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A coastal plain varying in width up to 100 miles borders the Gulf of Mexico in the southern portion of the United States. This plain is tilted 5 feet per mile toward the Gulf. This almost imperceptible slope extends out into the open water where the gradient is 8-12 feet per mile on top of the continental shelf, but steepens to 400-600 feet per mile off the edge of the shelf. This change in slope occurs at the 100-fathom line. The shelf is 50 miles wide south of Mobile Bay and 70 miles wide at the mouth of the Rio Grande, but reaches a maximum width of 150 miles between these points south of the Sabine River.
Several hundred salt domes have been discovered on the coastal plain, and domes have already been located by geophysical work in the open water. More than 140 dome-like topographic prominences with relief varying from 12 feet to 600 feet are present along the edge of the shelf.
The Mississippi River is building its delta across the shelf at the rate of one mile in 16 or 17 years and is now within 12 or 15 miles of the edge of the shelf. The natural levee along the Mississippi serves as a ramp from which oil operations have taken place, and ten or more domes are now producing from this ramp. These domes are well out on the shelf, thus there is actually nothing new about oil production from the shelf area.
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