GROUND WATER IN NORTH-CENTRAL TENNESSEE

170 GROUND WATER IN NORTH-CENTRAL TENNESSEE
BOBEBTSON COUNTY
[Area 455 square miles. Population, 28,191]
GENERAL FEATURES
Like Montgomery County, which it adjoins on the east (pi. 1),
Robertson County lies in the most densely populated and fertile
part of the Highland Rim plateau in north-central Tennessee. It
comprises extensive slightly undulating interstream tracts with a
local relief of 25 to 50 feet, which are remnants of the Highland Rim
peneplain (pp. 19-20), and mature and youthful drains, which trench
the plain to a maximum depth of about 250 feet. The peneplain
remnants range from about 875 feet above sea level at Ridgetop, at
the crest of the Highland Rim escarpment in the southeastern part
of the county, to about 625 feet along the northern boundary of the
county. The peneplain remnants also include many small undrained
depressions, of which a large number contain perennial ponds. These
water bodies occur at all altitudes, and most of them are derived
wholly from surface run-off; a few such ponds, which are generally
in the lower parts of the surface, may be supplied by ground water
through one or more submerged solution channels. All but a very
small part of the county is drained by tributaries of the Red River,
which head in relatively shallow mature drains along the crest of the
Highland Rim escarpment and flow northwestward and westward.
In their lower reaches these tributaries occupy youthful valleys 100
feet or more deep. A strip several miles wide along the southern
boundary of the county from Ridgetop westward is drained by
Sycamore and Half Pone Creeks.
Robertson County lies on the northwest flank of the Nashville dome
(pp. 62-63), so that the primary structure of the rocks is that of a mono­
cline dipping very gently northwestward. In all the upland tracts
the bedrock is the massive limestone that composes the St. Louis
limestone and Warsaw formation, although there are few outcrops
of these formations away from the youthful stream valleys. In the
peneplain tracts the bedrock is concealed by a mantle of residual
clay and soil, usually between 15 and 50 feet thick but in some places
100 feet thick. Over most of the county this material contains very
few chert fragments, unlike the residuum that overlies the Fort Payne
formation in other counties. In Robertson County the Fort Payne
formation is composed chiefly of thin-bedded earthy cherty limestone,
calcareous shale, and impure sandstone. It crops out only in the
valleys of the tributaries of the Red River south and east of Spring­
field and in the valley of Sycamore Creek from Ridgetop westward.
The stratigraphic relations of these rock formations are discussed on
pages 33-37. Their areal extent is shown on Plate 4.