GROUND WATER IN NORTH-CENTRAL TENNESSEE

78 GROUND WATER IN NORTH-CENTRAL TENNESSEE
CYCLES IN THE FORMATION OF UNDERGROUND DRAINAGE SYSTEMS
In any district underlain by limestone the joints and solution
passages are integral and tributary parts of the regional drainage
system. Furthermore, the work of the ground water modifies pro­
foundly the normal sequence of land forms that are cut elsewhere on
insoluble rocks by subaerial erosion. Several workers, including
Beede & and Cvija6,53 have sought to define the stages of the erosion
cycle in a limestone district in terms of the land forms produced
directly or indirectly by the solvent action of the ground water. As
thus defined the youthful stage of the cycle is characterized by pro­
gressive capture of surface drainage by the underground streams and
by the appearance of scattered solution sinks. In the mature stage
the drainage is virtually all underground and the surface valleys of
youth are entirely disorganized by solution sinks and scattered col­
lapse sinks, which together cover most of the region. At this stage
there exists the strongest topographic expression of the solvent action
of the ground water. In late maturity collapse sinks become numer­
ous and valleys are formed by foundering of the roofs above the major
solution channels, much of the drainage being brought to the surface
again. In the stage of old age the last solution channels are unroofed
and a plain drained by a normal system of surface streams is formed;
this plain is the final product of the cycle.
These definitions of the stages of the erosion cycle in terms of the
minor land forms produced by solution are somewhat unsatisfactory,
however, for they imply that certain minor forms invariably accom­
pany each of the major topographic forms by which the stages are
best known. Such a fixed association of major and minor forms is
not necessarily true. It seems more rational to analyze the cyclic
history of the surface and underground drainage systems independ­
ently, though by analogous stages.
In the first or youthful stage of its history a surface stream advances
by headward erosion, branches repeatedly, and encroaches upon an
undrained upland by expanding fanlike until a rough equilibrium is
reached between the erosive powers of adjacent streams. Its valleys
have steep longitudinal gradients and are V-shaped in transverse pro­
file; the valley slopes intersect the upland plain in sharp angles.
There are many undrained remnants of the upland between the tribu­
tary streams, but they are gradually reduced in size. In the stage of
late youth or adolescence the angular shoulders at the crests of the
valley walls disappear from the profiles and are replaced by curves
that are convex upward. The stage ends when the upland remnants
H Beede, J. W., The cycle of subterranean drainage as illustrated in the Bloomington, Ind., quadrangle:
Indiana Acad. Sei. Proc. for 1910, pp. 81-111,1911.
" Cvijafi, Jovan, Hydrographie souterraine et evolution morphologique du Karst: Inst. gfiog. alpine,
Recueil des travaux,vol. 6, pp. 375-426, Grenoble, 1918 (abstracted at length by Sanders, E. M., The cycle
of erosion In a karst region, after Cvija6: Geog. Review, vol. 11, pp. 593-604,1921).