Januaey 10, 1908] 



SCIENCE 



71 



SPECIAL ARTICLES 



ALTAMAHA FORMATION OF THE COASTAL PLAIN 

 OF QEORGLi' 



The name, Altamaha, was applied by Dr. W. 

 H. Dall'' in 1892 to a sandstone and gritty 

 clay formation prominently exposed along the 

 Altamaha and Ocmulgee Rivers of Georgia. 

 The study of the formation was confined 

 mainly to the above-mentioned rivers. Mr. R. 

 M. Harper" later studied the formation in 

 some detail from a phytogeographical stand- 

 point. The age of the Altamaha and its rela- 

 tion to other formations of the coastal plain, 

 however, have been unsettled. The writer has 

 spent considerable time during the past sum- 

 mer in studying and mapping the formation 

 and presents some conclusions concerning it. 



The Altamaha is the most widespread for- 

 mation of the coastal plain of Georgia, cover- 

 ing approximately 21,000 square miles or three 

 fifths of the entire coastal plain of Georgia. 

 Occurrences have been noted to within a few 

 miles of the Atlantic coast. Good exposures 

 of red sand and clay are found near Savannah, 

 Waynesville, Brunswick and Kingsland, while 

 the probabilities are that it occurs on the sea 

 islands. It thence extends northward into 

 Burke County to within twenty miles of 

 Augusta on the Fall Line. Thence going in 

 a southwestward course its northward extent 

 is marked by the towns of Tennille, Dublin, 

 Hawkinsville and Vienna to Flint River. 

 West of Flint River no formation has been 

 identified with certainty as the Altamaha, ex- 

 cept in Decatur County, in the extreme south- 

 western part of the state. Southward it ex- 

 tends into Florida. 



As a whole the Altamaha consists of yellow 

 and red sand and both massive and stratified 

 layers of gritty clay, with local areas of in- 

 durated grit or sandstone and clay. The sur- 

 face aspect, which is peculiarly characteristic 

 throughout the terrane, is a mottled or 

 " calico " effect ; that is, the weathered surface 

 is a splotched red, yellow and white and very 



' Published by permission of the state geologist 

 of Georgia. 



^Bull. No. 84 U. S. G. S. 



'Annals N. T. Acad. 8ci., Vol. XVII., Pt. I. 



frequently puiplish and white, due to unequal 

 weathering and oxidation of iron minerals. 

 At some localities small brown iron oxide ac- 

 cretions from the size of buckshot to walnuts 

 are abundant at the surface, and the land 

 where these are found is commonly referred 

 to as " pimple " land. These iron oxide peb- 

 bles are products of weathering of the Alta- 

 maha clay-sand and are almost certain evi- 

 dence, where found, that the underlying for- 

 mation is the Altamaha. The topography of 

 the Altamaha formation is rolling and unlike 

 that of any other coastal-plain formation. 



The sand of the formation is usually a 

 coarse quartz sand, red and yellow or orange 

 in color, and occasionally has a brownish tint. 

 It is always more or less argillaceous and con- 

 tains, frequently, layers of small quartz peb- 

 bles. A characteristic of these pebbles is 

 their angularity, some being lath-shaped, 

 showing scarcely any rounding of the angles. 

 The pebbly feature is nowhere very promi- 

 nent and is exceptional rather than gen- 

 eral. In a few localities, the pebbles are 

 rather large, attaining a diameter of four or 

 five inches. Near the Atlantic coast and in 

 the southwestern part of the state the sand 

 may be fine grained, rarely micaceous, cross- 

 bedded, and interstratified with thin layers 

 and leaves of plastic clays. These clay layers 

 may not exceed an inch in thickness. Such 

 structure is seen in the exposures of the Alta- 

 maha near Jesup and Waynesville in Wayne 

 County and near Whigham in Grady County. 



The clay of the Altamaha is fairly uniform 

 in texture and composition throughout the 

 Altamaha area. It is a greenish or drab, very 

 fine-grained and highly plastic and always 

 more or less sandy. It has a low specific 

 gravity and absorbs a high percentage of 

 water. It often has an acid or sour taste, 

 due, likely, to aluminum sulphates. It occurs 

 in thick irregular pockets or thin lenticular 

 layers or leaves, never persisting as in indi- 

 vidual beds over any large area. A tolerably 

 characteristic appearance throughout is green- 

 ish clay, fidl of coarse angular quartz particles 

 and subangular decomposed feldspar pebbles. 

 The clays may be locally indurated, the ce- 



