CLAY DUNES 755 
- and curls up, giving the wind a chance to get hold upon the material 
which is then broken up by its action into small grains and driven 
out upon the surrounding higher land. ‘The wind velocity in this sec- 
tion is rather high and prevailingly from the southeast; therefore the 
dunes are formed principally upon the northwest side of the lagoons. 
As soon as the small granules of clay are blown out of the lagoons 
they encounter vegetation and are stopped. ‘The first rain causes 
them to break down and coalesce, thus forming a compact mass 
which is no longer moved by the wind. By this process these clay 
ridges or clay dunes have been built up many feet above the sur- 
rounding lands. 
In many instances these lagoons have no outlet and the amount 
of material in the ridges, if placed in the depressions, would be 
sufficient to bring them to the level of the surrounding country. 
It looks very much as if the material forming the dunes had been 
scooped out and piled along the banks by a great dredge. In fact, 
many of these depressions have doubtless been formed, or at least 
enlarged, in just this manner, Nature having hollowed them out 
with her great scoop—the wind. 
These “clay dunes” are confined to the coast country from a point 
directly east of Raymondville southward to and possibly beyond the 
mouth of the Rio Grande. They are always found in the low coast 
country, generally within 5 miles of the Laguna Madre. The ques- 
tion why they occur here and not at the mouth of the Mississippi 
and other rivers requires some explanation. This will be found, 
it is believed, in the rather peculiar climate conditions of this section. 
A study of the precipitation records in this section will show that 
the rains are very irregular in their occurrence and that long periods 
of deficient rainfall may be followed by others of very heavy precipi- 
tation. The annual precipitation at Brownsville has varied from 
8.88 inches in 1870 to 60.06 in 1886, and the monthly from o.0 to 
as much as 30.57. During the long periods when there is little 
or no rainfall the lagoons nearly all become dry while in a more 
humid climate, where the rains are more frequent, sufficient time ° 
for drying out is not given and no clay dunes are found. The Rio 
Grande is the only large river in the country where such conditions 
exist and this is the reason why these clay dunes are confined to this 
locality. 
