The University of Tennessee at Chattanooga
Geology Program
Department of Physics, Geology and Astronomy
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Jonathan-Mies@utc.edu
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Jonathan W. Mies
April 5, 1999
Location
Signal Point is part of the Chickamauga and Chattanooga National Military Park and is located in the town of Signal Mountain, on the south end of Walden Ridge (Figure 1). From Chattanooga, travel north on US route 27 (Corridor J), 1.8 miles past Olgiati Bridge (Tennessee River), to the Signal Mountain Exit (US route 127). Follow US route 127 north to the town of Signal Mountain (6.4 mi). (All miles are cumulative and are measured from Olgiati Bridge.) Just before the traffic light in Signal Mountain (7 mi), turn left onto Signal Mountain Boulevard. At 7.2 miles, take the first left onto Mississippi Avenue. Follow Mississippi Avenue to Signal Point Road (8.2 mi) and turn left. Signal Point is as the end of Signal Point Road (8.5 mi from Olgiati Bridge). From the parking lot, walk approximately 70 meters along the paved path to the overlook. The best outcrops are a few hundred meters along the Cumberland trail, which starts at the edge of the woods northwest of the overlook. After a short, steep decent, the trail follows a hardwood forested ledge. Observations described here are from the cliffs above this ledge, northwest of Signal Point.
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Figure 1. Map showing the location of Signal Point (black arrow), relative to Chattanooga. (Modified from USGS 1:100,000-scale Chattanooga Tennessee Quadrangle.) |
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Outcrops at Signal Point are within a national park and are subject to national park rules and regulations, i.e. no hammers. Furthermore, access to these outcrops is limited to park hours.
Introduction
The southern Appalachians are divided into several tectonic belts that include (southeast to northwest) the Carolina terrane (or belt), the Inner Piedmont, the eastern and western Blue Ridge, the Valley and Ridge, and the Cumberland Plateau. The Valley and Ridge is otherwise known as the "foreland fold and thrust belt" for its characteristic geologic structures and its position in the foreland of the Appalachian orogen. Alleghanian deformation related to the development of the southern Appalachians, which is so conspicuous in the Valley and Ridge, is only scarcely evident in the Cumberland Plateau (Wilson and Wojtal, 1986).
The Valley and Ridge consists of folded and imbricately thrusted Paleozoic sedimentary rocks. The succession of valleys and ridges results from variable erosion of the dipping strata.
With the exception of areas adjacent to Sequatchie Valley and the Valley and Ridge, the Cumberland Plateau consists of nearly flat-lying sedimentary rocks; Pennsylvanian sandstone that forms the top of the plateau is underlain by Mississippian shale and limestone, and the middle to early Paleozoic (Devonian downward through Cambrian) sedimentary sequence.
The Cumberland Plateau is split by Sequatchie Valley and associated geologic structures. Major plateau segments southeast of Sequatchie Valley are known as Walden Ridge, Raccoon Mountain (also known as Elder Mountain), and Sand Mountain. (See the digital relief map on the Chattanooga Outcrops home page.)
Structural relief associated with the Sequatchie anticline and, in particular, the fact that Mississippian and Ordovician limestones occupy a higher position in the core of the anticline contributed to the erosional development of Sequatchie Valley. In Sequatchie Valley, the Pennsylvanian sandstone that serves as a resistant cap rock for the Cumberland Plateau has been breached and the underlying limestones are more extensively eroded.
The longitudinal edges of Walden Ridge and Sand Mountain are also the limbs of adjacent anticlines (Sequatchie anticline to the northwest and Lookout Valley anticline to the southeast). Sedimentary rocks along the northwest edge of Walden Ridge, for example, dip southeastward, toward the interior of the ridge; rocks on the southeast edge of the ridge dip toward the northwest. Thus, Walden Ridge and Sand Mountain are a broad synclinal trough that trends northeast-southwest, parallel to adjacent anticlines. In the Chattanooga area, the long axis of Walden Ridge has an azimuth of 035 degrees.
As described below, Signal Point provides an excellent opportunity to view characteristic landscapes of the Valley and Ridge and the plateaus and to study the Pennsylvanian sandstone that caps the plateaus. Joints, in particular, are plentiful at Signal Point and are geometrically related to larger geologic structures.
Site description
Signal Point provides a spectacular view. The view to the southeast, toward Chattanooga, includes Godsey Ridge, Stringers Ridge, Missionary Ridge, and other, more distant, ridges (Figure 2). These ridges are held up by resistant rocks (Mississipian Fort Payne Chert and Cambrian Knox-Group dolomite) and punctuate Valley-and-Ridge-style geologic structures (folds and thrusts). Looking toward the south and southwest, the view includes Lookout Mountain (in the distance, adjacent to Chattanooga) and Raccoon Mountain/Elder Mountain (directly across the river). These plateaus have flat tops and square shoulders and tiers of near-vertical cliffs. This landform is characteristic of flat-lying or shallowly dipping sandstone, underlain by less resistant rocks (shale and limestone). The uppermost cliffs across the river are equivalent to the cliffs below Signal Point and are made of the same sandstone. The view to the west looks downstream into the Tennessee River Gorge and to parts of the river that were once turbulent with rapids. These parts of the river were once known as the "Suck" (for which Suck Creek is named) and the "Kettle".
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Figure 2. Photograph taken from Signal Point looking southeast, toward Chattanooga. Note Godsey Ridge (foreground), Stringers Ridge, and Missionary Ridge (beyond Chattanooga), and the square shoulders of Lookout Mountain and Raccoon Mountain (also known as Elder Mountain). |
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Cliffs below Signal Point and along the upper part of the Cumberland escarpment consist of well-indurated Pennsylvanian sandstone belonging to the Gizzard Group (Milici et al., 1978). The white to light-gray to pale-yellowish-brown sandstone is fine to medium grained and locally contains a few well-rounded, quartz pebbles. Centimeter- to meter-scale bedding is nearly horizontal, except where the sandstone is cross bedded; cross beds dip up to 25 degrees to the northwest, suggesting a paleocurrent in that direction.
Editors note: there is a spectacular outcrop of cross-bedded sandstone along route 127 (Taft Highway), across from the "Welcome to Signal Mountain" sign.
Joints in the sandstone are nearly vertical and have strikes that define two distinct joint sets. Approximately 50 % of the joints strike between 305 degrees and 325 degrees (azimuth), perpendicular to Walden Ridge, and 40 % of the joints strike between 035 degrees and 065 degrees, parallel to the ridge (Figures 3 and 4). There are also bedding-parallel joints and a few near-vertical joints that strike between 070 degrees and 075 degrees. Joint surfaces are essentially featureless; they lack hackle and associated marks (plumose, fringes, arrest lines, etc.). The northwest-southeast trend of the cliff face in this area relates to the dominant joint set.
Neither the age of the joints nor their geometric relationship to larger strucures are well understood.
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Figure 3. Photograph of the outcrop at Signal Point showing two near-vertical joints with strikes of 305 degrees and 035 degrees. The compass rests on a bedding-parallel joint surface and points due north. |
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Figure 4. Rose diagram showing the orientations of joints at Signal Point. |
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Several large cavities, some of them nearly spherical and up to 3 meters in diameter, are found in the otherwise smooth and planar cliff face (joint surface) (Figure 5). The cavities have pitted surfaces that are coated with limonite (soft yellowish-brown mineral), suggesting that they relate to concentrations of iron.
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Figure 5. Photograph of a large cavity in the cliff face at Signal Point. The cliff is approximately 20 meters high and trends northwest-southeast, parallel to the dominant joint set. The cavity, midway up the cliff face, is approximately 3 meters in diameter. |
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Signal Point is also historically significant. The overlook was used as a signal station by the Federals during the Civil War, particularly after the Confederates had severed telegraph lines to Chattanooga.
Milici, Robert C., Wilson, Robert L., Maher, Stuart W., Leamon, Alvin R., Knox, Larry M., and Johnson, Robert W., Jr., 1978, Geologic Map of Hamilton County, Tennessee: State of Tennessee, Department of Conservation, Division of Geology, Bulletin 79, Plate 1.
Wilson, Robert L., and Wojtal, Steven F., 1986, Cumberland Plateau decollement zone at Dunlap, Tennessee: Geological Society of America Centennial Field Guide: Southeastern Section, p. 143-148.