topographical factors in epidemiology of malaria 
137 
United States the Atlantic Coastal Plain ex¬ 
tends from New England to the southern 
tip of Florida, rising slowly to the Appa¬ 
lachian Highlands. Beyond these moun¬ 
tains lie the Central Plains and Great 
Plains of the Middle West, which extend 
to the Rocky Mountains, the Western Pla¬ 
teaus and the Pacific Ranges. The Gulf or 
Southern Coastal Plain borders the Gulf of 
Mexico, and leads to the Ozark Plateau, 
part of the Central Plains and the Pied¬ 
mont Plateau in the north (Fig. 1). 
The regions richest in swamps and marshes 
are, for the most part, located in the coastal 
plains. Extensive natural breeding places 
for A. quadrimaculatus are in general re¬ 
stricted to this lowland belt. The Gulf 
Coastal Plain is greatly widened in the 
vicinity of the Mississippi River, extending 
north in a pronounced embayment to south¬ 
ern Illinois. This area is a great alluvial 
plain covering 300,000 square miles, in 
which optimum conditions occur frequently 
for the propagation of A. quadrimaculatus. 
Two topographical features encountered 
in the coastal plains, namely, meandering, 
old-age streams and “solution topography” 
described by Boyd and Ponton (1933), are 
chiefly responsible for the prevalence of 
malaria in this region. 
When a surface has been cut by erosion 
and solution to an elevation only slightly 
above that of the master streams of a region, 
the tributary streams flowing over the sur¬ 
face are unable to cut their channels deeper. 
Under these conditions the gradient of the 
tributary streams is lessened, they become 
sluggish, meandering across the flat surface 
in tortuous courses. Save for periods of 
flood, movement in these streams is slow 
and occasional pools may be almost stag¬ 
nant. In periods of drought, movement of 
the water is often stopped and the stream 
may be converted into a series of pools. 
During flood periods these old age streams 
frequently take more direct routes, cutting 
across the necks of loops in their normal 
courses. With the recession of the flood 
level, semi-circular ponds may be left where 
the former channel lay. The river, pur¬ 
suing a new course through the “cut-off,” 
leaves residual ponds in the old channel 
which are called “ox-bows” colloquially. 
These ponds may be refilled during flood 
Fig. 1. Principle physiographic divisions of the United States (from Compton’s Pictured Encyclope¬ 
dia 1937). 
