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Water tower on northeast rim of crater |
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Water tower on northeast rim of crater |
The road to the top of Bald Knob follows along apart of one of the ridges that define the northwest rim of the structure. Note the steep slopes on the north side of the road as you drive toward Bald Knob. These slopes are parallel to the foliation of the ridge rock as seen at Stop 1. The south side of the ridge is very steep and represents weathered fault scarps of the interior of the structure.
The Bald Knob stop provides a panoramic view of the concentric nature of the impact crater. Trees and vegetation have grown up during the past 20 years and now partially block the view. The above picture was taken from Bald Knob. The water tower is approximately 4 km away and is included to give the viewer a sense of scale. Elevation of the rim ranges from 308 m to nearly 462 m above sea level except where it is missing in the southwest quadrant of the structure. The elevation of the floor of the crater averages ~ 100 m. Near the center of the structure is a hill standing approximately 186 m. This feature coincides with the general position of a central rebound area of an impact structure.
If you look away from the Wetumpka impact crater, you may note the topographic elevation of the surrounding rock: to the north the peneplain of the Piedmont is clearly evident; and to the west and south you see the flat to gently dipping strata of the Cretaceous sedimentary units.
| Q5: Rock units that form the rim of the Wetumpka ipact crater are not necessarily any less resistant to weathering than the same units to the north yet there they stand in stark relief in a concentric arc well above both the peneplain of the Piedmont and the flat-lying beds of the Coastal Plain. WHY? |
The Rim
The exposed metamorphic rock units of the rim are primarily coarse- to medium-grained biotite-garnet-feldspathic schist and gneiss (metagraywacke), quartzite, quartz-graphic-muscovite schist. Metasedimentary rocks have been found northeast of the structure. Foliations of the rim rocks dip outward from the center to define an eroded circular dome-like structure.
Locally along the southern rim, a complex set of foliation attitudes suggest a possible pre-impact fold system was imprinted on the metamorphic rocks, similar to those seen farther northeast in the main body of the crystalline rock. Several concentric faults are suggested by the presence of steep scarps that face inward toward the center of the structure. Surface expressions of these faults are poor. However, brecciated zones and the close spacing of several mappable stratigraphic units exposed in stream beds and gullies indicate their location. Total relief of the rim is approximately 200 m above the Cretaceous-Piedmont nonconformity.
Central interior of the impact crater
Most of the sedimentary rock strata exposed along the road is of Quarternary or Late Cretaceous age. Many of the smaller hills contain chaotically oriented metamorphic rock fragments up to 30 m across. Good exposures of the rock strata in the interior part of the impact structure are few and difficult to see. The best exposures are found in nearly inaccessible borrow pits where steeply dipping, chaotically oriented unconsolidated sediments can be observed (STOP 3). Several exposures occur along the central access road and illustrate, to a small degree, the complex relationships among the various rock strata contained within the central interior part of the structure. Locally thick zones composed of fragmental blocks of metamorphic rock mixed with sedimentary material may represent massmovement (mega-slump) deposits off the horseshoe rim. In some areas the strata appear flat-lying. In other areas, the strata are vertical or are steeply inclined and may be overturned.
The floor of the structure consists of irregular, chaotically distributed blocks of sedimentary and metamorphic rock, which are locally covered with Late Pleistocene-Holocene alluvial sands and gravels. Sedimentary units recognized in the floor area includes the Coker and Gordo Formations of the Tuscaloosa Group, the Eutaw Formation, and the Mooreville Chalk.
At the time of the initial investigation in 1972, only five deep wells had been drilled in the interior of the crater. These wells penetrated unique stratigraphic relationships and abnormal rock-unit thicknesses, which Neathery et. al. (1976) interpreted to indicate the complex geometry of the observed structure. Neathery et. al., (1976) interpreted all these phenomena to indicate post-deposition disruption of the soft sediment deposits in response to a meteorite impact. The following three stop descriptions provide a brief look at the complex nature of the interior structure.
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