GROUND WATER IN NORTH-CENTRAL TENNESSEE

24 GROUND WATER IN NORTH-CENTRAL TENNESSEE
Although the rocks of central Tennessee are but slightly deformed
and are for the most part limestones, they vary widely in resistance
to abrasion and solution. Hence the streams tend to follow the less
resistant and more soluble beds, and the drainage pattern tends to
express the geologic structure of the subsurface rocks. This is
especially true of the smaller tributary streams, which at many places
follow the strike of the beds faithfully.
TOTOEROROTJND DRAINAGE
Many extensive tracts in north-central Tennessee, both on the
Highland Kirn plateau and on the floor of the Nashville Basin, have
no permanent surface streams but are drained into underground
channels in the limestone. Elsewhere minor streams flow for a dis­
tance on the surface, then disappear into sink holes and are added to
the subsurface drainage. These underground passages are by no
means fortuitous but tend to develop a definite drainage system which
is tributary to the surface streams and is an integral part of the
regional drainage mechanism. Under favorable circumstances the
underground streams may degrade their channels very rapidly and
so may even become pirate streams and capture other underground
channels or divert surface streams. The factors that govern the
development of such an underground system are discussed on pages
69-74. Here and there the roofs above the larger of these underground
channels are breached by collapse (see pi. 5, A, B), by solution, or
by stream erosion, so that sizable "caves" and galleries are exposed.
Locally these features are most striking.
STRATIGRAPHY
SEQUENCE AND GENERAL CHARACTER OF THE ROCKS
The rocks of north-central Tennessee include both unconsolidated
and consolidated sedimentary types, but no igneous rocks indigenous
to the region are known. The unconsolidated rocks are stream-bed,
stream-terrace, and coastal-plain deposit® of Upper Cretaceous and
Quaternary age, none of unquestioned Tertiary age being recognized.
Extensive Pleistocene deposits are lacking, so that the Quaternary
beds are for the most part of Kecent age. The consolidated sedimen­
tary rocks are chiefly limestone and cherty limestone, with some beds
of shale and a very few beds of sandstone. Those which are exposed
at the surface range in age from Lower Ordovician (Beekm&ntown) to
Mississippian, and all the geologic epochs of that interval are rep­
resented. The sequence is parted, however, by many minor dis-
conformities and by one major unconformity, which causes the omis­
sion of the entire Devonian and Silurian systems over an extensive
part of the region.