16 OIL FIELDS OF TEXAS-LOUISIANA COASTAL PLAIN, [bull.212. 
GEOLOGY OF THE WESTERN DIVISION. 
Little detailed study has been given to this division, and it lias 
little present importance in connection with oil production. Its main 
characteristics may, however, be briefly outlined. 
SURFACE FORMATIONS. 
Brown and gray sand appear to be the mantle covering everything 
from the border of the Fayette sandstones on the north to the coast. 
In going southward from San Diego a belt of brown sand, probably 
25 miles in width, is first crossed. This is a rolling country more or 
less covered with ruesquite and chaparral. South of this comes the 
gray sand belt, having a width varying from 50 to GO miles, and, with 
the exception of a few live oaks, practically destitute of trees. This 
is the region described by Loughridge, in his report on cotton pro- 
duction for the Tenth Census, as the great Texas desert. Across the 
face of these gray sands stretch two belts of moving sandhills, or 
medanos. These bells have an approximately east-west direction, as 
the prevailing winds are from the east. Eacli belt consists of a 
double rbw of dunes from a half mile to a mile apart, the one row 
overlapping the other. Near the coast most of these dunes or medanos 
appear only as white spots slightly elevated above the general level 
of the plain. As they travel westward the dunes gradually acquire 
size and elevation until in northern Starr County some of them have 
reached elevations of from 90 to 120 feet. In form these dimes are 
usually semilunar, having the heavy body of sand toward the center 
and the two ends tailing off to the west to form the horns of the cres- 
cent. Some of them are almost circular, with a depression in the 
center; others are oval, with their longer axis parallel with the course 
the hill may be traveling. Nothing appears to stop their westward 
course, and in several instances tall live oaks have been buried so 
deeply that only the dead tops of the highest branches show the fate 
of the grove. 
The sand forming these hills is extremely fine in texture and has a 
snow-white color. The crests of the dunes assume fantastic shapes, 
and the lightest wind sets the fine grains in motion. These dunes 
are very difficult to ascend, the climber sinking almost to the knee at 
every step, and, owing to the fineness of the particles, his eyes, mouth, 
and nose are almost constantly filled with sand. 
These dune belts lie almost parallel to each other, the first or north- 
ern line stretching across the country through the northern ends of 
Cameron, Hidalgo, and Starr counties, and the second or southern 
belt extending across the same counties about 50 miles farther south. 
The blue clays underlying the sands do not appear in many places 
and very little is known about their stratigraphy or areal extent. 
They may be the equivalents of the white and yellow clays forming 
