Geologic dating issues

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Younger rocks dating from the Pennsylvanian period are still preserved in down-folded synclines, or concave folds, like those that form Lookout and Pigeon mountains.

If ever deposited, Permian strata have been removed altogether.

Cambrian and Early Ordovician times, the source of sandstone and shale (sands and muds indicating eroded land) lay toward the center of the continent, but from the Middle Ordovician time the source shifted to the east.

This change in sediment source indicates, as does such other evidence as the occurrence of volcanic ash beds within Middle Ordovician rocks, both that the Iapetus Ocean was beginning to close as a consequence of subduction (when one plate descends below the edge of another) and that mountain uplift was occurring at the continental margin.

These sedimentary rocks now make up the Valley and Ridge Province, while metamorphosed rocks from the continental margin occur in the Piedmont and Blue Ridge provinces.

Deformation is most severe within the metamorphic rocks of the Piedmont and gradually decreases in intensity going westward across the Valley and Ridge, until the folds and faults die out beneath the Appalachian Plateau.

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Ridges are developed on resistant layers of sandstone or chert; valleys are underlain by shale or limestone.

Mountain building ended some 250 million years ago, and by 200 million years ago the Atlantic Ocean was beginning to open.

Two hundred million years of weathering and erosion have taken their toll on the Appalachians and upon the folded and faulted structures of the Valley and Ridge.

The spectacular Blue Ridge escarpment, north and west of Cartersville, marks the southeasterly dipping faults, formed where metamorphic rocks slid over folded sedimentary rocks.

Closer to Chattanooga the folds almost die out, and the strata are almost horizontal, although they are still more than 1,000 feet above the elevation at which they were originally deposited. Furcron, "The Georgia Story (The Geological History of Georgia)," Georgia Mineral Newsletter 1-5 (1948-1952). Gore and William Witherspoon, Roadside Geology of Georgia (Missoula, Mont.: Mountain Press, 2013). Abrams, Geology of the Greater Atlanta Region, Georgia Geologic Survey Bulletin 96 (1984).

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