Toccoa River Rocks Types

Corbin Metagranites are characterized by large quartz crystal development.

The Toccoa River watershed includes southwest Union County and most of Fannin County in northern Georgia on the Tennessee border. The Toccoa is 29 miles long, with an average channel slope of 30 feet per mile. This extreme southern limit of the Blue Ridge Mountains is underlain by metamorphic rocks that originated as Proterozoic or Paleozoic foreland sediments, as well as igneous massifs such as Corbin Metagranite, Fort Mountain Gneiss, mafic and ultramafic rocks, and the metavolcanics of the “gold belt” of the 1830s Georgia gold rush. Does this Spark an idea?

Great Smoky Group Sedimentary Rocks

The Toccoa River flows over the Great Smoky Group rock association. Mountain building occurred several times in eastern North America. Landmasses collided and mountains were thrust up. The mountains eroded and sediments accumulated on adjacent marine forelands that were later thrust up as mountains again. The sedimentary rocks of the Great Smoky Group formed mostly on deep marine terraces. Great Smoky metamorphics include coarse greywacke sandstone with more than 15 percent clay and coarse arkose sandstone that is more than 25 percent feldspar. Characteristic sediment bedding is also visible in dark slate that originated as silt deposits and in other schist found in the Great Smoky Group. Continental collisions were accompanied by the great heat and pressure of metaplutonic igneous massifs and metavolcanics that intruded into these sedimentary rocks.

Great Smoky Group Metaplutons

Great Smoky Group sediments were metamorphed by the Rowland Springs Formation, the Red Top Mountain Formation and the Corbin Metagranite igneous intrusions, A mountain chain as high as the Himalayas uplifted sediments from the original continental shield and stretched from Georgia to Canada. The Rowland Springs massif of diorite and andesite intruded into these sediments. Rowland Springs Formation was later heated and pressed by the Red Top Mountain Formation intrusion, so that the Rowland Springs Formation metamorphed at exceptionally high temperature to pyroxene granulite facies. Both of these formations are now part of the Corbin Metagranites, a more recent igneous massif. Corbin Metagranites, or Corbin Gneiss, is found as coarse-grained megacrystic granodiorite, quartz monzonite and other granites that display large quartz crystal development.

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Murphy Marble

The Great Smoky Group is associated with the Murphy Syncline belt of garnet schist, gray slates and conglomerates overlying quartzite, slate, calcareous schist and Murphy Marble, a fine- to medium-grained blue and white marble. Marble is a hard, metamorphosed limestone, and limestone originates in marine calcium-rich sediments. Around 1900, excavations for a railway bridge over the Toccoa River discovered marble boulders below the river’s surface level. The boulders were not structurally sound for building materials. Further reconnaissance established that Young Stone creek, a small tributary of the Toccoa, was widely underlain by a light-colored, fine-textured marble with few impurities.