66 OIL FIELDS OF TEXAS-LOUISIANA COASTAL PLAIN, [bull.212. 
CHANGES OF LEVEL DURING DEPOSITION OF COASTAL PLAIN 
FORMATIONS. 
At the close of the Eocene the Frio clays occupied a broad belt 
above sea level and formed the coast line of this portion of the conti- 
nent. The coast had been rising for a long time, and then remained 
practically 'stationary at a comparatively high level long enough to 
permit of the extensive erosion to which the Frio beds have been sub- 
jected, and at the same time to permit the deposition of some 900 to 
1,000 feet of sandstones and limestones in the comparatively shallow 
water offshore. During the deposition of these sandstones and lime- 
stones there appear to have been considerable oscillations, the land 
sinking toward the east and rising to the west. It formed a low sandy 
shore, with broad stretches covered at intervals by tide. 
FolloAving the deposition of these sandstones the land began to sink 
again, and continued sinking until a depth of at least 500 feet was 
reached throughout southeastern Texas and southwestern Louisiana. 
This sinking enabled the deposits then being laid down to cover a very 
wide stretch, extending as far northward as the old Cretaceous shore 
line in the Texas areas and in some places overlapping it. These 
deposits, however, did not completely submerge the whole of the 
Eocene of Texas. Some of the higher divides appear to have been 
above water at 1 hat time, as they are not covered with the brown sands 
and sandy clays of these later deposits and there are no evidences of 
their ever having been covered. It was during this period that the 
brown sands, sandy clays, ami gravels of the Lafayette were laid down. 
Another change took [dace and the land again began to rise. The 
elevation was apparently al a more rapid rate than the previous 
depression. This elevation gradually increased until the present 
shore line in the Texas areas had an approximate elevation of 400 
feet and the Lafayette shore lay at least 150 miles seaward. 
This elevation of the land continued for a period long enough to 
permit of the removal of these sands from a very large area and the 
formation of wide channels along the courses of the rivers and the 
deposition of the heavy beds of gravel which now everywhere cover 
the lower levels of the country. 
During this period the rivers then in existence had extended their 
channels seaward, and the Brazos, Trinity, and Sabine united in their 
lower reaches and flowed for miles through a single channel. 
Following this high continental condition was another depression, 
and the brown and gray sands and sandy clays of the Columbia epoch 
were deposited. Again the sea enroached upon the land, but not 
nearly to the extent it had done during the preceding Lafayette, as 
the present shore line sank only to a depth of about 100 feet below sea 
level. 
