The geologic section along the Chattahoochee River is one of the best and most complete in the Coastal Plain of the United States. It is the only continuous unweathered section of Cretaceous and Tertiary beds in southeastern Alabama and southwestern Georgia, and comprises a connecting link between the well-known standard section and type exposures to the west in Alabama and beds to the east in Georgia and South Carolina. It is also an important section because of its intermediate position between the clastic facies in the central and western Gulf Coastal Plain and the equivalent carbonate facies in the subsurface in Florida.

Upper Cretaceous, Paleocene, lower Eocene, and middle Eocene strata are exposed in an almost continuous section down the dip from the crystalline rocks at the Fall Line at Columbus, Ga., to upper Eocene exposures about 8 miles north of the Alabama-Florida boundary, a distance of 126 miles. This study of a part of that section supplies detailed stratigraphic and structural information on rocks of Tertiary age that are exposed southward from the Upper Cretaceous-Paleocene contact 15 miles south of Eufaula, Ala., to exposures of upper Eocene Crystal River limestone of Moore, 1955, 15 miles south of Columbia, Ala., a distance of 49 miles. The total thickness of Paleocene, lower Eocene, and middle Eocene strata in this part of the river is a little over 600 feet. The average dip is 15 feet per mile to the south, but in places the beds are horizontal for distances of as much as 3 miles.

The formations recognized in the river section are those in the standard Alabama stratigraphic section. From the bottom up they are the Clayton formation of Paleocene age, the Nanafalia formation, Tuscahoma

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sand, and Hatchetigbee formation of lower Eocene age, and the Tallahatta and Lisbon formations of middle Eocene age.

Stratigraphic sections for these studies were combined into longitudinal profiles for the left and right banks of the river, an arrangement that visually describes detailed lithology and structure. Horizontal control was from aerial photographs. Vertical control was from the river surface. The altitude of the river surface was determined from the stage of the river referenced to "the thalweg," as plotted by the U. S. Corps of Engineers, and bench marks of the U. S. Coast and Geodetic Survey. The use of "the thalweg" in determining altitudes of contacts permits rapid and accurate mapping of geologic sections along rivers.

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