GROUND WATER IN NORTH-CENTRAL TENNESSEE

32 GEOTJND WATEE IN NOETH-CENTEAL TENNESSEE
Dickson County. (See pi. 4.) In this district no paleonfcologic evi­
dence of the age of the formation has been found, so that the correla­
tion is based upon the lithology of the material, the geographic rela­
tion fco localities at which paleontologic evidence exists, and the
relation of the deposits to the Highland Bim peneplain. The geo­
logic map of Tennessee ** (see pi. 4) also shows a small outcrop of the
formation capping the divide between Long Creek and Cross Creek
near Bear Spring, Stewart County. Furthermore, the deposits of
waterworn chert and vein quartz gravel on the Highland Bim plateau
in southwestern Dickson County and adjacent portions of Hickman
County, which are described by Hayes and Ulrich,43 may also belong
to the Tuscaloosa formation.
The Tuscaloosa formation as exposed in a cut on the Nashville,
Chattanooga & St. Louis Railway about 2 miles east of McEwen,
Humphreys County, is described by Wade ** as consisting of about
30 feet of very compact white chert gravel which rests upon weathered
St. Louis limestone. The individual pebbles are well rounded, and
most of them are less than an inch in diameter, although some are
as large as 6 inches. Some sand is mixed with the gravel, although
very little clay is present. The gravel of the Tuscaloosa formation
can generally be differentiated by three criteria from the stream-
terrace gravel with which it may be associated. In the first place,
the individual pebbles and cobbles of the Tuscaloosa formation are
well rounded, and many of them are almost spherical, whereas those
of the terrace gravel are generally flat, elongate, or even subangular.
Small discoidal pebbles of quartzite are abundant in the terrace
gravel at many localities. Second, the Tuscaloosa gravel is composed
for the most part of chert from the Mississippian rocks, whereas the
terrace gravel is derived in large measure from quartzite and sand­
stone. Third, pellets of iron oxide are not known to occur in the
Tuscaloosa gravel, whereas they have been observed in the stream
deposits.
The Tuscaloosa is the oldest formation of the Upper Cretaceous
series in the East Gulf Coastal Plain province, although the deposits
that exist in north-central Tennessee probably represent only some
of the uppermost beds of the type section in the vicinity of Tusca­
loosa, in central-western Alabama. In north-central Tennessee the
formation was laid down as a coastal-plain deposit along the western
edge of the Cumberland peneplain, underwent planation during the
Highland Rim cycle, and subsequently has been almost wholly
eroded by the Tennessee River during the Nashville Basin and recent
erosion cycles.
« Nelson, W. A., Geologic map of Tennessee, 3d ed., Tennessee Qeol. Survey, 1923.
« Hayes, C. W., and Ulrich, E. O., U. S. Qeol. Survey Qeol. Atlas, Columbia folio (No. 95), p. 1,1903.
« Wade, Brace, op. cit., pp. 103-104.