Iowa River at Grafton in Calhoun County. Except for a temporary diversion by the Illinois glacier, the Ancient Mississippi River followed this course from Fulton to the big bend and southward along the Illinois River valley until the Wisconsin Glacial Episode. During the Woodfordian advance, the river was permanently diverted westward to occupy the valley of the Ancient Iowa River along the west side of what is now Illinois, a course it still follows. Its former valley from Fulton to the big bend, known as the Princeton Valley, was buried by Wisconsin glacial drift. Stop #3: Lost Mound/Abandoned Shelly Quarry, Hannover Bluff This abandoned quarry is located along the southern boundary of the Hannover Bluff Nature Preserve. This property consists of 88.4 acres and is know as the Lang Property. Contact Apple River Canyon State Park for access to the quarry (815-745-3302). You must secure permission in advance. This abandoned quarry allows close examination of the Silurian age dolomite and an opportunity to collect fossils. The units exposed here are also present at the Mississippi Palisades State Park. An exposure of Ordovician age shale is to the left near the base of the road. The view from the top of the highwall in the northeast comer of the upper bench in the quarry offers a great View overlooking the Mississippi River valley and the Savanna Army Depot. Suggested Activities:
Resources:
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