Exploration of the slope off the Magdalena River with reflection profiling has shown a series of large diapiric intrusions with hills rising as much as 600 ft above the sea floor. These diapirs mostly are bordered by down-bent formations in contrast to the usual, but not invariable, upbending around salt domes. Because salt domes are unknown in the entire area, and because plastic mud layers must be common in the delta-front beds, it seems likely that these are mud diapirs or possibly some type of underwater mud volcanoes.

The mud lump islands off the Mississippi delta generally have been considered rather unique. However, the finding of diapiric intrusions, which are probably mud, off the Magdalena delta and the finding of many hills off other deltas suggest that such intrusions may not be unusual.

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