CLAY DUNES 
GEORGE N. COFFEY 
Washington, D.[C. 
Numerous instances of dunes composed of sand can be found in 
almost all countries, especially along the coast or in dry regions 
where the geological formations are of a sandy character, but dunes 
made up of clay have never, so far as I have been able to ascertain, 
been reported. 
During the last winter the writer was engaged in making a recon- 
noissance soil survey of south Texas for the United States Depart- 
ment of Agriculture and in the prosecution of the work encountered 
near the mouth of the Rio Grande some long, narrow clay ridges, 
the formation of which was at first very puzzling. These ridges 
were sometimes several miles long, 30 feet high, and not over 200 
or 300 yards in width. Occurring in a section of very level, semi- 
marsh land and being often covered with mesquite, cactus, and 
palmetto, while the surrounding country shows only a growth of 
coarse salt grass, they form-a very striking feature of the landscape. 
How had they been formed? To what agency were they due? 
One day while driving north of the mouth of the Arroyo Colorado, 
an old outlet of the Rio Grande, there appeared in the distance what 
looked to be a mile or more stretch of almost white drifting sand. 
The soil around was a heavy, marshy clay. On reaching the ridge, 
which was probably 20 feet high, what appeared, at first glance, 
to be grains of sand were seen blowing across the bare surface. 
A closer examination, however, showed them to be granules of clay 
which could be easily broken down between the fingers and when 
moistened became very plastic and sticky. Here was the explanation 
of the formation of these clay ridges; they were really clay dunes. 
Further observation showed that these “clay dunes” were almost 
always associated with a lagoon. Where such relation does not exist 
at the present time it is doubtless due to the filling of the lagoon 
by sediments brought down by the river. During dry weather these 
lagoons become partially or entirely dry, and the mud then cracks 
754 
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