TOPOGRAPHICAL FACTORS IN EPIDEMIOLOGY OF MALARIA 
139 
the roof of a cave is near the ground surface 
and is not supported by other rock forma¬ 
tions it finally becomes so thin that it col¬ 
lapses from the w6ight of its overburden. 
A depression results, the size of which is 
determined principally by that of the 
cavern. The depth may be little or as 
much as 100 feet; the diameter may be a 
few feet or several hundred yards. A re¬ 
gion which is characterized by the presence 
of these depressions is referred to as having 
a “solution topography” (Fig. 2). Re¬ 
gions with little surface relief, which are 
underlain with limestone, offer the best 
opportunity for the development of solution 
topography. They are usually covered by 
deposits of clay which tend to hold rain¬ 
water under hydrostatic pressure, thus en¬ 
abling it to take into solution more carbon 
dioxide. 
The relationship between the bottom of a 
limesink depression and the ground water 
table and the nature of the region in which 
it occurred determine whether it will be 
filled with water of a permanent or semi¬ 
permanent character. If the bottom of the 
depression lies well above the level of the 
ground water and vertical drainage is not 
obstructed, surface water flowing into it 
may pass quickly into underground chan¬ 
nels. These conditions are seen frequently 
in eastern Kentucky, in the Tennessee 
Valley Area in Eastern Tennessee and in 
southwestern Virginia. In these places 
limesinks are common but, because of ver¬ 
tical drainage, they rarely hold water for 
more than a day or two, except during pe¬ 
riods of heavy rainfall. 
In situations where the surface run-off 
carries fine clay from the top soil into the 
depression, natural drainage may be im¬ 
peded or prevented even though the bottom 
of the depression lies well above the water 
table. Water filters through this clay very 
slowly and a pond forms which is main¬ 
tained by normal rainfall although its level 
may recede considerably during dry seasons. 
When the level of the natural drainage of 
a depression is the same as, or below, that 
of the ground water table a permanent 
pond is formed, which is fed by the water 
table in addition to the surface run-off. 
The size, age and permanency of collec¬ 
tions of water in limesinks determine the 
nature of the vegetation which they con¬ 
tain. Permanent spring-fed ponds of con¬ 
siderable size and age often support dense 
growths of trees, such as tupelo gum and 
cypress. Nearly all of them contain at least 
marginal growth of dense emergent vege¬ 
tation which consists of small trees and 
plants of various kinds. The pH of their 
waters depends in part upon the vegetation 
they contain. In the Tennessee Valley re¬ 
gion the waters are usually neutral or alka¬ 
line although occasionally acid waters, due 
to dissolved carbon dioxide, may be encoun¬ 
tered. In northern Florida, southern Geor¬ 
gia and Alabama small limesink ponds are 
sometimes acid in reaction, due to the de¬ 
composition of vegetable debris. 
With the exception of propagation in col¬ 
lections of water which have been produced 
artificially, such as impounded water proj¬ 
ects, irrigation, and water in excavations, 
the production of A. quadrimaculatus is 
associated principally with still water pro¬ 
duced by the two topographic conditions 
described above. Moreover, it has been 
shown that the foci of endemic malaria in 
the southeastern Atlantic seaboard states 
and soutji Alabama are also associated inti¬ 
mately with these topographic conditions 
(Boyd and Ponton 1933). They call atten¬ 
tion particularly to the occurrence of the 
most intense malaria in this region in the 
area lying between 100 and 500 feet above 
sea level. 
These authors, as well as Andrews (1938), 
mention another type of topography asso¬ 
ciated with Anopheles production and ma¬ 
laria endemicity in the area which lies less 
than 100 feet above sea level. They call this 
the “sandy flat woods section” and An¬ 
drews says of this area in Georgia, “This 
land is notable for its flatness, and many 
of the water-holding basins are man-made 
borrow pits fed by flowing wells. The ma¬ 
jority of these provide satisfactory condi¬ 
tions for the production of A. quadrimacu¬ 
latus.” Naturally, there is a low incidence 
of malaria in the sandy flat woods sections. 
Since the tree-grown ponds and swamps 
contain accumulations of vegetable matter 
